Assessment of Exercise-Associated Gastrointestinal Perturbations in Research and Practical Settings: Methodological Concerns and Recommendations for Best Practice

in International Journal of Sport Nutrition and Exercise Metabolism

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Ricardo J.S. CostaDepartment of Nutrition, Dietetics and Food, Monash University, Notting Hill, VIC, Australia

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Pascale YoungDepartment of Nutrition, Dietetics and Food, Monash University, Notting Hill, VIC, Australia

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Samantha K. GillSt Peter’s Hospital, Ashford and St Peter’s Hospitals NHS Foundation Trust, Surrey, United Kingdom

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Rhiannon M.J. SnipeCentre for Sport Research, School of Exercise and Nutrition Sciences, Deakin University, Burwood, VIC, Australia

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Stephanie GaskellDepartment of Nutrition, Dietetics and Food, Monash University, Notting Hill, VIC, Australia

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Isabella RussoDepartment of Nutrition, Dietetics and Food, Monash University, Notting Hill, VIC, Australia

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Louise M. BurkeMary MacKillop Institute for Health Research, Australian Catholic University, Melbourne, VIC, Australia

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Strenuous exercise is synonymous with disturbing gastrointestinal integrity and function, subsequently prompting systemic immune responses and exercise-associated gastrointestinal symptoms, a condition established as “exercise-induced gastrointestinal syndrome.” When exercise stress and aligned exacerbation factors (i.e., extrinsic and intrinsic) are of substantial magnitude, these exercise-associated gastrointestinal perturbations can cause performance decrements and health implications of clinical significance. This potentially explains the exponential growth in exploratory, mechanistic, and interventional research in exercise gastroenterology to understand, accurately measure and interpret, and prevent or attenuate the performance debilitating and health consequences of exercise-induced gastrointestinal syndrome. Considering the recent advancement in exercise gastroenterology research, it has been highlighted that published literature in the area is consistently affected by substantial experimental limitations that may affect the accuracy of translating study outcomes into practical application/s and/or design of future research. This perspective methodological review attempts to highlight these concerns and provides guidance to improve the validity, reliability, and robustness of the next generation of exercise gastroenterology research. These methodological concerns include participant screening and description, exertional and exertional heat stress load, dietary control, hydration status, food and fluid provisions, circadian variation, biological sex differences, comprehensive assessment of established markers of exercise-induced gastrointestinal syndrome, validity of gastrointestinal symptoms assessment tool, and data reporting and presentation. Standardized experimental procedures are needed for the accurate interpretation of research findings, avoiding misinterpreted (e.g., pathological relevance of response magnitude) and overstated conclusions (e.g., clinical and practical relevance of intervention research outcomes), which will support more accurate translation into safe practice guidelines.

Exercise-associated gastrointestinal symptoms (Ex-GIS), such as abdominal pain and nausea, have been reported in the scientific literature for almost a century (Burgess et al., 1924). However, awareness of disturbances to the gastrointestinal tract, and subsequent instigation of Ex-GIS, as a result of physical exertion, started its primary exploration in the 1980s, with landmark exploratory investigations reporting high incidence and severity of Ex-GIS in endurance-based exercise and with mechanistic explanations (Brouns et al., 1987; Rehrer et al., 1989, 1992; Rehrer & Meiger, 1991; Worobetz & Gerrard, 1985). These research outcomes prompted a flurry of in-depth research focused on the etiology and pathophysiology of Ex-GIS. With further follow-on and ongoing in-depth exercise gastroenterology research, the impact of exercise stress and adjusted external factors, such as ambient conditions, on gastrointestinal integrity and function, and subsequent systemic and Ex-GIS outcomes, are now better understood from an exercise performance and clinical significance perspective (Costa, Snipe, et al., 2017; Costa et al., 2020).

Perturbations to gastrointestinal integrity resulting from splanchnic hypoperfusion and neuroendocrine-instigated intestinal epithelial injury and hyperpermeability may lead to translocation of luminal bacteria and/or bacterial endotoxins, and subsequent local and/or systemic inflammatory responses. Simultaneously, gastrointestinal functions, such as, motility, digestion, and absorption may be impaired as a result of such epithelial injury and/or dysfunction. Exercise-associated deactivation of the submucosa and myenteric plexuses, or other functional influencing components of the gastrointestinal tract, such as the interstitial cells of Cajal, may also contribute to the impairment in gastrointestinal function (Gaskell, Rauch, et al., 2021). The established term “exercise-induced gastrointestinal syndrome” (EIGS) defines these primary causes and secondary outcomes that naturally occur with the onset and ongoing exercise (Figure 1a). The pathophysiological pathways and factors that exacerbate EIGS have been comprehensively described and updated by Costa, Snipe, et al. (2017), Costa et al. (2020), and Gaskell, Lis, and Costa (2021), with clinical and performance implications, and professional practice application, presented in Gaskell et al. (2021a, 2021b). From an exercise performance perspective, such pathophysiology can lead to minor inconveniences linked with incidence of Ex-GIS (Figure 1b), or to more severe Ex-GIS that negatively affects performance outcomes and may warrant medical attention to manage the incidence and severity (Costa et al., 2016; Costa, Miall, et al., 2017; Engebretsen et al., 2013; Pillay et al., 2022). Moreover, of clinical concern, such pathophysiology has been consistently linked to nonfatal gastroparesis and acute reversible colitis (Benmassaoud et al., 2014; Cohen et al., 2009; García Gavilán et al., 2021; Gaskell, Parr, et al., 2021; Grames & Berry-Caban, 2012; Horner et al., 2015), through to potentially fatal systemic inflammatory responses (Gill, Teixeira, et al., 2015; Laitano et al., 2019; Roberts et al., 2021).

Figure 1
Figure 1

—(a) Updated schematic description of EIGS, and associated extrinsic and intrinsic exacerbation factors. Physiological changes in circulatory and neuroendocrine pathways at the onset of exercise resulting in perturbed gastrointestinal integrity and function, and may lead to GIS, and/or acute or chronic health complications. aSpecialized antimicrobial protein-secreting (i.e., Paneth cells) and mucus-producing (goblet cells) cells, aid in preventing intestinal-originating pathogenic microorganisms entering systemic circulation. bIncrease in neuroendocrine activation and suppressed submucosal and myenteric plexus may result in epithelial cell loss and subsequent perturbed tight junctions (Barrett, 2012; Holzer et al., 2017). cSplanchnic hypoperfusion and subsequent intestinal ischemia and injury (including mucosal erosion) may result from stress induced direct (e.g., enteric nervous system, and/or enteroendocrine cell) or indirect (e.g., braking mechanisms) alterations to gastrointestinal motility (Grootjans et al., 2016; Shin et al., 2013). dGastrointestinal brake mechanisms: Nutritive and nonnutritive residue along the small intestine, and inclusive of terminal ileum, results in neural and enteroendocrine negative feedback to gastric activity (Layer et al., 1990; Miall et al., 2018; Shin et al., 2013; van Avesaat et al., 2015; van Citters & Lin, 2006). eAggressive acute or low grade, prolonged mechanical strain, is proposed to contribute toward disturbances to epithelial integrity (i.e., epithelial cell injury and tight junction dysregulation) and subsequent “knock-on” effects for gastrointestinal functional responses (Lin et al., 2014). fBacteria and bacterial endotoxin MAMPs and stress induced DAMPs are proposed to contribute toward the magnitude of systemic immune responses (e.g., systemic inflammatory profile; Fleshner & Crane, 2017). gPreexercise fecal bacterial and short chain fatty acid concentrations (Bennett et al., 2020; Gaskell, Gill, et al., 2021; Young et al., 2021). hFood and fluid intake tolerance level (Rauch et al., 2022). (b) Schematic illustration depicting the relationship between EIGS pathophysiology and GIS (Costa, Gaskell, et al., 2020). EIGS = exercise-induced gastrointestinal syndrome; GIS = gastrointestinal symptoms; MAMPs = microorganism molecular patterns; DAMPs = danger associated molecular patterns. Adapted with permission from “Systematic Review: Exercise-Induced Gastrointestinal Syndrome—Implication for Health and Disease,” by R.J.S. Costa, R.M.J. Snipe, C. Kitic, & P. Gibson, 2017, Alimentary Pharmacology and Therapeutics, 46(3), pp. 246–265. Copyright 2017 by John Wiley & Sons Ltd.

Citation: International Journal of Sport Nutrition and Exercise Metabolism 32, 5; 10.1123/ijsnem.2022-0048

Concern around the potential implications of EIGS and Ex-GIS on exercise performance and health of athletes undertaking strenuous exercise explains the recent exponential growth in exercise gastroenterology research. This research has focused on discovering the underlying pathophysiology of EIGS (Costa, Snipe, et al., 2017; ter Steege & Kolkman, 2012; van Wijck et al., 2011), exacerbation factors (Bennett et al., 2020; Costa, Miall, et al., 2017; Gaskell et al., 2020; Gaskell, Parr, et al., 2021; Horner et al., 2015; Snipe et al., 2018a, 2018b), and more recently, strategies to prevent and manage EIGS and Ex-GIS (Costa, Hoffman, & Stellingwerff, 2019; Gaskell et al., 2021b). However, it is evident that a large proportion of studies in this research area to date contain substantial limitations in experimental methodologies to a degree that may influence data outcomes and interpretations. Such methodological limitations appear to have been overlooked, and/or due to the rapid evolving nature of exercise gastroenterology research and practice field, have not previously been known, identified, or addressed. For example, while many studies have investigated the impact of various EIGS prevention and management strategies on singular or limited combinations of gastrointestinal status markers, with or without assessment of Ex-GIS; very few have provided a comprehensive assessment of gastrointestinal integrity and function, or circulating systemic markers, to inform data interpretation and translation implication (Costa, Snipe, et al., 2017; Costa et al., 2020). It is also notable in the mainstream published literature that a substantial proportion of studies rely on singular or limited indirect “gateway” (e.g., intestinal epithelial injury and/or permeability) markers with minimal magnitude of exercise-induced change. These minimal changes in magnitude on “gateway” marker are then interpreted to ascertain potential effects on key performance and clinical outcome markers, such as exercise cessation, systemic inflammatory cytokine profile, and/or medical incidence (Chantler et al., 2021, 2022; Walter et al., 2021). Research that includes a comprehensive combination of circulatory-gastrointestinal variable assessment such as intestinal epithelial and tight junction injury or dysfunction, and neuroendocrine-gastrointestinal variable assessment such as gastrointestinal transit and nutrient malabsorption, markers of systemic outcomes such as bacteria and bacterial endotoxin and/or cytokine profiles, and assessment of Ex-GIS using a validated tool, where appropriate and justified, is in minority. On another concerning note, a substantial proportion of exercise gastroenterology research has not provided adequate experimental control for known confounders of EIGS markers. These may include, but not limited to, preexercise dietary intake, food and fluid intake during exercise, hydration status, environmental conditions, circadian variation, individual predisposition, fitness status, age, biological sex, plasma and/or fecal bacterial, and short chain fatty acid composition (Bennett et al., 2020; Costa, Miall, et al., 2017; Costa, Camões-Costa, et al., 2019; Costa, Camões-Costa, et al., 2020; Gaskell et al., 2019, 2020; Gaskell, Gill, et al., 2021; Gaskell, Parr, et al., 2021; Gaskell et al., 2021b; Snipe & Costa, 2018a, 2018b; Snipe et al., 2017, 2018a, 2018b; Young et al., 2021). These limitations, including selecting erroneous measurement variables and analytical techniques, can lead to misguided and misinterpreted conclusions, as well as inappropriate and/or overstated translation into professional practice, such as advising a particular EIGS and/or Ex-GIS prevention or management strategy when no real world and consistent effect truly exists.

