Exogenous carbohydrate (CHO) intake during endurance and ultraendurance exercise (i.e., >2 hr) supports glucose supply and enhances performance. CHO ingestion during exercise improves exercise capacity in a dose-dependent manner (e.g. 9–78 g/hr), with the performance-enhancing effect diminishing with intakes >78 g/hr (Stellingwerff & Cox, 2014). Current recommendations for CHO ingestion during prolonged exercise suggest 30–90 g/hr, targeting individual needs and tolerance (Burke et al., 2019; Costa et al., 2019). Furthermore, individual whole-body oxidation capacity, tolerance, and practicality when determining CHO type and amount should also be considered (Burke et al., 2019; Costa et al., 2019; Gaskell, Rauch, & Costa, 2021; Rauch et al., 2022). High incidence of exercise-associated gastrointestinal symptoms (Ex-GIS) in endurance and ultraendurance athletes (Costa et al., 2016) necessitates a tailored approach to prevent symptom exacerbation (Costa et al., 2017; Gaskell, Rauch, & Costa, 2021).
Gut-training or feeding-challenge, exposing the gut to higher volumes of food or fluid around exercise, improves feeding tolerance, gut discomfort, and Ex-GIS severity (Martinez et al., 2023). It has also been demonstrated to improve intestinal CHO absorption and circulatory glucose availability (Costa et al., 2017; Miall et al., 2018). This reduces the risk of CHO malabsorption associated with the ileal brake mechanism, a negative feedback loop that is activated by exposure of the most distal part of the small intestine to energy-containing nutrients, resulting in the slowing down of gastric emptying and gut motility, and appetite suppression (Shin et al., 2013). Furthermore, an increase in CHO availability as a result of improved absorption may subsequently support whole-body CHO oxidation during steady-state exercise (Cox et al., 2010; Rauch et al., 2022), enhancing exercise performance (Costa et al., 2017; Miall et al., 2018). Several gut-training methods have been proposed (Jeukendrup, 2017); however, evidence supporting these is limited and mostly anecdotal. Gut-training or feeding-challenge protocols that have been scientifically investigated and have shown positive results involve the repeated ingestion of CHO in solid (Costa et al., 2017; Miall et al., 2018) or liquid forms (Lambert et al., 2008), or chronic high-CHO intake (Cox et al., 2010), before and during exercise. These protocols collectively show functional, symptomatic, and performance benefits in recreational athletes (Costa et al., 2017; Cox et al., 2010; Lambert et al., 2008; Miall et al., 2018). In elite athletes, however, King et al. (2022) found that a 2-week gut-training protocol (90 g/hr CHO, beverage) had minor effects on gastrointestinal status and exercise performance, likely due to daily high-CHO intakes, adaptations to feeding, or study design. This suggests a need to fine-tune protocols depending on the athlete’s fitness level.
The macronutrient composition of supplements ingested during a repetitive feeding-challenge can influence outcomes, as slowly-digested foods may facilitate functional adaptations. For instance, fat ingestion slows gastric emptying by triggering intestinal receptors, which prolong gastric distention and control nutrient release. It lowers proximal gastric tone, reduces antral and duodenal contractile activity, and increases pyloric pressures (Feinle et al., 2003). Alternatively, high-fat diets (40%–55% calories for 7–14 days) have been demonstrated to accelerate gastric emptying possibly via desensitization of small intestinal regulatory mechanisms (Castiglione et al., 2002; Clegg & Shafat, 2011; Cunningham et al., 1991). Furthermore, competitive speed eating case studies show improved tolerance to large food volumes from rapid stomach expansion, not increased gastric emptying (Levine et al., 2007; Smoliga, 2020). Applying this to exercise settings, a repetitive feeding-challenge with nutrient dense food may further challenge the gut, and improve gastric motor function (e.g., gastric accommodation and/or emptying) during exercise, potentially reducing Ex-GIS severity. Additionally, practical gut-training or repetitive feeding-challenge protocols must fit within the usual design of training programs, with participants anecdotally reporting symptomatic improvements within 1 week, but complaining of difficulties in following the 2-week repetitive feeding-challenge protocol (Costa et al., 2017; Miall et al., 2018). These previous studies also demonstrated that a repetitive feeding-challenge with a matched placebo had no effect on CHO absorption, glucose availability, Ex-GIS, fuel kinetics, and exercise performance suggesting that if there is no nutrient presence within the feeding-challenge protocol, then no adaptation and/or benefits are viable (Costa et al., 2017; Miall et al., 2018). Based on this, the current study aimed to investigate the effects of a 7-day repetitive feeding-challenge protocol using a high-fat versus high-CHO supplement on gastrointestinal function and tolerance, fuel availability and utilization, Ex-GIS, and performance in response to a high-CHO load (87 g/hr) during a gut-challenge test protocol. We hypothesized that a repetitive feeding-challenge with fat would further improve gastrointestinal function and tolerance, fuel availability and utilization, Ex-GIS, and performance compared with a repetitive feeding-challenge with CHO.
Methods
Study Participants
The study (Figure 1) was approved by Monash University Human Research Ethics Committee (MUHREC approval number: 35216). Fifty-five endurance and ultraendurance trained recreational athletes (body mass [BM]: 71.2 ± 9.2 kg, height: 173.6 ± 7.0 cm, training volume 7.3 ± 0.7 hr/week,
—Schematic illustration of the study design.
Citation: International Journal of Sport Nutrition and Exercise Metabolism 35, 3; 10.1123/ijsnem.2024-0145
Preliminary Assessment
Height (stadiometer, Holtain Limited), BM, and composition (mBCA 515, Seca) were measured before the experimental trial days. Maximal oxygen uptake (
Experimental Design
To avoid diet-associated artifacts on measured variables (Costa et al., 2022), participants were provided with all food to consume for 48 hr prior to each experimental trial day as a standardized low FODMAP control diet (overall mean ± standard deviation [SD]: energy, 12.9 ± 1.0 MJ/day; protein, 111 ± 16 g/day; CHO, 509 ± 65 g/day; fat, 51 ± 9 g/day; fiber, 23 ± 6 g/day; and 4.9 ± 0.1 g/day total FODMAP), along with a compliance log. Twenty-four hours prior to each trial day, an orocecal transit time test (OCTT) was completed in the morning after a 12 hr overnight fast, by ingesting 20 g lactulose (Actilax, Alphapharm) in 150 ml water and collecting breath samples before and every 15 min during the 3 hr test period.
Participants attended the laboratory the following morning after consuming a standardized control breakfast (overall mean ± SD: energy, 2.3 ± 0.5 MJ; protein, 16 ± 4 g; CHO, 65 ± 16 g; fat, 7 ± 4 g; fiber, 1 ± 1 g; and 0 g total FODMAP) with water (400 ml), at least 2 hr prior to the start of the exercise protocol. Participants were asked to void before measuring nude BM and total body water (mBCA 515, Seca). Preexercise whole blood (6 ml lithium heparin, Greiner Vacuette, Interpath) and breath samples (Wagner Analysen Technik) were collected, along with tympanic temperature measurement (TH839S, Omron). Participants were educated and then instructed to complete the modified visual analogue scale assessment tool for gastrointestinal symptoms (GIS; Gaskell et al., 2019) wherein 0 corresponds to no symptom, 1–4 indicative of mild GIS (i.e., sensation of GIS, but not substantial enough to interfere with exercise workload) and increasing in magnitude, 5–9 indicative of severe GIS (i.e., GIS substantial enough to interfere with exercise workload), and 10 indicative of extremely severe GIS warranting exercise cessation. Type of GIS included overall gut discomfort, upper (i.e., gastroesophageal: belching, heartburn, bloating [stomach fullness], stomach pain, urge to regurgitate, regurgitation, and/or projectile vomiting), lower (i.e., intestinal: flatulence, lower abdominal bloating [abdominal pressure], urge to defecate, left and right intestinal pain, defecation with or without abnormalities [e.g., watery stools, diarrhea, and/or fecal blood loss]), and other related symptoms (i.e., nausea, dizziness, stitch [acute transient abdominal pain]). In addition, a 10-point Likert-type rating scale was used to quantify self-reported perceptive feeding tolerance, with 0 indicative of no tolerance and 10 indicative of extremely high tolerance (5 indicative of moderate tolerance) for the following variables: taste fatigue, interest in food (I want to eat), interest in drink (I want to drink), tolerance to food (I could eat), tolerance to drink (I could drink), appetite (I’m hungry), and thirst (I’m thirsty). Last, participants completed a sport anxiety questionnaire (Miall et al., 2018; Smith et al., 1990) based on how they felt about the exercise trial they were about to participate in.
After the 2 hr steady-state run, a 1 hr self-paced distance test (DT; overall mean speed: 11.4 ± 2.1 km/hr, 21.3 ± 0.8°C, 50.0 ± 7.1 relative humidity, dual-fan wind speed of 10.6 km/hr), with ad libitum water was completed. A self-paced distance performance test was selected based on previous studies assessing intervention effect on performance outcomes in endurance running (Oliver et al., 2007; Scrivin et al., 2024), which is more reflective of endurance and ultraendurance racing. HR, RPE, speed, and distance covered were recorded every 10 min during the DT. Post-DT, GIS, and tympanic temperature were assessed, while participants completed an additional 5 min cool down (60%
Repetitive Feeding-Challenge Intervention
Using a single-blind, randomized, parallel group design, participants were allocated using a randomization plan (http://www.randomization.com) to either the fat (fat feeding-challenge [FFC]), or the CHO (CHO feeding-challenge [CFC]) repetitive feeding-challenge group after the first gut-challenge trial (T1). Researchers interacting with the participants were blinded to the intervention assignments of the participants. Participants completed a repetitive feeding-challenge protocol consisting of 1 hr of either running or cycling exercise (majority running) estimated to equate to 60%
Sample Analysis
Hydrogen (H2) and methane (CH4) were measured in breath samples in duplicate (CV: <1.0%) using a gas sensitive analyzer (Breathtracker Digital Microlyzer, QuinTron; Peters et al., 1994). Using heparin whole blood samples, hemoglobin (HemoCue Hb 201+), and blood glucose (HemoCue Glucose 201 RT) were measured in duplicate (CV: <1% and <1%, respectively). Hematocrit was measured in triplicate (CV: <1%) using the capillary method and a microhematocrit reader to estimate plasma volume changes from baseline and correct plasma variables (Dill & Costill, 1974). The remaining blood samples were centrifuged (10 min, 1,500 g, 4°C), and plasma osmolality was determined in duplicate (CV: 1.2%) by freeze-point osmometry (Osmomat 030, Gonotec). The remaining plasma was aliquoted and stored at −80°C until further analysis.
OCTT at rest was determined as the time interval between lactulose ingestion and breath H2 rise (≥10 ppm above baseline, on two consecutive readings) or breath CH4 rise for non-H2 producers (Bate et al., 2010).
Statistical Analysis
Using mean, SD, and effect size data for breath H2 to detect malabsorption differences between interventions in previous studies (Costa et al., 2017; Miall et al., 2018) and applying a standard α (.05) and β (0.8) values, a sample size of 44 was determined for sufficient statistical power (G*Power, version 3.1). Statistical analysis was done using SPSS (version 29.0, IBM, SPSS, IBM Corp.) with a significance level set at p < .05. Singular data point variables were examined using t tests or nonparametric equivalents, where applicable. A two-way repeated-measures analysis of variance with time and trial as factors was used to examine variables with multiple data points. Assumptions of homogeneity and sphericity were checked and, when appropriate, adjustments to the degrees of freedom were made using the Greenhouse–Geisser correction method. Significant main effects were analyzed using a post hoc Tukey’s honestly significant difference test. Spearman’s rank correlation analysis was used for associations. Data presented in-text and in tables are mean ± SD or mean and range, and figures show mean ± standard error of the mean and/or individual responses, unless otherwise specified.
Results
Participant Characteristics
Among the enrolled participants, there were n = 3 dropouts prior to T1 and n = 4 after completion of T1 (n = 2 FFC, n = 2 CFC) due to personal circumstances (i.e., relocation, injury, and loss to follow-up). Forty-eight participants completed the study; however, n = 4 had low intervention compliance and were excluded from data analysis. The final sample size was 44 with no significant differences in baseline characteristics observed between groups (Table 1). Repetitive feeding-challenge intervention compliance were 98 ± 5% in FFC and 99 ± 3% in CFC, with majority of sessions completed using running as the exercise modality (FFC: 6 ± 2 run, 1 ± 2 cycle; CFC: 5 ± 2 run, 1 ± 1 cycle).