The limitations within the available exercise gastroenterology research raise the possibility that different outcomes would occur if more comprehensive scientifically rigorous and robust experimental procedures were applied. The risk of misrepresenting research outcomes can have significant translational implications for practitioners, with outcomes ranging from ineffective interventions to the risk of fatalities. Among athlete populations, these may include, but not limited to, septic shock instigated from luminal originating pathogenic agents translocating into systemic circulation and subsequent systemic inflammatory responses (Hodgin & Moss, 2008; Laitano et al., 2019; Roberts et al., 2021), which can also affect occupational populations exposed to exertional heat stress, such as military personnel and the mining and agricultural industry. Given the increasing interest in exercise gastroenterology research, reflection on methodologies, data analysis, and practical interpretation is critical to enhance the reliability and safety of future scientific and translational advancement. The primary aim of this review is to characterize methodological concerns and recommend standards of best practice within exercise gastroenterology research.

Participant Screening

Concern

Individuals with established or predisposition to gastrointestinal inflammatory or functional diseases/disorders may present greater magnitude of EIGS and associated Ex-GIS in response to exercise (Costa, Snipe, et al., 2017). A substantial number of exercise gastroenterology research studies have been observed to not provide comprehensive information on protocols for screening individuals for predefined inclusion/exclusion criteria, including providing information of participant characteristics known as potential confounding factors of EIGS and Ex-GIS. These may include, but not limited to, specified and targeted population demographics, medical conditions, restrictive dietary practices and/or supplement use, and/or pharmaceutical prescription.

Recommendation

Participants’ inclusion and/or exclusion criteria should be reported. This may include declaration of, but not limited to (a) gastrointestinal infections, diseases, disorders, past history of gastrointestinal surgery, and other self-reported gastrointestinal issues; and (b) dietary intake of nutrients known to alter integrity and function of the gastrointestinal tract, such as fermentable oligo-, di-, monosaccharides and polyols (FODMAP), fiber, and/or macronutrient, nonnutritive nutritional supplements (e.g., prebiotics, probiotics, synbiotics, amino acids, and/or antioxidants), and/or pharmaceutical agents (e.g., nonsteroidal anti-inflammatory drugs, antibiotics, laxatives, antidiarrhea agents, antacids, and/or antiemetics). In addition, the participants reported gastrointestinal condition lifespan, adherence time course of any altered dietary regime, nonnutritive supplement/s, and/or pharmaceutical agent/s, where applicable.

Exertional Stress Load

Concern

A minimal degree of exertional stress is needed for disturbances to gastrointestinal integrity and function, associated systemic immune responses, and Ex-GIS to reach levels of clinical significance, meaning are comparable to values synonymous with diseases or disorders of the gastrointestinal tract and/or warranting medical management (Al-Saffar et al., 2017; Gaskell et al., 2021b; Haas et al., 2009; Jekarl et al., 2015; Linsalata et al., 2018; Martinez-Fierro et al., 2019; Pelsers et al., 2005; Power et al., 2021; Surbatovic et al., 2015); or performance debilitating significance, such as reducing workload, temporary cessation, or full withdrawal from exercise (Costa et al., 2016; Costa, Miall, et al., 2017; Miall et al., 2018; Walter et al., 2021). It is clear from the available literature that a substantial number of laboratory-controlled exercise gastroenterology research using steady-state and/or high-intensity exercise protocols <2 hr in duration, performed in temperate ambient conditions (∼20 °C), have reported none to modest perturbations to markers of gastrointestinal integrity and function, and none to minimal reports of Ex-GIS. Indeed, it is now well established that the incidence and severity of Ex-GIS are prominent only in ultra-endurance activities and/or exertional heat stress, and there appears to be of no to little meaning with lesser exertional stress loads (i.e., <2 hr of exercise) in temperate conditions, irrespective of the exercise intensity (Costa et al., 2016; Engebretsen et al., 2013; Pfeiffer et al., 2012; ter Steege et al., 2008), and modality, albeit in running versus cycling comparisons only (Costa et al., 2022; unpublished data).

Recommendation

The generalized minimal criteria for inducing gastrointestinal and systemic disturbances (Figures 2 and 3) in controlled-laboratory experimental models or individual gastrointestinal assessment in clinical practice appear to be 2 hr of exercise (e.g., running or cycling) at ≥60% V˙O2max in hot (i.e., ≥35.0 °C) ambient conditions, or 3 hr of exercise in temperate conditions, or equivalent. Assessment of lower exertional loads/duration, irrespective of exercise intensity or modality, appears to be generally redundant as it results in none to minimal gastrointestinal disturbance, lacks clinical and/or performance relevance, and is not associated with reports of severe or medical management requiring Ex-GIS in real-life sports competition. However, considering the infancy of exercise gastroenterology research and practice, lesser exertional and/or exertional heat stress models may have a useful place by providing benefit in exploring mechanistic aspects of EIGS and Ex-GIS and/or exploring newer prevention and management strategies prior to employing more aggressive exploration experimental models. Such an approach has its advantage by reducing burdens of more profound exertional and/or exertional heat stress models, which are subject to participant recruitment difficulties and potentially higher participant drop-out rates. It is also noted that EIGS and Ex-GIS have not been adequately investigated in a number of endurance and ultra-endurance sports, including open water swimming, rowing and kayaking, adventure or obstacle racing, cross-country skiing, and motor enduro events. Exploratory research in these modalities should study the magnitude of gastrointestinal perturbations in actual events or mirrored simulations of their characteristics if using laboratory settings. Researchers should acknowledge the challenges of undertaking randomized-controlled trials of strategies to prevent or manage gastrointestinal issues in field settings (i.e., competitive events or field simulations). The difficulty of controlling the many potential confounding factors in such settings threatens research rigor and robustness, subsequently increasing the risk of misinterpretation of results. This can be seen when attempting to investigate prevention and/or management strategies (e.g., nutritional supplementation and physical manipulation) to exercise-associated gastrointestinal disturbances and Ex-GIS in field-based settings. Whereby discrepancies in hydration status between comparison groups is likely and difficult to control, food and fluid intake during competition is difficult to accurately and comprehensively obtain and standardize, and/or optimal sample collection timing may be difficult to standardize (Pugh et al., 2019; Roberts et al., 2016; Zadow et al., 2021).

Figure 2
Figure 2

—Exercise-associated disturbances to markers of gastrointestinal integrity, systemic responses, and GIS. Plasma (a) I-FABP, (b) sCD14, (c) LBP, (d) gram-negative bacterial endotoxin, (e) antiendotoxin IgM, (f) exercise-associated SIR profile, (g) incidence, and (h) severity of GIS using standardized experimental procedures. *Carbohydrate provisions preexercise, frequently, and consistently throughout the exercise bout. Reporting of GIS incidence and severity in accordance with Gaskell et al. (2019). Extracted data (mean ± SEM) from: aCosta, Camões-Costa, et al. (2019), bGaskell et al. (2020), cGaskell, Rauch, et al. (2021), dGill, Allerton, et al. (2016), eKing et al. (2022), fRusso et al. (2021a), gRusso et al. (2021b), hRusso et al. (2021c), iSnipe et al. (2017), jSnipe et al. (2018a), kSnipe et al. (2018b), lSnipe and Costa (2018a), and mWallett (2021). Percentage value indicative of peak magnitude of change (%) from preexercise baseline. Reported ambient temperature (°C) rounded to the nearest whole number. EndoCAb = endogenous endotoxin antibodies; GIS = gastrointestinal symptoms; HITT = high intensity targeted training; I-FABP = intestinal fatty acid-binding protein; Ig = immunoglobulin; LBP = lipopolysaccharide-binding protein; sCD14 = soluble CD14; and SIR = systemic inflammatory response.

Citation: International Journal of Sport Nutrition and Exercise Metabolism 32, 5; 10.1123/ijsnem.2022-0048

Figure 3
Figure 3

—Exercise-associated systemic inflammatory response to (a) exertional stress (extracted data from Costa, Camões-Costa, et al., 2019; Gaskell, Rauch, et al., 2021; Russo et al., 2021a, 2021b, 2021c; Snipe et al., 2018a) and (b) exertional heat stress (Gaskell et al., 2020; Gill, Allerton, et al., 2016; Snipe et al., 2017, 2018a). Data are presented as mean ± SEM. TNF = tumor necrosis factor; IL = interleukin.

Citation: International Journal of Sport Nutrition and Exercise Metabolism 32, 5; 10.1123/ijsnem.2022-0048

Exertional Heat Stress Load

Concern

It is now well established that EIGS is exacerbated by heat stress during physical exertion (Pires et al., 2017, 2018; Snipe & Costa, 2018a; Snipe et al., 2017, 2018a, 2018b). However, this appears to occur substantially only when exertional heat stress overrides the body’s thermoregulatory capabilities, and peak core body temperature reaches ≥39.0 °C (Gaskell et al., 2020; Gill, Allerton, et al., 2016; Snipe & Costa, 2018a; Snipe et al., 2017, 2018a, 2018b). A substantial number of exercise gastroenterology research studies investigating gastrointestinal disturbances during exercise with heat exposure have shown to not provide sufficient exertional heat stress due to inadequate exercise load and/or insufficient heat stress. For example, those studies that use of warm ambient conditions (e.g., ∼30.0 °C; Sheahen et al., 2018; Snipe et al., 2018b), or walking protocols with heat exposure (Ogden et al., 2020a, 2020b), resulting in mild nonproblematic increases in core body temperature (e.g., <39.0 °C), and none to modest disturbance to gastrointestinal epithelial integrity or function, systemic immune responses, and/or Ex-GIS. Consequently, it is well defined that exercise research protocols that do not produce sufficient thermal strain (e.g., core temperature <39.0 °C) fail to elicit enough intestinal epithelial injury, systemic endotoxin, and inflammatory responses, and Ex-GIS to clearly detect differences between intervention and control trials or groups. This is compared to scenarios involving sufficient exertional heat stress, such as 2 hr of running at 60% V˙O2max in ∼35 °C ambient conditions that consistently results in core temperature ≥39.0 °C (Figures 2 and 3).

Recommendation

Protocols should ensure sufficient exercise stress and ambient temperature is applied to achieve a maximum core body temperature of ≥39.0 and/or ≥2.0 °C from preexercise baseline core body temperature. A minimum exertional heat stress model that consistently induces thermoregulatory strain above the body’s thermal threshold capacity and substantial EIGS and Ex-GIS involves ≥2 hr of indoor treadmill running exercise at  ≥60% V˙O2max in ≥35.0 °C ambient temperature, irrespective of relative humidity (Figures 2 and 3). It also appears that indoor cycling ergometer exercise at the same relative intensity accounts for a similar exertional heat stress load (Costa et al., 2022; unpublished data); and therefore, would also be an appropriate modality to attain a sufficient thermoregulatory strain. Environmental conditions and core body temperature during exertional or exertional heat stress protocols should be monitored in real time every 10–15 min and reported accordingly. Traditionally, in both clinical and exercise settings, the gold standard protocols for measurement of core body temperature include the use of a wired thermistor inserted ∼12 cm, but up to 15 cm, past the anal sphincter (Cattaneo et al., 2000; Costa et al., 2010; Costa, Crockford, et al., 2014; Miller et al., 2017; Schmitz et al., 1995). More recently, the ingested or suppository application of wireless telemetry thermoregulation measurement devices has been established as presenting similar valid and reliable methods for determining real time in vivo core body temperature (Gosselin et al., 2019; Hosokawa et al., 2016, 2017). The use of other body temperature monitoring sites (e.g., oral, tympanic, temporal, and/or skin) should be used with caution or not at all (Hosokawa et al., 2017; Mazerolle et al., 2011; Morrissey et al., 2021).

Dietary Control

Concern

The consistent lack of dietary control in a substantial number of exercise gastroenterology research studies is concerning, particularly in view of the acute and rapid plasticity of the gastrointestinal tract and emerging evidence that preexercise dietary intake can influence the magnitude of EIGS and Ex-GIS (Costa, Miall, et al., 2017; Costa, Camões-Costa, et al., 2019; Gaskell et al., 2020; Gaskell et al., 2021b; Lis, 2019; Wallett, 2021; Young et al., 2021). Indeed, most previous studies have either failed to implement or report any methods of dietary control, have neglected to report data on pre or during trial dietary intake, and/or have used uncontrolled simplistic instruction such as asking participants to record a food–fluid log and attempt to duplicate intake on any subsequent trial/s. Such methods are considered inadequate for standardizing or reporting on pretrial dietary intake (Jeacocke & Burke, 2010), and are unacceptable within exercise research with a dietary or nutritional intervention component. Thus, findings and conclusions from previous exercise gastroenterology research studies that have failed to implement robust dietary control and/or provide a reliable reporting tool warrant caution in their translational application.