Baseline Participant Characteristics
| Group | Fat feeding-challenge group | CHO feeding-challenge group | p |
|---|---|---|---|
| N (males, females) | 23 (18, 5) | 21 (17, 4) | — |
| Age (year) | 37 ± 11 | 34 ± 10 | .189 |
| Weight (kg) | 71.5 ± 9.2 | 71.4 ± 9.4 | .540 |
| Height (cm) | 173.2 ± 7.0 | 174.1 ± 7.2 | .316 |
| Fat mass (kg) | 15.0 ± 5.8 | 14.6 ± 10.8 | .978 |
| Skeletal muscle mass (kg) | 27.5 ± 4.1 | 28.1 ± 4.7 | .265 |
| Fat-free mass (kg) | 57.6 ± 7.4 | 56.8 ± 11.7 | .723 |
| Running | 55.1 ± 7.3 | 54.9 ± 4.7 | .908 |
| Training load (min/week) | 435 ± 180 | 443 ± 267 | .922 |
| Running only (n) | 17 | 10 | — |
| Cycling only (n) | 0 | 0 | — |
| Running and cycling (n) | 6 | 11 | — |
| Usual CHO intake during exercise (g/hr) | 40 ± 22 | 39 ± 19 | .754 |
| Usual fluid intake during exercise (ml/hr) | 446 ± 189 | 453 ± 319 | .498 |
Note. Data presented as mean ± SD (n = 44). CHO = carbohydrate.
Hydration Status and Thermo-Physiological Strain
No differences were observed within and between groups for preexercise total body water, total fluid consumption during exercise, exercise-induced BM loss, and pre- and postexercise plasma osmolality. Pre- to postexercise plasma osmolality did not differ between CFC trials (T1: p = .427, T2: p = .344), but significantly decreased from pre- to postexercise in FFC on T2 (1.4%, p = .007) only.
No Trial × Time interactions were observed for HR and RPE in both groups. A main effect of time for HR and RPE was observed in both groups across trials (p < .001), whereby both variables were higher during the DT compared with the first 5 min of the steady-state run (Figure 2A and 2B).
—HR (A1: FFC, A2: CFC) and RPE (B1: FFC, B2: CFC) recorded during 2-hr steady-state run at 60%
Citation: International Journal of Sport Nutrition and Exercise Metabolism 35, 3; 10.1123/ijsnem.2024-0145
Feeding Tolerance
No Trial × Time interactions were observed for subjective feeding tolerance markers in both groups (Table 2). There was a main effect of time (p < .001) for all feeding tolerance variables in both groups and trials. Tolerance to food decreased during the recovery period (9%, p = .048) in CFC. Appetite during the second hour of exercise was greater in CFC versus FFC after the intervention (42%, p = .046).
Feeding Tolerance Markers in Response to 2-hr Steady-State Run at 60%
| Feed tolerance variables | Before exercise | First hour during exercise | Second hour during exercise | Third hour during exercise | Total during exercise | First hour during recovery | Second hour during recovery | 6 hr postexercise | Total during recovery |
|---|---|---|---|---|---|---|---|---|---|
| Taste fatigue | |||||||||
| FFC | |||||||||
| T1 | 0 | 1 (0–7) | 5 (0–21) | 1 (0–9) | 7 (0–30) | 4 (0–34 | 2 (0–24) | 1 (0–10) | 7 (0–55) |
| T2 | 0 | 1 (0–8) | 6 (0–26) | 1 (0–9) | 8 (0–39) | 3 (0–25) | 1 (0–20) | 0 (0–2) | 4 (0–47) |
| CFC | |||||||||
| T1 | 0 | 2 (0–8) | 5 (0–21) | 1 (0–7) | 8 (0–31) | 3 (0–24) | 2 (0–18) | 1 (0–8) | 6 (0–42) |
| T2 | 0 | 4 (0–28) | 8 (0–30) | 3 (0–10) | 14 (0–68) | 5 (0–30) | 2 (0–24) | 0 (0–5) | 7 (0–54) |
| Interest in food | |||||||||
| FFC | |||||||||
| T1 | 2 (0–10) | 2 (0–12) | 2 (0–17) | 2 (0–10) | 6 (0–35) | 17 (0–40) | 26 (0–40) | 6 (0–10) | 49 (0–90) |
| T2 | 1 (0–4) | 2 (0–12) | 2 (0–15) | 1 (0–9) | 5 (0–34) | 15 (0–40) | 25 (0–40) | 6 (0–10) | 45 (4–87) |
| CFC | |||||||||
| T1 | 2 (0–8) | 4 (0–20) | 4 (0–18) | 2 (0–10) | 11 (0–34) | 24 (1–40) | 33 (8–40) | 7 (0–10) | 64 (24–88) |
| T2 | 1 (0–10) | 4 (0–26) | 5 (0–22) | 2 (0–8) | 12 (0–56) | 20 (1–40) | 31 (14–40) | 7 (2–10) | 57 (17–89) |
| Interest in drink | |||||||||
| FFC | |||||||||
| T1 | 3 (0–10) | 5 (0–14) | 4 (0–16) | 4 (0–33) | 14 (0–36) | 17 (0–40) | 17 (0–40) | 5 (0–10) | 39 (0–87) |
| T2 | 3 (0–10) | 8 (0–30) | 7 (0–26) | 4 (0–10) | 19 (0–56) | 19 (0–56) | 17 (0–40) | 5 (0–10) | 39 (3–90) |
| CFC | |||||||||
| T1 | 3 (0–8) | 8 (0–20) | 6 (0–19) | 4 (0–10) | 18 (0–44) | 20 (3–40) | 20 (0–40) | 5 (0–10) | 46 (7–84) |
| T2 | 3 (0–8) | 6 (0–16) | 4 (0–17) | 4 (0–10) | 13 (0–33) | 16 (0–38) | 19 (0–40) | 5 (1–10) | 40 (7–86) |
| Tolerance to food | |||||||||
| FFC | |||||||||
| T1 | 7 (0–10) | 15 (0–30) | 13 (0–30) | 5 (0–10) | 33 (0–70) | 29 (0–40) | 33 (11–40) | 7 (1–10) | 69 (16–90) |
| T2 | 7 (0–10) | 15 (0–30) | 13 (0–30) | 5 (0–10) | 34 (0–70) | 28 (6–40) | 34 (16–40) | 7 (0–10) | 69 (33–90) |
| CFC | |||||||||
| T1 | 7 (0–10) | 19 (3–30) | 17 (2–30) | 5 (0–10) | 41 (10–70) | 34 (16–40) | 38 (31–40) | 7 (0–10) | 79 (54–90) |
| T2 | 7 (1–10) | 18 (3–30) | 17 (0–30) | 5 (0–10) | 39 (11–70) | 29 (10–40)a | 36 (24–40) | 8 (2–10) | 72 (39–90)a |
| Tolerance to drink | |||||||||
| FFC | |||||||||
| T1 | 9 (2–10) | 20 (2–30) | 17 (0–30) | 8 (0–10) | 45 (2–70) | 33 (16–40) | 31 (12–40) | 7 (1–10) | 71 (36–90) |
| T2 | 9 (3–10) | 22 (6–30) | 18 (0–30) | 7 (0–10)a | 46 (9–70) | 31 (10–40) | 31 (7–40) | 6 (0–10) | 68 (23–90) |
| CFC | |||||||||
| T1 | 9 (0–10) | 22 (7–30) | 16 (0–30) | 7 (0–10) | 45 (11–70) | 31 (9–40) | 29 (0–40) | 7 (1–10) | 67 (19–90) |
| T2 | 8 (3–10) | 19 (6–30) | 16 (3–30) | 7 (1–10) | 43 (13–70) | 28 (4–40) | 30 (10–40) | 7 (1–10) | 64 (18–90) |
| Appetite | |||||||||
| FFC | |||||||||
| T1 | 2 (0–7) | 3 (0–15) | 3 (0–15) | 2 (0–10) | 8 (0–33) | 18 (0–37) | 27 (0–40) | 6 (0–10) | 52 (0–87) |
| T2 | 2 (0–5) | 4 (0–15) | 4 (0–18)b | 2 (0–10) | 10 (0–42) | 16 (0–40) | 26 (0–40) | 6 (0–10) | 49 (8–86) |
| CFC | |||||||||
| T1 | 2 (0–7) | 5 (0–15) | 6 (0–16) | 3 (0–10) | 14 (0–36) | 25 (0–40) | 32 (0–40 | 6 (0–10) | 62 (4–90) |
| T2 | 3 (0–10) | 7 (0–26) | 7 (0–22) | 3 (0–8) | 16 (0–56) | 21 (1–40) | 32 (16–40) | 7 (2–10) | 60 (19–88) |
Note. Feeding tolerance markers rated on a 10-point Likert-type rating scale preexercise, during running with carbohydrate provision, and during recovery (Miall et al., 2018). Participants reported summative accumulation of rating scale at measured time periods, and range (n = 44). First hour during exercise: from 0 to 60 min of exercise. Second hour during exercise: from 60 to 120 min of exercise. Third hour during exercise: from 120 to 180 min of exercise. Total during exercise: from 0 to 180 min of exercise. First hour during recovery: from 0 to 60 after exercise. Second hour during recovery: from 60 to 120 after exercise. 6 hr postexercise: from 120 to 380 min after exercise. Total during recovery: from 0 to 380 min after exercise. CFC = CHO feeding-challenge; FFC = fat feeding-challenge.
ap < .05 versus T1. bp < .05 versus CFC.
Orocecal Transit Time
OCTT at rest did not differ between (T1: p = .395, T2: p = .961) or within FFC (T1: 150 [30, 180] min, T2: 165 [15, 180] min; p = .617) and CFC (T1: 165 [15, 180] min, T2: 135 [45, 180] min; p = .297]. Absence of breath H2 (or CH4) response was observed in both FFC (T1: 39%, T2: 35%) and CFC (T1: 29%, T2: 33%) (Figure 3).
—OCTT (A1: FFC, A2: CFC) and breath H2 turning point (A1: FFC, A2: CFC). Individual responses for OCTT (n = 44) at T1 (
) and T2 (
) and total number of participants with H2 turning point at T1 (▪) and T2 (□). #Participants who failed to present a breath H2 turning point in the 3 hr testing period at T1. >More than 130 min (FFC) and 116 min (CFC) at T1 and 127 min (FFC) and 128 min (CFC) at T2 due to participants failing to present a breath H2 turning point. **p < .01 and *p < .05 vs. 0 min. OCTT = Orocecal transit time at rest; CFC = CHO feeding-challenge; FFC = fat feeding-challenge.
Citation: International Journal of Sport Nutrition and Exercise Metabolism 35, 3; 10.1123/ijsnem.2024-0145
CHO Malabsorption
At T1, clinically significant CHO malabsorption incidence was 13% in FFC and 10% in CFC, and was reduced after the repetitive feeding-challenge to 1% and 0%, respectively. A main effect of time (p < .001) for breath H2 was observed in both groups across trials, whereby breath H2 during recovery increased from baseline. Breath H2 area under the curve during recovery did not significantly differ within FFC (p = .127) and CFC (p = .501) and between groups (T1: p = .290; T2: p = .085). In FFC, peak breath H2 during recovery at T2 was not significantly different from T1 (p = .061), but was lower compared with CFC (p = .028) (Figure 4).
—Mean ± SEM (n = 44) CHO malabsorption as indicated by breath H2 levels over time (A1: FFC, A2: CFC) in response to 2 hr steady-state run at 60%
Citation: International Journal of Sport Nutrition and Exercise Metabolism 35, 3; 10.1123/ijsnem.2024-0145
Blood Glucose
A main effect of time (p < .001) for blood glucose was observed in both groups across trials; whereby blood glucose during exercise and postexercise was higher than preexercise. In FFC, blood glucose level immediately after exercise was higher after the repetitive feeding-challenge (T1: 5.0 ± 1.0 mmol/L vs. T2: 5.7 ± 1.2 mmol/L, p < .01; Figure 5).