Recommendation

Gold standard experimental procedures should include the actual provision of all foods and fluids at least 24 hr before and throughout the experimental period, with an accompanying intake plan, and a subsequent prospective reporting method to assess and confirm compliance. Considering the potential impact of certain dietary carbohydrate types, such as FODMAP, nonstarch polysaccharides, and resistant starches on markers of EIGS and Ex-GIS (Gaskell & Costa, 2019; Gaskell, et al., 2020; Gaskell, Gill, et al., 2021; Halmos et al., 2015; Lis, 2019; Lis et al., 2016, 2018; Rehrer et al., 1992), it would seem imperative to standardize the intake of these highly fermentable food and fluid components leading up to and during the experimental trial/s, and subsequently reporting intake quantity. Using updated nutritional analysis software and/or validated mobile apps (e.g., www.monashfodmap.com/ibs-central/i-have-ibs/get-the-app/) may provide a useful tool for quantification and reporting of these gastrointestinal impacting foods and fluid components. Nevertheless, to prompt the greatest gastrointestinal integrity disturbance, to avoid nutrient influencing protection against EIGS, and to avoid dietary induced artifact of Ex-GIS, standardizing dietary intake prior to interventions involving the exploration and management of EIGS is advised. Such standards should employ and report a low-FODMAP diet (i.e., <5 g/day total FODMAP, with overall reductions in fructooligosaccharides, galactooligosaccharides, fructose in excess of glucose, lactose, and sugar alcohols), and meet fiber, energy, and macronutrient requirements for at least 24 hr pretrial (Gaskell et al., 2020; Gaskell, Gill, et al., 2021; Snipe et al., 2017, 2018a, 2018b). This does not apply to experimental procedures that are investigating the effects of dietary FODMAP and/or fiber, or investigations of real-life practices in field studies.

With the recent exponential advancement in technologies, including image-assisted methods for dietary assessment, traditional paper-based dietary assessment methods may become obsolete. Although in its infancy phase of development and implementation, various image-assisted software packages and mobile apps have shown promising outcomes for validity and reliability in energy and nutrient intake quantification (Boushey et al., 2017; Gemming et al., 2015; Höchsmann & Martin, 2020). However, participant burden and underestimation are still withstanding issues within image-assisted methods that warrant attention. Nevertheless, such methods may provide a useful and future approach to robust dietary control in exercise gastroenterology research. For example, the common approach in studies of advocating participants to report intake and duplicate intake, or simply record their intake before each trial is considered to achieve poor dietary control. However, using these advancing technologies in dietary tracking and compliance may provide more robustness and rigor in controlling this potent EIGS and Ex-GIS confounding factor.

Pre and During Exercise Nutrient Provisions

Concern

The intake of carbohydrate or protein, including protein derivatives, such as L-citrulline and glutamine, before and/or frequently during exercise have been consistently reported to reduce exercise-associated perturbations to gastrointestinal integrity (Alcock et al., 2018; Flood et al., 2020; Jonvik et al., 2019; Lambert et al., 2001; Pugh et al., 2017; Rehrer et al., 2005; Snipe et al., 2017; van Wijck et al., 2014). The mechanisms for these outcomes appear to be linked with maintenance of villi microvascular perfusion and/or intestinal epithelium extracellular and intracellular signaling linked to cellular function and membrane stability (de Moura et al., 2013; Grootjans et al., 2016; Kip et al., 2021; Kotler et al., 2013; Rehrer et al., 2005). However, in certain circumstances, for example, when intake load is greater than the individual’s tolerance level, macronutrient intake before and/or during exercise may exacerbate gastrointestinal functional issues (e.g., gastric or intestinal trafficking, and malabsorption) and Ex-GIS as an artifact of the experimental design (Costa, Miall, et al., 2017; King et al., 2022; Miall et al., 2018). Therefore, the total energy and nutrient spectrum of the intervention and/or placebo may interfere with EIGS markers and the interpretation of results if not consistent within and between trials, and within the tolerance level of participants. Factors that may affect gastrointestinal function and tolerance include food and fluid volume, concentration, texture, osmolality, and pH (Costa, Miall, et al., 2017; Costa, Camões-Costa, et al., 2019; Horner et al., 2015; McCubbin et al., 2020; Miall et al., 2018; Guillochon & Rowlands, 2017). However, reporting of these factors has been conspicuously absent in exercise gastrointestinal research studies that have provided food or fluid before and/or during exercise, or in studies investigating the effects of nonnutritional supplement interventions on markers of gastrointestinal status in response to exercise.

Recommendation

Intervention studies involving nutrient intake, an energy-matched placebo and/or a nonnutritive control trial should be included in the research design. In addition, it is advised that participants should be screened for: (a) carbohydrate malabsorption (Bate et al., 2010; Rezaie et al., 2017), and (b) gastrointestinal tolerance in the absence of exercise using the same in-trial assessment tool (Miall et al., 2018; Rauch et al., 2022), of the proposed food and fluid provision, before initiating the experimental trials, as some individuals may present carbohydrate malabsorption and intolerance at rest, which will result in an artifact response to exercise (Gaskell et al., 2020; King et al., 2022; Russo et al., 2021a, 2021b, 2021c). The intake of protein (and derivatives) before and during the exercise protocol should be limited unless specific to the research question and experimental design. Furthermore, characteristics of food/s and/or fluid/s that could influence gastric factors should be reported, for example, food and/or fluid nutritional composition, volume, texture, osmolality by freeze point osmometry, and pH by digital pH meter (Costa, Miall, et al., 2017; Guillochon & Rowlands, 2017; McCubbin et al., 2020).

In exercise protocols of ≤ 2 hr, carbohydrate intake during exercise is unnecessary and should be avoided in the study design. In such scenarios, only water should be provided in accordance with experimental procedures (Hoffman et al., 2018), noting that dehydration and fluid overload both have the potential to induce Ex-GIS (Costa, Camões-Costa, et al., 2019). In exercise protocols of longer duration, to aid exercise bout completion, carbohydrate intake may be applied by providing small and frequent feedings within gastrointestinal tolerance levels to avoid experimental design artifact induced Ex-GIS. However, this should be withdrawn in the last hour of exercise, to avoid any attenuating effects the nutrients may have on EIGS biomarkers (Alcock et al., 2018; Flood et al., 2020; Jonvik et al., 2019; Lambert et al., 2001; Rehrer et al., 2005; Snipe et al., 2017). Here, it is important to note that the FODMAP (e.g., fructose and/or polyols) content of carbohydrate-rich food and/or fluid provisions during exercise may influence EIGS and Ex-GIS outcomes. For example, although glucose solutions ≤ 60 g/hr appear to be tolerated by the vast majority of study participants, higher intakes of multitransportable carbohydrates (e.g., ≥90 g/hr) have been shown to increase feeding intolerance and Ex-GIS, with or without evidence of carbohydrate malabsorption, in the majority of study participants (Costa, Miall, et al., 2017; Gaskell, Parr, et al., 2021; Gaskell et al., 2021b; Miall et al., 2018; Rauch et al., 2022; Snipe et al., 2017). This is particularly the case in recreationally competitive athletes, who constitute the majority of participants in sport and exercise, explaining a greater incidence and severity of Ex-GIS.

It is well established that higher carbohydrate intake rates, such as ≥90 g/hr, using multiple transportable carbohydrates, appear to enhance circulating glucose availability, exogenous carbohydrate oxidation, and improve exercise performance outcomes, with greater gastrointestinal tolerance seen in cycling compared with running modality (Costa et al., 2022; Hearris et al., 2022; O’Brien et al., 2013; Pfeiffer et al., 2012; Stellingwerff & Cox, 2014). However, such outcomes are observed mainly in highly trained endurance athletes accustomed to carbohydrate intake during exercise with or without gut training (Costa, Miall, et al., 2017; Gaskell et al., 2021b). In addition, a recent field study reported a lack of Ex-GIS in elite ultra-endurance runners who consumed up to 120 g/hr of a 2:1 glucose–fructose gel formulation during a mountain marathon, and were supposably “gut trained,” but no data evidence was presented to support this claim (Viribay et al., 2020). Within this study, three participants withdrew due to gastrointestinal issues; however, there was no reporting of participant group/s of these withdrawals nor formal measure of Ex-GIS and/or feeding tolerance in real time or retrospectively. Caution is, therefore, warranted in using such an experimental design to interpret the impact of high rate carbohydrate intake during exercise on Ex-GIS and feeding tolerance. Considering the ability to consume carbohydrate intake during exercise, and to tolerate intake without instigating Ex-GIS, varies in accordance with fitness status, biological sex, and with differences between recreational/amateur and elite athletes frequently observed (Costa et al., 2016; Costa, Miall, et al., 2017; Jentjens, Achten, & Jeukendrup, 2004; Jentjens, Moseley, et al., 2004; King et al., 2022; Pfeiffer et al., 2012; Snipe & Costa, 2018b), it is not surprising that recent guidelines and recommendations for prolonged exercise (i.e., ≥3 hr) focus on an individually tailored approach, rather than a blanket value (Burke et al., 2019; Costa, Knechtle, et al., 2019). Individual tolerance to carbohydrate intake warrants consideration when planning carbohydrate intake rates during exercise gastrointestinal research in order to avoid artifact Ex-GIS and misrepresentation of results due to the experimental design.

Hydration Status

Concern

Preexercise hydration status, fluid intake during exercise, and postexercise rehydration are not consistently measured or accurately reported in a large number of exercise gastroenterology research studies. Although modest in its effect, hydration status has been shown to impact the magnitude of some EIGS integrity and functional markers, and subsequently Ex-GIS (Costa, Camões-Costa, et al., 2019).

Recommendation

Unless the aim of the research question and experimental procedure is to investigate preexercise hypohydration, participants should commence the exercise protocol in a euhydrated state. Additionally, hydration during exercise should be supported by allowing ad libitum drinking habits and/or programmed “small and frequent” strategies that cater for participants’ gastric tolerance and comfort (Hoffman et al., 2018). Programmed drinking that attempts to match sweat losses and/or limit body mass loss may increase the risk of fluid overconsumption and subsequent increases in intragastric pressure and artificially induced Ex-GIS (Costa, Camões-Costa, et al., 2019). Programmed drinking regimens that regulate and/or challenge hydration status, and potentially introduce a gastric burden, may be justified when investigating specific feeding requirements (e.g., gut training investigation or feeding challenge protocols) and/or exercise-induced dehydration. The results should identify any limitations and practical implications of such protocols, especially for Ex-GIS outcomes. Direct measures of either total body or extracellular hydration status using only validated and reliably checked methods should be used. These include multifrequency bioelectrical impedance analysis with established validation against total body water determination through deuterium oxide and mass spectrometry analysis, or blood-based measurements such as plasma osmolality and/or plasma volume change. The strengths and limitation of such methods have previously been explored (Armstrong, 2007). Indirect measures for determining hydration, such as, body mass change or urine measure of hydration, which are fundamentally governed by renal function, are not considered reliable, especially in exertional and exertional heat stress testing, and should be avoided for research purposes (Cheuvront et al., 2015; Costa et al., 2013; Costa, Gill, et al., 2014; Hoffman et al., 2019; Maughan et al., 2007; McCubbin et al., 2019).