—Mean ± SEM (n = 44) of blood glucose level (A1: FFC, A2: CFC) in response to 2 hr steady-state run at 60%
Citation: International Journal of Sport Nutrition and Exercise Metabolism 35, 3; 10.1123/ijsnem.2024-0145
Whole-Body CHO and Fat Oxidation
A main effect of time (p < .001) for whole-body CHO oxidation and whole-body fat oxidation was observed in both groups across trials. CHO oxidation decreased, and fat oxidation increased over time during exercise versus the first 20 min of exercise. Whole-body fat oxidation area under the curve during exercise was lower at T2 in CFC (19%, p = .025) but not significantly different from FFC (p = .245; Figure 6).
—Mean ± SEM (n = 44) whole-body oxidation rate of carbohydrate (A1: FFC, A2: CFC) and fat (C1: FFC, C2: CFC) in response to 2 hr steady-state run at 60%
Citation: International Journal of Sport Nutrition and Exercise Metabolism 35, 3; 10.1123/ijsnem.2024-0145
Gastrointestinal Symptoms
During the preintervention experimental trial, GIS with highest prevalence during exercise were upper abdominal bloating (98%), belching (82%), and lower abdominal bloating (59%). The same symptoms were the most prevalent after the intervention, but of lesser severity (Table 3). Gut discomfort severity was similarly improved between groups during exercise (p = .075), and recovery (p = .556), after the repetitive feeding-challenge. Total GIS severity during exercise was reduced by 27% in FFC (p = .005) and 38% in CFC (p = .001), with no group differences (p = .664). Upper GIS severity during exercise was reduced by 26% in FFC (p < .001) and 40% in CFC (p < .001), with no group differences (p = .391). Severity of lower GIS during exercise was significantly reduced only in FFC (32%, p = .022). During recovery, both groups had similar reductions in severity of total (p = .787), upper (p = .426), lower (p = .645), and other GIS (p = .703). Somatic trait anxiety scores decreased in FFC (T1: 14 ± 5, T2: 12 ± 4; p < .001) and CFC (T1: 14 ± 4, T2: 13 ± 3; p = .002) after the repetitive feeding-challenge, with no difference between groups in both trials (T1: p = .339 and T2: p = .271). Somatic trait anxiety scores were not correlated to total-GIS during exercise at T1, r(41) = 0.110, p = .493, and T2, r(41) = −0.092, p = .568.
Incidence and Severity of GIS in Response to 2 hr Steady-State Run at 60%
| Incidence (%) | Summative accumulation of rating scale (severity) | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Before exercise | During exercise | During recovery | Before exercise | First hour during exercise | Second hour during exercise | Third hour during exercise | Total during exercise | First hour during recovery | Second hour during recovery | 6 hr post-exercise | Total during recovery | |
| Gut discomfort1 | N/A | N/A | N/A | |||||||||
| FFC | ||||||||||||
| T1 | 4 (1–8) | 12 (3–26) | 13 (2–30) | 3 (2–10) | 29 (7–66) | 7 (2–20) | 4 (2–15) | 3 (1–10) | 14 (1–44) | |||
| T2 | 4 (1–10) | 11 (1–27) | 12 (4–24) | 3 (1–8) | 26 (6–54) | 5 (1–18)bb | 2 (1–16) | 2 (1–10) | 10 (1–44)bb | |||
| CFC | ||||||||||||
| T1 | 2 (1–6) | 11 (3–30) | 13 (3–26) | 3 (1–10) | 27 (4–60) | 4 (1–16) | 2 (1–16) | 3 (1–10) | 9 (1–32) | |||
| T2 | 3 (1–7) | 10 (1–30) | 9 (2–30)a | 2 (1–10) | 21 (5–70)a | 0 (0–2)aa | 4 (1–12)a | 3 (1–10) | 4 (1–12) | |||
| Total-GIS2 | ||||||||||||
| FFC | ||||||||||||
| T1 | 92 | 100 | 92 | 8 (1–31)b | 26 (7–61) | 32 (4–74) | 8 (1–37) | 66 (11–129) | 15 (3–56) | 9 (1–59)b | 7 (1–24) | 31 (3–122)bb |
| T2 | 96 | 100 | 92 | 5 (1–16) | 19 (1–45)a | 22 (7–50)aa | 7 (2–33) | 48 (9–92)aa | 8 (2–29)bb | 3 (2–20)bb | 5 (1–26) | 16 (1–59)bb |
| CFC | ||||||||||||
| T1 | 79 | 100 | 100 | 4 (1–12) | 21 (4–44) | 31 (4–76) | 8 (2–20) | 60 (16–118) | 7 (1–22) | 3 (3–27) | 8 (1–25) | 18 (1–60) |
| T2 | 79 | 100 | 100 | 3 (1–9) | 17 (1–40) | 15 (2–39)aa | 5 (2–20) | 37 (7–79)aa | 2 (1–9)a | 0 (2)a | 5 (3–24) | 7 (1–32)a |
| Total upper-GIS3 | ||||||||||||
| FFC | ||||||||||||
| T1 | 92 | 100 | 79 | 4 (1–12)b | 17 (5–45) | 21 (2–56) | 4 (1–14) | 42 (7–106) | 7 (1–26) | 3 (1–14) | 1 (1–6) | 11 (1–40) |
| T2 | 92 | 100 | 75 | 4 (1–8) | 13 (1–31) | 14 (3–35)a | 4 (1–14) | 31 (2–66)aa | 5 (1–18)b | 2 (1–16)bb | 1 (2–5) | 8 (1–34)b |
| CFC | ||||||||||||
| T1 | 71 | 100 | 67 | 2 (1–7) | 16 (4–40) | 22 (2–52) | 5 (1–13) | 42 (6–92) | 5 (1–16) | 2 (1–16) | 1 (1–5) | 7 (1–32) |
| T2 | 75 | 100 | 54 | 2 (1–8) | 12 (1–34) | 11 (2–39)aa | 3 (2–18) | 25 (5–79)aa | 2 (1–9)aa | 0 (2)a | 1 (1–16) | 3 (1–24)a |
| Belching | ||||||||||||
| FFC | ||||||||||||
| T1 | 17 | 79 | 38 | 0 (1–2) | 3 (2–8) | 5 (1–15) | 1 (1–5) | 9 (2–21) | 1 (1–3) | 0 (1–2) | 0 (1–1) | 1 (1–5) |
| T2 | 8 | 67 | 25 | 0 (2–2) | 2 (1–15)a | 3 (1–15)aa | 1 (1–8) | 5 (1–33)aa | 0 (1–1)a | 0 | 0 (1–2) | 0 (1–2)a |
| CFC | ||||||||||||
| T1 | 13 | 88 | 38 | 0 (1–3) | 5 (2–20) | 6 (3–14) | 1 (1–4) | 12 (2–29) | 1 (2–6) | 0 (3–3) | 0 (1–3) | 1 (1–10) |
| T2 | 8 | 83 | 29 | 0 (1–3) | 3 (1–9)a | 3 (1–10)a | 1 (1–6) | 6 (1–18)a | 0 (1) | 0 | 1 (1–5) | 1 (1–5) |
| Heartburn (gastroesophageal reflux) | ||||||||||||
| FFC | ||||||||||||
| T1 | 4 | 17 | 8 | 0 (1) | 1 (6–9) | 0 (3–6) | 0 (1–2) | 1 (2–13) | 0 (1–6) | 0 (4–4) | 0 (1–1) | 1 (1–11) |
| T2 | 4 | 17 | 13 | 0 (2–2) | 1 (1–10) | 0 (2–5) | 0 (1–6) | 1 (4–11) | 0 (2) | 0 (4) | 0 (1) | 0 (1–7) |
| CFC | ||||||||||||
| T1 | 4 | 33 | 0 | 0 (1) | 0 (2–4) | 0 (2–4) | 0 (3–4) | 1 (2–8) | 0 | 0 | 0 | 0 |
| T2 | 0 | 13 | 0 | 0 | 1 (3–6) | 0 | 0 | 1 (3–6) | 0 | 0 | 0 | 0 |
| Upper abdominal bloating (fullness) | ||||||||||||
| FFC | 92 | 96 | 63 | 4 (1–9)bb | 12 (3–21)b | 11 (2–24) | 2 (1–7) | 24 (6–44) | 6 (3–20) | 3 (2–14) | 1 (1–4) | 9 (2–36) |
| T1 | 88 | 100 | 58 | 3 (1–8) | 10 (1–25) | 10 (3–24)b | 2 (1–7)b | 22 (2–54)b | 5 (1–18)b | 2 (1–16)bb | 0 (1–2) | 7 (4–34)b |
| T2 | ||||||||||||
| CFC | ||||||||||||
| T1 | 71 | 100 | 50 | 2 (1–5) | 8 (2–13) | 10 (2–22) | 2 (1–5) | 21 (4–37) | 4 (1–16) | 2 (1–16) | 0 (1–2) | 6 (1–32) |
| T2 | 71 | 92 | 42 | 2 (1–5) | 7 (1–18) | 5 (2–16)aa | 1 (1–4)a | 13 (3–34)a | 2 (1–9)a | 0 (2–2) | 0 (1–3) | 2 (1–11) |
| Upper abdominal pain | ||||||||||||
| FFC | ||||||||||||
| T1 | 0 | 8 | 21 | 0 | 0 | 0 | 0 (2–7) | 0 (2–7) | 0 (2–2) | 0 | 0 (1–3) | 0 (1–3) |
| T2 | 0 | 4 | 8 | 0 | 0 | 0 | 0 (7) | 0 (7) | 0 (1) | 0 | 1 (5) | 1 (1–5) |
| CFC | ||||||||||||
| T1 | 0 | 13 | 13 | 0 | 0 | 1 (2–9) | 0 (2–3) | 1 (2–9) | 0 | 0 | 0 (1) | 0 (1) |
| T2 | 0 | 13 | 4 | 0 | 0 (2–7) | 0 (2–2) | 0 (2–2) | 1 (4–9) | 0 | 0 | 0 (4) | 0 (4) |
| Urge to regurgitate | ||||||||||||
| FFC | ||||||||||||
| T1 | 0 | 17 | 4 | 0 | 0 (1)b | 1 (4–20)bb | 0 (1) | 1 (1–20)b | 0 | 0 | 0 | 0 |
| T2 | 0 | 13 | 0 | 0 | 0 (1) | 1 (1–11) | 0 | 1 (1–12) | 0 | 0 | 0 | 0 |
| CFC | ||||||||||||
| T1 | 4 | 50 | 8 | 0 | 1 (1–6) | 3 (1–14) | 0 (3–3) | 4 (1–17) | 0 (3) | 0 | 0 (1) | 0 (1–3) |
| T2 | 0 | 17 | 4 | 0 | 0 | 0 (2–3)a | 1 (5–6)aa | 1 (2–6) | 0a | 0 | 0 (4) | 0 (4) |
| Regurgitation | ||||||||||||
| FFC | ||||||||||||
| T1 | 0 | 38 | 4 | 0 | 2 (10–20) | 4 (10–30) | 1 (10) | 6 (10–60) | 0 (6) | 0 | 0 | 0 (6) |
| T2 | 0 | 8 | 0 | 0 | 1 (10) | 0 (10)a | 0 | 1 (10–20)aa | 0 | 0 | 0 | 0 |
| CFC | ||||||||||||
| T1 | 0 | 21 | 0 | 0 | 1 (30) | 2 (10–20) | 1 (10) | 4 (10–50) | 0 | 0 | 0 | 0 |
| T2 | 0 | 21 | 0 | 0 | 2 (12–20) | 2 (10–30) | 0 (10) | 4 (10–60) | 0 | 0 | 0 | 0 |
| Total lower-GIS4 | ||||||||||||
| FFC | ||||||||||||
| T1 | 54 | 92 | 88 | 3 (1–34) | 9 (2–34) | 10 (1–26) | 3 (1–15) | 22 (3–50) | 7 (2–35)b | 5 (1–33)b | 5 (1–16) | 16 (2–71) |
| T2 | 33 | 71 | 63 | 2 (2–10) | 6 (1–25)a | 7 (2–20) | 2 (1–15) | 15 (7–48)a | 2 (1–11)a | 1 (1–8)a–b | 4 (1–21) | 7 (1–25)a |
| CFC | ||||||||||||
| T1 | 33 | 83 | 67 | 1 (1–8) | 5 (2–28) | 7 (2–35) | 2 (2–9) | 14 (2–46) | 2 (2–10) | 1 (5–8) | 6 (1–21) | 9 (1–38) |
| T2 | 33 | 75 | 50 | 1 (1–6) | 5 (1–17) | 4 (2–20) | 1 (1–7) | 10 (2–32) | 0 (1–2) | 0 | 3 (1–15) | 3 (1–15)a |
| Flatulence | ||||||||||||
| FFC | ||||||||||||
| T1 | 25 | 58 | 54 | 0 (1–3) | 2 (2–9) | 2 (1–11) | 0 (1–7) | 5 (1–20) | 1 (1–16) | 1 (1–12) | 1 (1–2) | 2 (1–29) |
| T2 | 8 | 33 | 17 | 0 (2–2) | 1 (2–5) | 1 (2–9) | 0 (3–4) | 2 (2–16) | 0 (2–2) | 0 | 0 (1–2) | 0 (1–2)a |
| CFC | ||||||||||||
| T1 | 13 | 38 | 46 | 0 (1–2) | 1 (2–7) | 1 (3–13) | 0 (2) | 2 (4–13) | 0 (1) | 0 | 1 (1–5) | 1 (1–5) |
| T2 | 8 | 42 | 25 | 0 (1) | 1 (3–6) | 1 (3–7) | 0 (2) | 2 (2–10) | 0 | 0 | 0 (1–2)a | 0 (1–20)a |