Temperature of Fluid Provisions

Concern

There is evidence that the temperature of fluids consumed during exercise influences the magnitude of EIGS, with lower drink temperatures (i.e., 0 and 7 °C) providing some modest reduction in exercise-associated gastrointestinal epithelial injury, and associated Ex-GIS (Snipe & Costa, 2018a). The mechanism for these outcomes appears to be linked to an attenuation of the rise in core body temperature associated with internal percooling interventions. Meanwhile, the impact of varied pre- and/or percooling using internal cooling strategies in isolation or in combination with external cooling on EIGS and Ex-GIS still warrant a substantial amount of exploration (Bongers et al., 2017; Morris & Jay, 2016; Tan & Lee, 2015).

Recommendation

The temperature of fluids consumed before and during exercise should be standardized, measured, and reported. It is recommended that beverages are provided at room temperature (∼20 °C) as a standardized approach, since cool and cold temperatures may cause artifact impact on markers of EIGS and Ex-GIS. However, lower beverage temperature may be justified in accordance to the research focus and/or aims. For example, in internal pre- or percooling methods implemented within the study’s experimental design.

Exercise-Associated Gastrointestinal Perturbation Assessment

Concern

Many exercise gastroenterology research focused publications involve exploratory, mechanistic, and intervention studies in which the measurement of singular or limited numbers of biomarkers of EIGS are used to make inference on systemic or clinical outcomes without direct measurement or sufficient justification (see Costa, Gaskell, et al., 2020; Costa, Snipe, et al., 2017). For example, intestinal epithelial injury or permeability may be assessed via plasma intestinal fatty acid-binding protein and dual-sugar tests, respectively; but are claimed to impact luminal to circulation bacterial endotoxin translocation and systemic immune responses in the absence of any respective direct analysis. In addition, the markers of interest may be measured at inappropriate time points within the experimental designs, failing to capture postexercise peak responses and/or leading to misrepresented area under the curve calculations. Such methodological concerns are consistently observed in investigations that focus on the impact impact of dietary supplements on the managing of exercise-associated gastrointestinal integrity disturbances (Marchbank et al., 2011; Ogden et al., 2021; Pugh et al., 2017; Roberts et al., 2016). This warrants attention when translating study outcomes into practical implementation and clinical relevance. Unfortunately, a scarce number of exercise gastroenterology research studies collect a comprehensive global assessment of luminal, epithelial, basolateral membrane, and systemic statuses; and adjunct gastrointestinal functional status (Table 1). In addition, as the field of exercise gastroenterology has evolved, it has gained awareness that some gastrointestinal assessment tools or biomeasures do not have good validity or reliability within experimental models, fail to provide meaningful data of clinical relevance, and/or potentially lead to the overstatement or misinterpretation of results. These outcomes are likely due to lack of the marker response to the inadequate exertional or exertional heat stress model used, analysis issues with indirect marker, and/or biomarker dynamics (i.e., clearance, neutralization, and/or turnover) constituting a confounding factor (Table 1, and Figures 2 and 3).

Table 1

Gastrointestinal Integrity and Function, and Systemic Immune Response Assessment Techniques Previously and Currently Used Within Exercise Gastroenterology Research

TechniqueTechnique invasivenessaApplication (A), translational or research relevance (TR), and identified issues (I)
(a) Gastrointestinal integrity
 Splanchnic hypoperfusion and ischemiaTonometryHighA: Gastric tonometry to determine gastric CO2 appearance due to splanchnic hypoperfusion/ischemia (van Wijck et al., 2011).

TR: Modest association with intestinal epithelium injury (plasma I-FABP concentration: r = .59, n = 9, p < .001), but associations with intestinal hyperpermeability are less clear (Jonvik et al., 2019; van Wijck et al., 2011).

I: Invasive technique during an exercise protocol. Does not determine intestinal microvilli hypoperfusion or neuroendocrine-gastrointestinal pathway factors linked to intestinal integrity perturbations.
 Intestinal epithelial injury/damageScanning and endoscopic techniquesHighA: Histological evidence of epithelial injury.

TR: Reversible ischemia colitis has been reported after prolonged strenuous exercise via CT scanning or colonoscopy (Cohen et al., 2009; Grames & Berry-Caban, 2012; Kyriakos et al., 2006). However, data fails to demonstrate link between histological evidence of blood-based gastrointestinal perturbation markers and Ex-GIS.

I: Although adding clear evidence of damage to the intestinal epithelium, the technique is invasive during exercise and fails to quantify overall magnitude of local and systemic inflammatory impact.
Plasma I-FABP ELISAMediumA: Surrogate marker for intestinal epithelial cell injury, but not indicative of gastric or intestinal hyperpermeability.

TR: Active healthy population resting values (n = 134, CV = 5.6%): mean and 95% confidence interval 418 (326–510) pg/ml (HK406, Hycult Biotech).b Δ pre- to postexercise indicates exercise-associated intestinal epithelial injury (Figure 2a). Clinical relevancec: Δ consistently associated with perturbed systemic endotoxin and inflammatory profiles ≥ 1,000 pg/ml (n = 104; Costa, Gaskell, et al., 2020; Costa, Snipe, et al., 2017). Exercise-induced values peak immediately postexercise; however, splanchnic reperfusion injury may result in further small increase after 30–60 min into the postexercise recovery period.

I: Large interstudy and intrastudy variation, potentially associated with different ELISA kits and laboratory analysis procedures. Lack of consistency in reporting (i.e., raw values, Δ pre- to postexercise units and percentage, or area under the curve) between studies. Other biomarkers have been used concurrently, such as I-BABP and/or L-FABP (van Wijck et al., 2011). Urinary I-FABP has also been used in research studies (Ma et al., 2020); however, appears erroneous in application and increases risk of results misinterpretation, since urinary production and output is fundamentally under renal control that is regulated by hydration, electrolyte (e.g., sodium), and thermoregulatory status (McCubbin & Costa, 2018; McCubbin et al., 2019).
 Intestinal epithelial inflammationFecal calprotectin ELISAMediumA: Surrogate marker for intestinal epithelial inflammation, but not indicative of gastric or intestinal epithelial injury/damage or intestinal hyperpermeability.

TR: Active healthy population resting values (n = 79, CV = 9.0%): 3.2 (2.6–3.8) μg/g (DE843, Demeditec).b Δ pre- to postexercise: 1.0 (0.3–1.8) μg/g. Clinical relevancec: inflammatory diseases of the gastrointestinal tract: >100 μg/g (Bressler et al., 2015).

I: Not sensitive as biomarker of transient EIGS inflammation versus consistent gastrointestinal inflammatory diseases (e.g., Crohn’s disease and ulcerative colitis). Difficult to obtain precise sample associated with exercise perturbations with further confounding due to heterogeneity of collection timing, volume, and processing methods (Costa, Camões-Costa, et al., 2019; Snipe et al., 2018a).
 Intestinal tight junction injuryFecal or plasma claudin-3 ELISAMediumA: Surrogate marker for epithelial tight junction injury underpinning hyperpermeability and subsequent translocation of pathogenic luminal contents into systemic circulation.

TR: Active healthy population resting values (plasma, n = 26, CV = 7.8%): 18.0 (14.8–21.1) ng/ml (SEF293Hu, Cloud-Clone).b No significant change in plasma claudin-3 concentration with prolonged strenuous exercise despite substantial intestinal epithelial injury and modest bacterial endotoxin translocation (e.g., 3-hr running at 60% V˙O2max in 23.1 °C; claudin-3: ↑ 6.7%, but I-FABP ↑ 135% and sCD14 ↑ 12%) or small intestinal hyperpermeability (Gaskell et al., 2020; Gaskell, Rauch, et al., 2021; Snipe et al., 2017, 2018a, 2018b).

I: Fecal sample collection issues as above. Plasma changes do not follow magnitude of change to other markers of exercise-associated perturbations to intestinal integrity. Claudin-3 in circulation may indicate systemic response rather than specific gastrointestinal tract effect (Garcia-Hernandez et al., 2017).
 Gastric and/or intestinal permeabilityNonmetabolizable sugars in urine or plasma:

 Gastric permeability—sucralose

 Small intestinal permeability—lactulose

 Large intestinal permeability—rhamnose or mannitol

Multisugar test for full gastrointestinal permeability profile

Dual-sugar test for small intestinal permeability determination (e.g., lactulose:rhamnose ratio)
HighA: Multi or dual nonmetabolizable sugars with measures in plasma/serum (appearance) or urine (excretion) are commonly used as a marker to determine perturbation of gastric and/or intestinal integrity. Considered a “gateway” marker of changes in tight junction space.

TR: For small intestine permeability resting normative values from health controlsc: ≤0.020. Clinical relevancec: Δ lactulose: rhamnose or mannitol ratio ≥0.040. Δ pre- to postexercise indicates a change in intestinal epithelial permeability only, and not indicative of a state of “intestinal hyperpermeability of pathogenic luminal content.” Increases in exercise-associated intestinal permeability not associated with intestinal epithelial cell injury, postpermeability systemic endotoxin, and inflammatory cytokine profiles (Snipe et al., 2018a, 2018b).

I: Large interstudy and intrastudy variation due to dose/timing of sugars around exercise and urine/plasma collection, processing, and chromatography analysis. Plasma appearance may detect smaller changes in permeability, but time course of the absorption of different sugars is currently unknown. Changes in intestinal permeability occur naturally without clinical relevance (e.g., post nutrient-rich meal) with modest changes expected with exercise that have no clinical or performance meaning (i.e., instigation and impact of Ex-GIS) or issues warranting action (Costa, Gaskell, et al., 2020; Costa, Snipe, et al., 2017).
Fecal or plasma zonulin ELISAMediumA: Precursor to haptoglobin-2, proposed as surrogate for change in intestinal permeability.

TR: Currently lacks validation and verification in healthy active populations (Tatuca-Babet et al., 2020).

I: Fecal sample collection issues stated above. ELISA assessment kits for zonulin found to provide flawed analysis (Ajamian et al., 2019; Scheffler et al., 2018).
 Luminal bacterial endotoxin translocationPlasma LAL end point assay for gram negative bacterial endotoxin (e.g., LPS)MediumA: Assesses translocation of gram-negative luminal bacterial (e.g., LPS). Most common method used in exercise gastroenterology research for determination of exercise-associated systemic endotoxemia.

TR: Active healthy population resting values (n = 39, CV = 7.2%): 119 (101–137) pg/ml (HIT302, Hycult Biotech).b Clinical definition of endotoxemia: Δ >5 pg/ml with depressed EndoCAb such as IgM (Barclay, 1995; Bosenberg et al., 1988; Brock-Utne et al., 1988; Camus et al., 1997, 1998). Clinical relevancec: Pre- to postexercise Δ >10 pg/ml with reductions in EndoCAb are associated with greater postexercise I-FABP and systemic inflammatory profile, compared with <10 pg/ml. Unable to detect gram-positive and bacterial pathogenic and immune activating components. Peaks 0–1 hr postexercise.
I: Efficient immune/hepatic clearance of luminal-originated PAMP, such as LPS, in systemic circulation invokes small changes in gram-negative endotoxin via LAL end point assay (Table 3 in Costa, Snipe, et al., 2017, and Figure 5 in Costa, Gaskell, et al., 2020), unless substantial exertional stress (e.g., +30 pg/ml in 5-day multistage ultra-endurance run, and +122 pg/ml in 24-hr continuous ultramarathon; Gill, Hankey, et al., 2015; Gill, Teixeira, et al., 2015). Should be accompanied by EndoCAb determination or substituted with plasma LBP and/or sCD14 concentrations to provide accurate depiction of luminal-originated PAMP translocation (Barclay, 1995; Camus et al., 1997; Gnauck et al., 2016; Snipe et al., 2017, 2018a; Young et al., 2022). LAL end-point assay risks contamination with environmental pyrogenic endotoxins and β-glucan during sample collection, processing, and analysis procedures (Buchacher et al., 2010; Cooper et al., 1997; Gnauck et al., 2015a, 2015b; Nalepka & Greenfield, 2004).
Plasma sCD14 ELISAMediumA: A surrogate marker of systemic endotoxemia. sCD14 is produced by phagocytic cells, and acts as a broad spectrum PAMP receptor, that initiates an immune response, such as phagocytic chemotaxis, phagocytosis, and degranulation, with associated inflammatory responses to support systemic clearance of pathogenic endotoxins. sCD14 generally increases proportionally in response to luminal originated bacterial endotoxin translocation into systemic circulation.