| Lower abdominal bloating | ||||||||||||
| FFC | ||||||||||||
| T1 | 42 | 67 | 54 | 1 (1–9) | 4 (2–14) | 4 (2–19) | 1 (1–5) | 9 (1–26) | 3 (2–18) | 1 (1–9)b | 0 (1–3) | 5 (1–26) |
| T2 | 21 | 58 | 33 | 1 (2–4) | 2 (3–13) | 4 (2–19) | 1 (1–9) | 7 (2–38) | 2 (1–11) | 0 (1–4)b | 0 (1) | 2 (1–15) |
| CFC | ||||||||||||
| T1 | 25 | 46 | 38 | 1 (1–6) | 1 (4–8) | 3 (2–13) | 1 (2–6) | 5 (2–23) | 1 (1–7) | 0 (1) | 0 (1–2) | 1 (1–10) |
| T2 | 21 | 46 | 21 | 1 (2–6) | 2 (1–12) | 2 (2–13) | 0 (1–3) | 4 (1–22) | 0 (1–2) | 0 | 0 (1–3) | 1 (1–5) |
| Urge to defecate | ||||||||||||
| FFC | ||||||||||||
| T1 | 17 | 46 | 63 | 1 (2–6) | 2 (1–10) | 2 (2–12) | 0 (2–7) | 5 (2–29) | 2 (3–15) | 2 (1–15) | 1 (1–3) | 5 (1–29) |
| T2 | 13 | 38 | 29 | 0 (2–6) | 1 (1–12) | 1 (1–12) | 0 (3) | 3 (1–24) | 0 (2–5)a | 0 (4) | 1 (1–9) | 2 (1–11)a |
| CFC | ||||||||||||
| T1 | 8 | 54 | 38 | 0 (2–3) | 2 (1–13) | 2 (3–10) | 0 (8) | 4 (2–20) | 0 (1–5) | 1 (4–8) | 1 (1–4) | 2 (2–15) |
| T2 | 8 | 29 | 21 | 0 (3) | 1 (1–7) | 1 (1–6) | 0 (1) | 2 (1–13) | 0 (1–2) | 0 | 0 (1–2)a | 0 (1–2)a |
| Lower abdominal pain | ||||||||||||
| FFC | ||||||||||||
| T1 | 9 | 22 | 17 | 0 (5–6) | 0 (4) | 0 (2) | 1 (2–7) | 1 (2–7) | 0 (2–5) | 0 (1–4) | 0 (1) | 1 (2–5) |
| T2 | 0 | 9 | 0 | 0 | 0 | 0 (1) | 0 (3) | 0 (1–3) | 0 | 0 | 0 | 0 |
| CFC | ||||||||||||
| T1 | 0 | 19 | 10 | 0 | 0 (1–2) | 0 (2) | 0 (9) | 0 (9) | 0 (5) | 0 | 0 (1) | 0 (1–5) |
| T2 | 0 | 14 | 5 | 0 | 0 (2) | 0 | 0 (2–7) | 0 (2–7) | 0 | 0 | 0 (2) | 0 (2) |
| Abnormal defecation5 | ||||||||||||
| FFC | ||||||||||||
| T1 | 0 | 21 | 38 | 0 | 1 (10) | 1 (10) | 0 | 2 (10) | 0 (10) | 0 | 3 (10) | 3 (10) |
| T2 | 0 | 21 | 33 | 0 (10) | 1 (10) | 1 (10) | 0 | 3 (10–20) | 0 | 0 | 3 (10) | 3 (10) |
| CFC | ||||||||||||
| T1 | 0 | 17 | 42 | 0 | 1 (10) | 1 (10) | 0 | 2 (10) | 0 | 0 | 4 (10) | 4 (10) |
| T2 | 0 | 13 | 17 | 0 (10) | 1 (10) | 0 (10) | 0 | 3 (10) | 0 | 0 | 2 (10) | 2 (10) |
| Other GIS6 | ||||||||||||
| FFC | ||||||||||||
| T1 | 0 | 42 | 25 | 0 | 1 (2–9) | 1 (3–6) | 1 (1–9) | 2 (1–11) | 2 (4–14) | 2 (3–24) | 0 (1–2) | 4 (1–40) |
| T2 | 0 | 46 | 25 | 0 | 0 | 0 (2–3) | 2 (1–11) | 2 (1–11) | 1 (1–13) | 0 | 0 (1–2) | 1 (1–14) |
| CFC | ||||||||||||
| T1 | 4 | 63 | 21 | 0 (2) | 0 (1–7) | 1 (2–6) | 2 (3–10) | 3 (1–15) | 1 (1–10) | 1 (3–19) | 0 (1–2) | 2 (2–21) |
| T2 | 0 | 21 | 21 | 0 | 0 (4) | 0 (2–3) | 1 (4–10) | 1 (2–10)a | 0 (1–3) | 0 | 0 (3–4) | 1 (1–4) |
| Nausea | ||||||||||||
| FFC | ||||||||||||
| T1 | 0 | 13 | 21 | 0 | 0 (2) | 0b | 0 (1–4) | 0 (1–6) | 1 (1–13) | 1 (19) | 0 (1–2) | 1 (1–33) |
| T2 | 0 | 13 | 8 | 0 | 0 | 0 (2) | 0 (4) | 0 (2–4) | 0 (4) | 0 | 0 (1) | 0 (1–4) |
| CFC | ||||||||||||
| T1 | 4 | 29 | 4 | 0 (2) | 0 (1) | 0 (1–3) | 0 (3–6) | 1 (1–6) | 0 (3) | 0 | 0 | 0 (3) |
| T2 | 0 | 8 | 4 | 0 | 0 (4) | 0 | 0 (1) | 0 (1–4) | 0 | 0 | 0 (2) | 0 (2) |
| Acute transient abdominal pain (stitch) | ||||||||||||
| FFC | ||||||||||||
| T1 | 0 | 21 | 4 | 0 | 0 (2) | 0 (3–6) | 0 (1–5) | 1 (1–8) | 0 (2) | 0 | 0 | 0 (2) |
| T2 | 0 | 29 | 8 | 0 | 0 | 0 (3) | 1 (1–5) | 1 (1–6) | 0 (1–9) | 0 | 0 | 0 (1–9) |
| CFC | ||||||||||||
| T1 | 0 | 25 | 0 | 0 | 0 (2) | 1 (1–6) | 1 (3–10) | 2 (1–14) | 0 | 0 | 0 | 0 |
| T2 | 0 | 17 | 8 | 0 | 0 | 0 (2–3) | 1 (4–10) | 1 (2–10) | 0 (3) | 0 | 0 (3) | 0 (3) |
Note. Gastrointestinal symptoms (GIS) rated on a 10-point modified visual analogue scale (mVAS) pre-exercise, during running with carbohydrate provision, and during recovery. Incidence (%): total number of participants reporting GIS ≥ 1 on the mVAS for any GIS type before exercise, during exercise, and after exercise. Severity: summative accumulation of rating scale of measured time periods, and range (n=44). First hour during exercise: from 0–60 min of exercise. Second hour during exercise: from 60–120 min of exercise. Third hour during exercise: from 120–180 min of exercise. Total during exercise: from 0–180 min of exercise. First hour during recovery: from 0–60 after exercise. Second hour during recovery: from 60–120 after exercise. 6 hr post-exercise: from 120–380 min after exercise. Total during recovery: from 0–380 min after exercise.