TR: Active healthy population resting values (n = 75, CV = 5.7%): 2.9 (2.5–3.4) μg/ml (HK320, Hycult Biotech).b Intense endurance exercise in temperate conditions modestly increases plasma-level postexercise. Substantial increases observed only in response to prolonged duration exercise (i.e., 3 hr) and sufficient exertional heat stress. Clinical relevance values in exercise gastroenterology research have not yet been established, since a limited number of studies have investigated the magnitude response to exercise (Figure 2b).

I: Negligible response if magnitude of exertional or exertional heat stress model is insufficient.
Plasma LBP ELISAMediumA: A surrogate marker of systemic endotoxemia. Acute phase protein LBP binds to LPS allowing presentation to PAMP-recognizing receptors such as TLR4 and CD14 that initiate an immune response to support systemic clearance of LPS. LBP is proposed to increase proportionally in response to luminal originated LPS translocation into systemic circulation.

TR: Active healthy population resting values (n = 65, CV = 4.9%): 10.8 (9.5–12.1) μg/ml (HK315, Hycult Biotech).b Endurance exercise in temperate or hot conditions modestly increases plasma level postexercise, with greatest change occurring immediately after cessation of exercise. Reduced concentration only observed with prolonged exercise (i.e., 3 hr), likely due to LBP utilization overwhelming replacement capacity. Clinical relevance values in exercise gastroenterology research have not yet been established, since a limited number of studies have investigated the magnitude response to exertional stress (Figure 2c).

I: In isolation may promote misinterpretation, since magnitude of exercise stress will determine its utilization and subsequently direction of change (i.e., increase or decrease). Predominately focused on detecting LPS luminal translocation, and thus does not quantify other potential pathogenic agents.
 Luminal bacteria translocationMicrobial DNA detection and gene sequencing techniques in plasma samplesHighA: Using a microbial DNA extraction and purification kit (e.g., QIAamp UPC pathogen extract kit, Qiagen, MoBio Laboratories), extracted microbial DNA quantification via fluorometer, and subsequent bacterial gene sequencing procedures (e.g., V3–V4 16S gene sequencing), to detect bacterial OTU in plasma, with blank controls for cross-contamination detection and correction factors. Such identification may provide an indication of whole bacterial translocation from lumen into circulation, “bacteremia,” which may be subject to prolific systemic immune responses.

TR: Plasma extracted microbial DNA quantification for active healthy population resting values (n = 56)b: SEI equating to bacterial groups with >0.05% relative abundance of 0.134 (0.124–0.145) for phyla (predominant pre- and postexercise microbial phyla: Proteobacteria, Firmicutes, Actinobacteriota, and Bacteroidota, respectively), 0.247 (0.236–0.257) for family (Pseudomonadaceae, Beijerinckiaceae, Yersiniaceae, Sphingomonadaceae, and Halomonadaceae, respectively), and 0.249 (0.230–0.259) for genus (Pseudomonas, Methylorubrum, Halomonas, Acinetobacter, and Anaerobacillus), respectively; Gaskell, Gill, et al., 2021; Young et al., 2021), which is in accordance with previous literature (Castillo et al., 2019; Villarroel et al., 2022). Exercise-associated bacteremia and a reduced SEI, i.e., reduced plasma microbial diversity at the expense of selected enhanced bacterial translocation (e.g., Proteobacteria, and associated family and genus; Gaskell, Gill, et al., 2021; Young et al., 2021).

I: Recent studies has attempted to quantify luminal to systemic bacterial translocation in response to exertional and exertional heat stress using gene sequencing techniques (March et al., 2019; Ogden et al., 2020b). However, concerns over the lack of exertional and exertional heat stress models used, lack of confounder control, and suboptimal microbial reporting (i.e., limited to Bacteroides to total ratio) appears to provide a limited contribution to the scientific literature and translation into professional practice. Therefore, to dates, the clinical and performance relevance of exercise-associated bacteremia, and its links with EIGS, still requires further exploration. In addition, such methodological procedures are high risk of cross-contamination during sample collection, processing, and analysis.
(b) Gastrointestinal function
 Gastric emptyingGastric aspirationHighA: Ingestion or injection of known solution or bolus volume. Aspiration of total contents after a specified period of time to determine solution or meal half-life.

TR: A common procedure performed in sports and exercise nutrition research to determine gastric emptying rate of sports focus formulated beverages. Normally done in resting conditions as difficult to perform technique during exercise. Provides an indication of emptying rate of beverage solution or meal in relation to its volume, pH, energy content, macronutrient profile, and/or osmolality.

I: Invasive technique for participant due to nasogastric tubing in situ or other external to gastric tubing/devices. Only provides indication of gastric emptying rates per se and negates any intestinal activity and mechanistic reasoning for changes in rate. In addition, for accurate interpretation, determination of gastric secretions is required.
13C-labeled tracerMediumA: Ingestion of 13C tracer, followed by subsequent breath collection and analysis of 13C enrichment in breath CO2. Rate of enrichment reduction provides an indication of gastric emptying rate.

TR: Not commonly used in exercise gastroenterology research, but an alternative option to gastric aspiration for determining baseline and changes in gastric emptying rate (van Nieuwenhoven, Wagenmakers, et al., 1999). Can be used to assess pre, during, or post exercise beverage or meal gastric emptying responses. However, requires participant pretest preparation, breath collection consumables, and analysis via continuous flow isotope ratio mass spectrometry.

I: There is no available and defined established response rates for exertional or exertional heat stress. Requires strict pretesting dietary control (i.e., avoidance of 13C-rich foods–fluids) to improve accuracy and reliability of results. Only provides indication of gastric emptying rates per se and negates any intestinal activity and mechanistic reasoning for changes in rate.
 Gastrointestinal transitLactulose challenge test for OCTTLowA: Determination of mouth to cecum gastrointestinal transit, inclusive of gastric emptying, and small intestine transit.

TR: Ingestion of a lactulose solution 30 min prior to cessation of exercise. Breath samples are collected immediately postexercise and every 15 min for 3 hr after exercise. The time intervals between ingestion of lactulose and the first sustained rise in breath hydrogen concentration (e.g., at least 10 ppm, with two consecutive readings, above baseline) is used as a measure of OCTT, on which gastric emptying will exert some influence (Bate et al., 2010; Gaskell, Parr, et al., 2021).

I: Despite not commonly used in exercise gastroenterology research, OCTT may act as a useful noninvasive tool in helping to provide an overall picture of one part of gastrointestinal function. Limitations of the test need to be recognized, including but not limited to (a) The application of lactulose solution may hasten orocecal transit (Miller et al., 1997), and therefore needs to be considered in data interpretation procedures. (b) Caution is needed in interpreting results in athletes and/or research participants with SIBO, which will result in rapid rise in breath hydrogen not related necessarily to OCTT. Therefore, screening for SIBO is advised (Rezaie et al., 2017). (c) Hydrogen production may be reduced because of low colonic pH (Perman et al., 1981). There is a proportion of the population who are occasional nonproducers of hydrogen, which could be related to normal shifts of bacterial flora (La Brooy et al., 1983) measuring methane alongside hydrogen is advocated but the variability of methane production is unknown (Harvie et al., 2019). (d) There is poor reproducibility of lactulose breath testing (Yao & Tuck, 2017). The reproducibility with liquid solution is poorer than with liquid or solid meal preparations (Gasbarrini et al., 2009; La Brooy et al., 1983). (e) It is not clear whether the menstrual cycle influences OCTT (Gasbarrini et al., 2009). In summary, the interpretation of the data is not straightforward and requires experience in the field of research gastroenterology.
Oral ingestion of telemetry pillLowA: Measurement of gastrointestinal transit. Uses gas (i.e., H2, CO2, and O2), pH, and/or pressure sensing technology to determine transit along the gastrointestinal tract (Weitschies et al., 2021). The profile of gases, pH, and/or pressure provides information of pill location within the gastrointestinal tract, and thus provides information of gastrointestinal transit speed over the time course of the assessment period. Includes temperature sensor that can provide data on core body temperature.

TR: A novel technique in clinical gastroenterology. Currently, no exercise gastroenterology research has used this gastrointestinal assessment technique in exertional or exertional heat stress experimental designs.

I: Currently, no exercise gastroenterology research has used this gastrointestinal assessment technique in exertional or exertional heat stress experimental designs.
 Enterocyte carbohydrate absorption3-OMG and d-xylose tracerHighA: A proposed technique for detecting the nonabsorption of ingested carbohydrate during exercise, targeting active (SGLT by 3-OMG) and passive (GLUT-5 by d-xylose) carbohydrate transporters on the enterocyte apical border, using nonmetabolized sugars.

TR: Clinical relevance values in exercise gastroenterology research have not yet been established, since few studies have used this technique to determine exercise-associated intestinal carbohydrate transporter impairment (Lang et al., 2006; van Nieuwenhoven, Brouns, & Brummer, 1999; van Nieuwenhoven, Brouns, & Brummer, 2004; van Nieuwenhoven, Vriens, et al., 2000), in response to an endurance exercise (1-hr running at 70% VO2max) synonymous with only minor disturbance to gastrointestinal integrity and function.

I: Specialized technique not repeatedly used in exercise gastroenterology research, and therefore, the performance and clinical significance is not yet established. In addition, it is postulated that nonmetabolizable sugars would compete for intestinal carbohydrate transporters with pre and/or during exercise carbohydrate provisions. Therefore, avoiding ingestion of carbohydrate when applying this methodological technique is recommended.
 Carbohydrate malabsorptionBreath H2 and CH4LowA: Breath testing is used to identify the presence of undigested and/or malabsorbed fermentable carbohydrates. Breath collection and analysis measures the amount of H2 and/or CH4 in the breath after a specific dose and type of carbohydrate/s consumed. The test relies on the principle that nonabsorbed carbohydrate passes into the large intestine, then undergoes fermentation by bacteria, producing H2, and CH4 gas as a by-product. The gas is absorbed from the large intestine, into circulation, and transported for pulmonary exhalation.

TR: A noninvasive tool to assess one part of gastrointestinal function—carbohydrate malabsorption and feeding tolerance in response to carbohydrate quantity and quality challenge before, during, or after exercise (Rauch et al., 2022; Russo et al., 2021a, 2021b, 2021c). Substantial rise in breath H2 (≥10 ppm from baseline) or methane (≥25 ppm from baseline) is indicative of carbohydrate malabsorption (Bate et al., 2010) and linked to GIS incidence and severity (Costa, Miall, et al., 2017; Gaskell et al., 2020). Breath samples should be collected at baseline, immediately postexercise, and every 15 min of postexercise for approximately 3 hr depending on intervention. Prior to carbohydrate challenge and/or feeding tolerance assessment, dietary control measures are needed (e.g., 24 hr of low FODMAP diet and standardized preexercise snack/meal 2 hr beforehand).

I: A substantial amount of poorly controlled confounding factors may have influential outcomes, such as, dietary intake, gastrointestinal modifying nutritional supplementation, pharmaceutical prescriptions, exercise behavior, psychophysiological factors—anxiety, gut microbiota composition, and/or gastrointestinal diseases/disorders (Hill et al., 2017; Kalantar-Zadeh et al., 2019; Tuck et al., 2014).
 Protein malabsorptionL-[1-13C]phenylalanine-labeled protein ingestion with continuous intravenous L-[ring-2H5] phenylalanine infusionHighA: A proposed technique for detecting the nonabsorption of ingested protein.

TR: Clinical relevance values in exercise gastroenterology research have not yet been established, since only a few studies have used this technique to determine postexercise recovery protein absorption (Churchward-Venne et al., 2020; Mazzulla et al., 2017; van Wijck et al., 2013), in response to an exercise stress model (i.e., a resistance exercise session, 90 min of cycling at 60% Wmax, and 60 min of running at 70% VO2peak) that are known to not substantially disturb the integrity and function of the gastrointestinal tract.

I: Specialized technique not repeatedly used in exercise gastroenterology research, and therefore the performance and clinical significance is not yet established.
 Gastrointestinal smooth muscle activityEGGLowA: EGG is used to assess motor function of the gut (gastric myoelectrical activity). This myoelectrical activity is referred to as gastric slow waves and is a surrogate of gastric motility. The frequency of normal gastric slow waves is 3 cpm (e.g., normal: 2–4 cpm, bradygastria: 0.5–2 cpm, and tachygastria: 4–9 cpm) in humans. The gastric slow wave determines the maximum frequency and propagation of gastric contractions.

TR: Not commonly used in exercise gastroenterology research but may be a useful adjunct in helping provide an overall picture of gastrointestinal function, especially related to gastroparesis. The EGG is sensitive to motion artifacts and electrical interferences (e.g., skeletal and cardiac muscle contractions), and it is therefore important to follow strict procedures and analysis methods to help reduce disturbance of data acquired.

I: EGG should not be used and interpreted in isolation, and potentially in conjunction with OCTT. Specialized technique not repeatedly used in exercise gastroenterology research, and therefore the performance and clinical significance is not yet established.
(c) Systemic immune responses
 Inflammatory cytokine profileSingle assay or multiplex ELISAMediumA: Determination of mediators that regulate systemic inflammatory responses induced by, but not limited to, tissue injury, intramuscular, or intracellular signaling, and/or systemic pathogenic presence and immune activation (Peake et al., 2015). Exercise-associated SIR characterized by none to modest increases in plasma proinflammatory (e.g., IL-1β and TNF-α) and immune modulating cytokines (e.g., IL-6 and IL-8) increases, followed by substantial increase in anti-inflammatory cytokines (e.g., IL-10 and IL-1ra) (Figure 4), differs to responses observed in infection incidence (e.g., sepsis), characterized by substantial raised proinflammatory and immune modulating plasma cytokines (Walsh et al., 2011)

TR: Active healthy population resting plasma values for systemic inflammatory cytokines (n = 103, CV = 8.0%, HCYTOMAG-60K Milliplex human cytokine/chemokine magnetic bead panel) includeb IL-1β 4.0 (2.4–5.6) pg/ml, TNF-α 14.2 (10.0–18.3) pg/ml, IL-6 14.8 (8.1–21.5) pg/ml, IL-8 11.3 (8.3–14.4) pg/ml, IL-10 12.5 (9.6–15.4) pg/ml, and IL-1ra 49.3 (36.0–62.6) pg/ml. Magnitude of Δ pre- to postexercise is indicative of exercise-associated SIR and appears to be proportional to exertional and exertional heat stress (Figures 2e and 4). Considering the large intra- and interindividual response in SIR to exertional and exertional heat stress, for comparative purposes, an exercise-associated SIR profiled has previously been proposed (Figure 2f; Bennett et al., 2020; Russo et al., 2021a, 2021b, 2021c).

I: It is common in a substantial number of exercise gastroenterology research studies to only measure a subset of inflammatory associated cytokines (e.g., IL-6 and TNF-α), unknowing that such systemic inflammatory markers are consistently shown to present none to modest response to exertional or exertional heat stress (Figure 4), in healthy active populations. Only in cases of extreme physical exertion (e.g., ultramarathon competition in hot ambient conditions) do proinflammatory and immune modulating cytokines show substantial rises from baseline in healthy active populations (Gill, Hankey, et al., 2015; Gill, Teixeira, et al., 2015). It is now well recognized that anti-inflammatory cytokine responses (e.g., IL-10 and IL-1ra) are more sensitive to exertional and/or exertional heat stress (Figure 3a and 3b), but are rarely adopted, and when adopted incorrect sample timing is applied (e.g., immediately postexercise), since initial peak values are consistently seen only >1 hr of postexercise.
 Immune cell responsesCirculatory leukocyte traffickingMediumA: Determination of whole blood total and differential (e.g., lymphocytes, neutrophils, and monocytes) leukocyte counts by automated cell counter (e.g., Coulter counter) or bench top analysis (e.g., HemoCue).

TR: Resting pre- to postexercise (e.g., 2 hr of running at 70%–75% VO2max in temperate conditions) change in leukocyte counts: total leukocyte 3–8 × 109/L, neutrophil 5–12 × 109/L, lymphocytes 1.5–2.5×109/L followed by a lymphopenia in acute recovery, and neutrophil-to-lymphocyte ratio 2–8 (Costa et al., 2009, 2011; Costa, Camões-Costa, et al., 2020; Costa, Camões-Costa, et al., 2019).

I: Only provides general overview of immune cell distribution in circulation. Does not quantify immune cell maturity, cell transit into tissues, and functional responses.
Immune cell function responses (e.g., in vitro LPS challenged neutrophil elastase release).MediumA: Whole blood (1 ml) incubation with E. coli LPS (e.g., 50 μl of a 1 mg/ml LPS concentration) to determine neutrophil functional response (in vitro model) to potential luminal originated LPS translocation.

TR: Exercise-associated reduction in elastase release reported at −20% to −30% below baseline and remaining depressed throughout recovery (∼4 hr; Costa et al., 2009, 2011; Costa, Camões-Costa, et al., 2020; Costa, Camões-Costa, et al., 2019; Russo et al., 2021a).

I: Limited number of exercise gastroenterology studies has included an immune functional response to support luminal bacterial translocation and SIR profile.
 Antibody responsesEndoCAb ELISAMediumA: Produced by lymphocyte B cells and/or plasma cells, IgM, IgA, and IgG are considered EndoCAb. Within a healthy individual, plasma concentrations are abundant and of no clinical relevance (e.g., immunosuppression), and primarily act to tag bacterial endotoxins (e.g., LPS and/or lipid-A) for immune tagging, neutralization, and/or clearance by innate and/or adaptive immune responses. Circulating levels increase in response to stress (Barclay, 1995). However, it is proposed that if utilization overrides baseline starting levels and in situ production levels, stress associated reductions in circulating concentrations are reported (Barclay, 1995; Snipe et al., 2017, 2018a, 2018b; Young et al., 2022).

TR: Active healthy population resting values includeb IgM (n = 74, CV = 6.5%): 149 (123–175) MMU/ml, IgA (n = 44, CV = 7.7%): 37 (29–44) AMU/ml, and IgG (n = 44, CV = 13.2%): 79 (48–109) GMU/ml (arbitrary units represented as median-units [MU] based on minimal detection levels). Circulating levels of EndoCAb modestly increase in response to a variety of exertional and exertional heat stress models, but are of no clinical relevance since any modest exercise-associated changes remain within resting normative (35–250 MU/ml) values (Young et al., 2022).

I: Negligible response if magnitude of exertional or exertional heat stress is insufficient. Limited number of exercise in gastroenterology research studies has included EndoCAb, namely IgM, to support systemic endotoxin and inflammatory profiles.
(d) Others
 Fecal bacterial compositionGene sequencing techniques in fecal samplesHighA: Using a feces-specific microbial DNA extraction and purification kit (e.g., PowerFecal DNA isolation kit, Qiagen, MoBio Laboratories), and subsequent bacterial gene sequencing procedures (e.g., V3-V4 16S gene sequencing), to detect bacterial OTUs in fecal samples, with blank controls for cross-contamination detection and correction factors. Such identification provides a generalized overview of the potential bacterial composition (i.e., diversity and relative abundance) along the gastrointestinal tract, termed “gut microbiome.”

TR: The role of gene sequencing technology in determining fecal microbial taxa in response to exercise stress, the outcomes relating to EIGS, and potential performance and clinical implications have previously been highlighted in Mailing et al. (2019) and researched by Bennett et al. (2020). Although there has recently been an exponential growth in “gut microbiome” exploratory research in athlete groups, the research outcomes have shown no consistency in the relative abundance of specific OTUs (e.g., phylum, family, genus, and/or species) between studies, with each study favoring and promoting a selected few OTUs. Moreover, a substantial amount of individual variation within athlete population studies has been observed. This has led to no definitive normative “gut microbiome” profile for athlete populations being established to date. However, common characteristics reported among healthy athlete populations include increased α-diversity, high relative abundance of SCFA producing bacterial groups, and low relative abundance of potential pathogenic bacterial groups.

I: Considering the rapid plasticity of the bacterial composition along the gastrointestinal tract, the observed inconsistencies between studies, the OTU profiles reported are likely associated with substantial methodological differences, such as (a) fecal sample management including collection, processing, and storage; (b) experiment procedures and control of confounding factors that includes acute and prolonged dietary control; and (c) sample analytical procedures including sequencing, identification platform, and bioinformatics (Bennett et al., 2020; Brown et al., 2012; Gilbert et al., 2018; Mailing et al., 2019). Changes in the “gut microbiome” that favor increases in bacterial metabolite production, such as SCFA acetate, butyrate, and propionate are the prime factors that have been suggested to have implications in performance and health (Bongiovanni et al., 2021; Clauss et al., 2021), including EIGS and Ex-GIS (Gaskell, Gill, et al., 2021; Young et al., 2022). However, SCFA determination in exercise gastroenterology research is scarce. It would appear logical to directly measure these metabolic by-product indicative of collaborative bacterial activity (e.g., fermentation) along the gastrointestinal tract, instead of attempting to establish the normative values within the enormity of the “gut microbiome” and then making assumption and inference that such profiles influence SCFA production without measuring, which is a common flaw in resent studies exploring the “gut microbiome” in athlete populations.
 Plasma and fecal short chain fatty acid compositionGas chromatographyHighA: Fecal and plasma SCFA including acetate, butyrate, propionate, valeric acid, and caproic acid measured by gas chromatography. In addition, BSCFA, such as isovaleric and isobutyric acids, can be measured concurrently (i.e., BSCFA to SCFA ratio) and provide an indication of nitrogenous fermentation and potential harmful outcomes (Diether & Willing, 2019; Rios-Covian et al., 2020).

TR: Active healthy population resting fecal values includeb (n = 83, CV: 5.4%): acetate 58.0 (49.9–66.2) μmol/g, butyrate 12.9 (12.3–16.0) μmol/g, propionate 15.8 (13.5–17.4) μmol/g, valeric acid 1.9 (1.6–2.2) μmol/g, caproic acid 0.8 (0.7–1.0) μmol/g, isobutyrate 2.1 (1.9–2.4) μmol/g, isovaleric 1.9 (1.7–2.2), and BSCFA to SCFA ratio 6.6 (5.2–8.0). Active healthy population resting plasma values includeb (n = 83, CV: 10.9%): acetate 161 (148–179) μmol/L, butyrate 1.9 (1.3–2.8) μmol/L, and propionate 6.1 (4.3–6.7) μmol/L. Associations between fecal and plasma SCFA with EIGS and Ex-GIS have been reported, but require further exploration (Gaskell, Gill, et al., 2021; Young et al., 2021).

I: A limited number of exercise gastroenterology research studies have included measurements of SCFA, and therefore the performance and clinical implications of modifying the “gut microbiome” to alter fecal or plasma SCFA still requires further investigation. Therefore, caution is advised with precipitated and overstated conclusions from a range of studies and reviews suggestion SCFA enhanced exercise performance (Bongiovanni et al., 2021; Clauss et al., 2021; Scheiman et al., 2019), considering their primary origins and evidence from animal experimental and/or theoretical models, and discount the potential role the “blood microbiome” in plasma SCFA profile and performance contributions (Castillo et al., 2019; Gaskell, Gill, et al., 2021; Villarroel et al., 2022; Young et al., 2022).

Note. A = application; TR = translational or research relevance; I = identified issues; cpm = cycles per minute; CT = computerized tomography; CV = coefficient variation; EGG = electrogastrography; EIGS = exercise-induced gastrointestinal syndrome; ELISA = enzyme-linked immunosorbent assay; EndoCAb = endogenous endotoxin antibodies; Ex-GIS = exercise-associated gastrointestinal symptoms; FODMAP = fermentable oligo-, di-, monosaccharides and polyols; GIS = gastrointestinal symptoms; L-FABP = liver fatty acid-binding protein; I-FABP = intestinal fatty acid-binding protein; Ig = immunoglobulin; LAL = limulus amebocyte lysate; LBP = lipopolysaccharide-binding protein; LPS = lipopolysaccharide; OCTT = orocaecal transit time; 3-OMG = 3-O-methylglucose; OTU = operational taxonomic units; PAMPs = pathogen associated molecular patterns; sCD14 = soluble CD14; SEI = Shannon Equitability Index; SGLT = sodium glucose transporter; SIBO = small intestinal bacterial overgrowth; SCFA = short chain fatty acids; SIR = systemic inflammatory response; TLR = toll like receptor; H2 = hydrogen; O2 = oxygen; CO2 = carbon dioxide; CH4 = methane; TNF = tumor necrosis factor; IL = interleukin; BSCFA = brached short chain fatty acids.

aThe subjective combined extent of participant burden, technical training and application required, and financial cost of technique. bResting baseline values within are reported as mean and 95% confidence interval. Data extracted from previously published research (Costa, Camões-Costa, et al., 2019; Costa, Miall, et al., 2017; Gaskell et al., 2020; Gaskell, Gill, et al., 2021; Gaskell et al., 2021a, 2021b; Russo et al., 2021a, 2021b, 2021c; Snipe & Costa, 2018a; Snipe et al., 2017, 2018a, 2018b; Young et al., 2021, 2022). cComparative values observed in gastrointestinal inflammatory or functional disease/disorders and/or symptomatic characteristics warranting medical management, with or without healthy controls (Al-Saffar et al., 2017; Costa, Miall, et al., 2017; Costa, Snipe, et al., 2017; Costa et al., 2020; Gaskell, Parr, et al., 2021; Haas et al., 2009; Jekarl et al., 2015; Linsalata et al., 2018; Martinez-Fierro et al., 2019; Pelsers et al., 2005; Power et al., 2021; Surbatovic et al., 2015). dCombined magnitude of peak Δ pre- to postexercise proinflammatory (i.e., IL-1β and TNF-α), immune modulating (i.e., IL-6 and IL-8), and anti-inflammatory (i.e., IL-10 and IL-1ra) cytokines has been proposed as a comparative arbitrary unit to assess within- and between-exertional and exertional heat stress models and intervention differences (Bennett et al., 2020; Russo et al., 2021a, 2021b, 2021c).

Recommendation

Experimental designs should provide a comprehensive global assessment of EIGS markers, unless the justified research aim is to target one specific pathophysiological pathway. This includes selecting analytical methods that have proven reliability and validity for gastrointestinal integrity (Table 1a), gastrointestinal functional responses (Table 1b), and systemic inflammatory responses (Table 1c). Biomarkers identified as erroneous (e.g., zonulin), having a limited magnitude of response to exercise stress (e.g., endogenous endotoxin core antibodies), or suffering from confounding effects of clearance, neutralization, and/or turnover rate such as endogenous endotoxin core antibodies, lipopolysaccharide-binding protein, direct bacteria, and/or bacterial endotoxin determination should be avoided in isolation or used as supportive biomarkers in adjunct to more established and robust biomarkers. It is acknowledged that comprehensive assessment of gastrointestinal integrity and functional markers, and relevant systemic responses, known to be perturbed by exertional or exertional heat stress is challenging. Issues may include expense, time, and increased burden within experimental procedures in respect to data and sample collection and/or using more complex methodologies warranting technical specialization. However, avoiding and/or attempting to excuse the absence of essential markers due to these factors is considered unacceptable, because it compromises research integrity, quality, and thoroughness. To overcome such barriers associated with undertaking comprehensive and thorough quality research, it is suggested that research groups collaborate to combine methodological strengths, capabilities, and associated costs.

Gastrointestinal Symptom Assessment Tool

Concern

It is now well established that the onset of Ex-GIS is multifactorial and dynamic in nature (Figure 1a). The type, incidence, and severity of GIS vary according to rapid versus delayed onset, acute and transient versus chronic and prolonged, and the instigation and magnitude of pathway innervation (Figure 1b). Unfortunately, within exercise gastroenterology research, the majority of studies have used a diversity of GIS assessment tools that have predominantly been developed in-house rather than validated and/or checked for reliability. Many have included symptoms that are not fully in accordance with clinically established signs and symptoms (e.g., ROME criteria), and/or used nebulous and erroneous terminology such as “gastrointestinal distress,” as discussed in Gaskell et al. (2019). In addition, some tools have been applied retrospectively (Costa et al., 2016; Jeukendrup et al., 2000; Pfeiffer et al., 2012; Stuempfle & Hoffman, 2015), and in some cases, online and long after cessation of the exercise bout (Pugh et al., 2018). Such data collection is notorious for participant bias and misreporting, and fails to capture acute and transient changes in Ex-GIS during exercise. Indeed, very few studies have provided in-depth and real-time descriptions of the temporal and site-specific nature of GIS; including type, incidence, and severity at baseline, at intervals during exercise, and in the recovery period. Yet distinct patterns of Ex-GIS responses have been identified, including rapid onset and transient upper-GIS during exercise with delayed onset and prolonged lower-GIS into the recovery period (Figures 2g and 2h).

Recommendation

A valid and reliable GIS assessment tool adapted from established clinical gastroenterology methodologies should be employed, such as a visual analogue scale (VAS) and ROME criteria (Bengtsson et al., 2013; Drossman, 2006; Gaskell et al., 2019; Table 2). The GIS tool should be applied pre, during, and post exercise in real time. For consistency in this self-reported parameter, all participants should be educated in a standardized manner in how to complete the GIS assessment tool, prior to data collection. Reported GIS metrics should include, but not limited to the following:

  1. (a)Total number of study participants (%) reporting ≥1 incidence of Ex-GIS, irrespective of type or severity.
  2. (b)Total number of study participants (%) reporting ≥1 severe incidence of Ex-GIS, indicated by ≥5 on a 10-point VAS.
  3. (c)Peak Ex-GIS severity for main categories; which includes, gastrointestinal discomfort, total-GIS, upper-GIS, lower-GIS, and nausea.
  4. (d)Individual symptom types within each main category.
  5. (e)Relative sum of Ex-GIS severity defined by x = (sum [VAS scores of each time point])/total time periods measured (n).
  6. (f)Various terms broadly describing GIS, such as “gastrointestinal or gut distress,” “burdened gut” or similar fabricated terms should be avoided and streamlined to established terms within the clinical gastroenterology literature and as described in Gaskell et al. (2019) within the exercise gastroenterology setting.

Table 2

mVAS for Determining GIS Incidence and Severity During and in Response to Exertional or Exertional Heat Stress With and Without Dietary Intervention

No symptomsMild symptomsSevere symptomsExtremely severe symptoms
Overall gut discomfort012345678910
 Upper GIS
  Belching012345678910
  Heartburn012345678910
  Bloating (stomach fullness)012345678910
  Stomach pain012345678910
  Urge to regurgitate012345678910
  Regurgitation010
  Projectile vomiting010
 Lower GIS
  Flatulence012345678910
  Lower abdominal bloating (abdominal pressure)012345678910
  Urge to defecate012345678910
  Left intestinal pain012345678910
  Right intestinal pain012345678910
  Defecation
   Normal consistency010
   Abnormal loose stools consistency010
   Diarrhea010
   Bloody stools0
Other GIS
 Nausea012345678910
 Dizziness012345678910
 Stitch (acute transient abdominal pain)012345678910

Note. 1–4 indicative of mild GIS (i.e., sensation of GIS, but not substantial enough to interfere with current activity) and increasing in magnitude, 5–9 indicative of severe GIS (i.e., GIS substantial enough to interfere with current activity) and increasing in magnitude, and 10 indicative of extreme GIS warranting activity cessation. If no specific GIS was reported, this is indicative of 0, and subsequently no rating is warranted. mVAS = modified visual analog scale; GIS = gastrointestinal symptom. Extracted from “Test Re-Test Reliability of a Modified Visual Analogue Scale Assessment Tool for Determining Incidence and Severity of Gastrointestinal Symptom in Response to Exercise Stress,” by S.K. Gaskell, R.M.J. Snipe, and R.J.S. Costa, 2019, International Journal of Sports Nutrition and Exercise Metabolism, 29(4), pp. 411–419. Copyright 2019 by Human Kinetics.

It is imperative that retrospective assessment of Ex-GIS should be avoided, as this method may result in erroneous and phantom values. For field-based observational or exploratory research, where retrospective GIS assessment may be the only option, the limitations of these data should be highlighted in reporting and discussed accordingly.

Circadian Variation

Concern

There is emerging evidence that suggests diurnal variation can influence the magnitude of EIGS in response to exertional stress, particularly involving the gastrointestinal-neuroendocrine pathway, and subsequent gastrointestinal functional disturbances and associated Ex-GIS. For example, 3 hr of running exercise at 60% V˙O2max in temperate ambient conditions at night (i.e., 2100 hr to 0000 hr) results in a more prolonged orocecal transit time compared with the same exertional stress performed from 0900 to 1200 hr (Gaskell, Rauch, et al., 2021). In addition, at night, a greater number of participants presented with no orocecal transit over the 3-hr postexercise monitoring period compared with day. In addition, three participants withdrew from the exercise protocol due to acute onset projectile vomiting or abnormally loose defecation on the night trial, but not on the day trial. These observed disturbances to gastrointestinal functional responses have practical implications for events scheduled in the nocturnal time period, multistage ultra-endurance events with a night section, and/or ≥24 -hr ultra-endurance events, especially considering such events require food and fluid provisions during competition (Costa, Hoffman, & Stellingwerff, 2019; Hoffman et al., 2019).

Recommendation

The time of initiation and cessation of exertional or exertional heat stress should be reported. The participant exercise commencement time for experimental trials should be standardized within each study. In respect to the optimal time of the day to undertake research protocols, and considering nearly all previous exercise gastroenterology research that has reported exercise protocol initiation time has been prior to midday, it is suggested for comparative purposes that the morning period appears the best practice for standardization. However, it is worth noting that unlike gastrointestinal integrity and systemic markers of EIGS, gastrointestinal functional responses and Ex-GIS are affected by circadian variation and should be permanently fixed to the morning for investigation (Gaskell, Parr, et al., 2021). Measurements of plasma or salivary cortisol concentrations may provide additional information on the impact of circadian associated gastrointestinal-neuroendocrine pathway disturbances and/or provide a correctional indicator of stress induced impact (Gaskell, Rauch, et al., 2021; Wright et al., 2015). It is important to note that plasma provides a more reliable medium for assessing resting and exercise-associated cortisol responses, compared with salivary due to the salivary cortisol’s dependence on saliva flow rate that is subject to multiple impacting factors (Gill et al., 2013, 2014; Gill, Price, & Costa, 2016; Gill, Teixeira, et al., 2016).

Other Considerations

  • – The size of study cohorts should be justified from a priori power calculations based on appropriate experimental designs; data analysis tests to be used; metrics of performance and/or clinical significance; and the potential for confounding factors such as age, biological sex, and/or fitness status (Bennett et al., 2020; Costa, Miall, et al., 2017; Snipe & Costa, 2018b).

  • – Given the potential for biological sex differences in gastrointestinal marker outcomes in response to exertional and exertional heat stress (Costa, Miall, et al., 2017; Russo et al., 2021d; Snipe & Costa, 2018b), undertaking exercise gastroenterology research in female populations requires distinctive considerations. Special considerations include a rigorous assessment of the menstrual or contraceptive cycle, with confirmation by biomarkers, such as plasma oestradiol and progestogen concentrations (Emmonds et al., 2019; Schaumberg et al., 2017; Smith et al., 2022).

  • – Data should be reported using methods that reduce reporting bias. For example, baseline values, response magnitude (i.e., Δ pre- to postexercise), and/or raw values over time should be reported for each EIGS marker. Adjunct and supportive data may include Δ % and area under curve (Table 1 and Figure 2). It is noted that these metrics are heavily influenced by the choice of baseline from which calculations are derived and may cause misleading/hyperinflated results (Figure 2).

  • – Changes in hydration status affect markers of EIGS in response to exertional and exertional heat stress. All blood borne variables should be corrected for change in plasma volume, as reported in Dill and Costill (1974). This will correct errors associated with exercise-induced hemoconcentration, or dilution linked with hyperhydration in response to postexercise rehydration.

  • – Although previous research has investigated links between preexercise anxiety and Ex-GIS (Wilson, 2018; Wilson et al., 2021), subjective anxiety scales should not be used in isolation due to the risk of participant bias/misreporting, and the failure to consider the multifactorial pathophysiology and the timescale onset of Ex-GIS. Nevertheless, using a validated assessment tool that captures anxiety (e.g., The State-Trait Anxiety Inventory [STAI], Perceived Onset Mood State [POMS], or anxiety VAS) may provide useful data (Rossi & Pourtois, 2012), especially when combined with objective stress biomarkers, such as plasma or salivary concentrations of cortisol, and corrected for circadian variation and collection medium (e.g., saliva flow rate and/or plasma volume change).

  • – Many studies containing an exercise model determine stress biomarkers from saliva samples (e.g., salivary cortisol, α-amylase, and other antimicrobial proteins) rather than plasma stress hormones (e.g., cortisol and catecholamine) to lessen participant burden (Gill et al., 2013, 2014; Gill, Teixeira, et al., 2016). It is important to use saliva collection methods that are valid and reliable, such as the unstimulated dribble method, to avoid confounding effects of saliva dilution or concentration within and between time points of collection (Gill, Price, & Costa, 2016).

  • – Several themes related to EIGS research are showing increasing interest, but currently lack adequate evidence to allow for interpretation and translation to practice. These themes include: (a) the roles of exosomes, plus bacteria and danger associated molecular patterns (BAMPs and DAMPs, respectively) on EIGS response magnitude (Fleshner & Crane, 2017); (b) the role of metabolomic analysis in identifying novel markers of EIGS response magnitude and potential health risk (Nieman et al., 2018; Sakaguchi et al., 2019); (c) the role of gut microbiota α-diversity and relative abundance of short chain fatty acid (i.e., butyrate, acetate, and propionate) producing commensal and pathogenic bacteria on EIGS response magnitude (Bennett et al., 2020; Gaskell, Gill, et al., 2021; Young et al., 2021); and (d) the role of blood microbiome on EIGS response magnitude (Gaskell, Gill, et al., 2021; Young et al., 2021). Other biomarkers, currently being explored for validity and reliability to determine stress induced disturbances to gastrointestinal integrity and/or function, include syndecan-1 (Bode et al., 2008; Wang et al., 2015).

Figure 4 provides a general overview exemplar of an experimental design that meets the rigors and robustness of an experimental design within exercise gastroenterology research, and focuses on removal or reducing the impact of potential confounding factors on primary and secondary targeted variables. Additionally, Table 3 provides a template checklist to assist researchers in designing their experimental procedures.

Figure 4
Figure 4

—Schematic illustration of exemplar experimental design. BM = body mass; CHO = carbohydrate; EIGS = exercise-induced gastrointestinal syndrome; FODMAP = fermentable oligo-, di-, monosaccharides and polyols; GIS = gastrointestinal symptoms; HR = heart rate; PRO = protein; RH = relative humidity; RPE = rating of perceived exertion; Tamb = ambient temperature; TCR = thermal comfort rating; Tre = rectal temperature; w/v = water volume equivalent.

Citation: International Journal of Sport Nutrition and Exercise Metabolism 32, 5; 10.1123/ijsnem.2022-0048

Table 3

Experimental Design Checklist for Researching the Impact of Exercise on Markers of Gastrointestinal Integrity and Function, Systemic Responses, and Ex-GIS

Examples of key informationYesNoNA
Participant screening
 • Athlete population demographics.Age, biological sex, body mass, stature, body composition, VO2max and/or other fitness status variables, social considerations such as culture, race, and/or religion.
 • Previous and current habitual dietary practices.Vegetarian type, vegan, and/or LCHF
 • Previous and current habitual exercise training and/or competitive practices.Modality, intensity, duration, topographical and environmental location, and/or competition level
 • Gastrointestinal healthGastrointestinal disease, disorder, and/or any other gastrointestinal complications.
 • History of GISPre, during, and/or post exercise.
 • History of gut trainingFeeding during exercise frequency, duration, type, texture, and water volume equivalent.
 • Gastrointestinal impacting dietary modification and/or nonnutritive supplementation.FODMAPs, dietary fiber, resistant starches, prebiotics, probiotics, macro- or micro-nutrients and derivatives, colostrum, and curcumin.
 • Gastrointestinal impacting pharmaceutical agents.NSAIDs, antibiotics, laxatives, anti-diarrhea agents, antacids, and/or antiemetics.
 • Female menstrual or contraceptive cycle, and confirmation by biomarker determination.Plasma estradiol and progestogen concentrations.
Confounder control
 • Physical activity control.Limit physical activity at a minimum of 24 hr before exercise protocol.
 • Dietary control.Energy and macronutrient balanced low FODMAP and/or low residue diet, otherwise indicated.
Standardized food and fluid provided (minimum 24 hr before exercise protocol).
Food and fluid log to support compliance of provisions.
 • Hydration status.Euhydration prior to onset (otherwise indicated).
 • Timing of exertional stress.Diurnal or nocturnal.
 • Specific event/match/competition exploration.Reporting of pre-activities food and fluid intake, hydration status, medications, and/or supplementation.
Exercise protocol
 • Sufficient exertional stress.≥3 hr at ≥60% VO2max or Wmax equivalent; or ≥2 hr at 60% VO2max or Wmax equivalent with heat exposure.
 • Sufficient heat stress.≥35.0 °C, plus report ambient temperature and relative humidity, and thermal strain markers: core body temperature (≥39.0 °C), heart rate, and thermal comfort rating.
Exploration of warm (∼30.0 °C) or temperate (∼20.0 °C) ambient temperatures.
 • Specific event/match/competition exploration.Reporting of event/match/competition topography/characteristics, whatever applicable.
 • Pre-exercise nutrient provisions.≥2 hr before commencement
 • During exercise nutrient provisions.No carbohydrate provisions during exercise ≤2 hr. Provisions of carbohydrate during exercise >2 hr if required to complete exercise bout (e.g., 3 hr exercise bout with carbohydrate feeding in first and second hour, and cease feeding in third hour).
 • Post-exercise nutrition.Exercise recovery nutrition based on research focus and intervention aim/s.
 • Euhydration maintenanceMonitor and record combined techniques including total body water, plasma osmolality, plasma volume change and nude body mass. Avoid using nude body mass solely and avoid urine measures of hydration.
 • Fluid provisionsMeasure, analyze, and report characteristics such as: volume, composition, % w/v, temperature, osmolality, and pH.
Sample collection, processing, and analysis
 • Fecal sample collection times∼30 g mid-flow; immediately pre-exercise, and if applicable first defecation ≥4 hr post-exercise, and immediate processing and/or storage (e.g., −80 °C or liquid nitrogen storage).
 • Blood sample collection times and processes.Immediately pre-exercise, and immediately, 1 hr, and 2 hr post exercise.
 • Appropriate vacutainers for blood collection.Lithium heparin, EDTA, and/or serum.
 • Dual sugars test for intestinal permeability.Dual sugars test given 30 min before exercise cessation, plus 5 hr total urine collection; and/or standard blood collection and timing.a
 • OCTT∼20–30 g lactulose solutionb in water to ∼150 ml total solution. Breath sample collection immediately pre- and post-exercise, and every 15 min post-exercise for 3 h.
 • Carbohydrate challenge or malabsorption detectionBreath sample collection immediately pre- and post-exercise, and every 15 min post-exercise for 3 h.
 • Sample collection.Pyrogen free sample collection, processing and analysis consumables for bacteria and/or bacterial endotoxin biomarkers.
 • EGG.30 min pre- and post-exercise recording period in resting supine position. Preparation and allocation of electrode placement and respiratory sensor. Use established protocols for during exercise real-time EGG.
 • Ex-GIS assessment.Validated and reliability checked GIS assessment tool (e.g., mVAS).

Note. If answered ‘no’ to any criteria provide rational and justification. EDTA = ethylenediaminetetraacetic acid; EGG = electrogastrography; Ex-GIS = exercise-associated gastrointestinal symptoms; FODMAP = fermentable oligo- dis- mono-saccharide and polyols; GIS = gastrointestinal symptom; LCHF = low carbohydrate and high fat; NA = not applicable; mVAS = modified visual analogue scale; OCTT = orocaecal transit time; w/v = water volume.

aDual-sugars test for intestinal permeability determination and OCTT for gastrointestinal transit assessment cannot be performed simultaneously due to lactulose conflict within technique procedures. bDependant on product used (target to 20 g lactulose content).

Due to the recent advancement in scientific knowledge within exercise gastroenterology research, certain issues and limitations in experimental methodologies and practical translation of outcomes in the past and more recent published research have been highlighted. Practitioners supporting athletes with gastrointestinal complaints and/or complications need to be aware of these research limitations and be appropriately cautious when accepting and/or applying study outcomes to practice. Indeed, incorrect or misguided application can result in no, minor, or extreme to fatal outcomes in real-world practice. With the implementation of more rigorous standardization and/or controls in future exercise gastroenterology research, communications pertaining to past research, which may not have met these rigors, should be judiciously translated.

The field of exercise gastroenterology is still considered to be in its infancy stage of knowledge and understanding. Therefore, a substantial amount of research work is still required to unveil the full extent to which exercise stress impacts the various components of the gastrointestinal tract, explore undiscovered extrinsic and/or intrinsic exacerbation factors of EIGS and Ex-GIS, and strategies to prevent or manage EIGS and associated Ex-GIS. In supporting best practice for this research stream, future exercise gastroenterology researchers are encouraged and advised to guide themselves with the research recommendations detailed throughout this methodological review and ensure a comprehensive understanding and implementation of performance and clinical and translational factors.

Acknowledgments

The authors combined have contributed to >40 original study peer-reviewed publications (i.e., exploratory, methodological, and prevention and management intervention, including randomized control trials) in the area of exercise gastroenterology, and >100 original study peer-reviewed publications in the topic areas of sports dietetic, exercise physiology, and sports medicine. The material within is the result of work supported with resources and the use of facilities at BASE Facility, Monash University, Department of Nutrition Dietetics & Food, Melbourne, Australia; Coventry University Sport & Exercise Applied Research Group; and the Supernova collaborations of the Australian Institute of Sport-Mary MacKillop Institute for Health Research, Australian Catholic University. Funding support from Coventry University Sport & Exercise Applied Research Group and the Department of Health Professions; BASE Facility, Monash University, Department of Nutrition Dietetics & Food; Monash University, Faculty of Medicine Nursing & Health Sciences, Strategic Grant Scheme; Sports Medicine Australia Research Foundation Grant; Lion Dairy & Drinks as part of Monash University Food & Dairy Graduate Research Industry Partnership program; Ultra Sports Science Foundation Research Project Grant; and Australian Catholic University Research Funds and the Australian Institute of Sport’s Applied Sports Research Program. Author Contributions: All authors contributed to various aspects of the research procedure that led to the generation of data presented within. Costa was responsible for compiling the manuscript. All authors reviewed the full manuscript and approved the final version. Conflicts of Interest: Costa, Snipe, Gaskell, and Russo have contributed to the development of exercise gastroenterology educational material presented on an online learning platform (https://www.futurelearn.com/courses/fam-exercise-gut), targeting health and exercise professionals, as part of Monash University continued professional development educational packages. RC is responsible for the delivery and management of the online educational resource. RC is lead of the Monash University Exercise & Nutrition Clinic that offers specialized gastrointestinal assessment services to athletes with clinical justification.

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  • Peake, J.M., <