1Total gut discomfort: maximum reported severity score for any GIS (excluding nausea, dizziness, and stitch). 2Total-GIS: total severity scores for each GIS. 3Total Upper-GIS: total severity scores for all upper-GIS. 4Total Lower-GIS: total severity scores for all lower-GIS. 5Abnormal stools: refer to loose watery, diarrhoea and/ or bloody stools. 6Other GIS: total severity scores for nausea, dizziness, and stitch.
aap< 0.01 and ap< 0.05 vs. T1. bbp< 0.01 and bp< 0.05 vs. CFC.
Exercise Performance
FFC covered more distance in T2 than T1 (11.08 ± 2.08 vs. 11.51 ± 2.02 km, p = .013). Distance covered between groups was not significantly different in T1 (p = .963) and T2 (p = .683; Figure 2D). Differences in speed during the DT were observed at 20 and 30 min in FFC (p < .01) and at 50 min in CFC (p < .01; Figure 2C1 and 2C2).
Discussion
The current study aimed to investigate the effects of a 7-day repetitive feeding-challenge protocol using a high-fat versus high-CHO supplement on gastrointestinal function and tolerance, fuel availability and utilization, Ex-GIS, and performance in response to a high-CHO load (87 g/hr) during a gut-challenge test protocol. On the contrary our hypothesis, total and upper GIS severity during exercise was not further reduced in FFC compared with CFC. Interestingly, lower GIS severity during exercise significantly reduced from T1 to T2 in FFC only, but the magnitude of reduction was not different from CFC. Moreover, CHO dose (87 g/hr) and single CHO blend (e.g., maltodextrin) was well-tolerated by athletes before and after the intervention. Peak breath H2 level at T2 was lower in FFC compared with CFC. Postexercise blood glucose level was higher in FFC after the repetitive feeding-challenge, without significant group differences observed in glucose availability during exercise. The repetitive feeding-challenge for 7 days with fat did not differ compared with CHO in terms of feeding tolerance, OCTT, substrate oxidation, and exercise performance. In previous gut-training studies, use of a nonnutritive supplement in the placebo group demonstrated no effect on gastrointestinal function, Ex-GIS, glucose availability, fuel kinetics, and exercise performance. Therefore, a control group was not included in the current study, with focus being on repetitive feeding-challenge between two differing nutrient provisions. However, inclusion of a control group in the current study may have yielded insight into “gut-training” per se.
Feeding tolerance impacts fueling during exercise and recovery nutrition (Costa et al., 2013, 2014). The 7-day repetitive feeding-challenge with fat did not change subjective feeding tolerance markers, when compared with a repetitive feeding-challenge with CHO. Tolerance to food/drink progressively declined in both groups during exercise, likely due to upper abdominal bloating, and increased intragastric pressure from the high fluid intake (43% above usual), likely exceeding gastric emptying capacity. Nutrients passing along the intestinal tract activate ileal break mechanisms, delaying gastric emptying, and causing upper GIS, consequently suppressing food/drink tolerance (Miall et al., 2018). The authors speculate that the CHO dose was well-tolerated at baseline and inadequate to challenge the gut (relative dose: 1.4 g/kg BM in females; 1.2 g/kg BM in males), potentially limiting magnitude of improvement. Future research could explore higher doses (up to 120 g/hr) documented in endurance and ultraendurance athletes, but careful consideration of such high-CHO intake rates is needed based on an athlete’s training status, individual tolerance, and performance goals (Costa et al., 2016). If gut-training is a suitable strategy, personalizing CHO supplementation (i.e., dose and blend) and fluid volume according to individual needs and tolerance is best practice (Gaskell, Rauch, & Costa, 2021; Rauch et al., 2022).
Gastrointestinal motility influences exogenous fuel delivery and is a potent contributor to Ex-GIS. OCTT, measured via lactulose ingestion and breath H2 (and/or CH4), is a common but clinically limited method for quantifying gastrointestinal motility (Costa et al., 2022; Rezaie et al., 2017; Yao et al., 2017). In controlled experimental settings, OCTT shows repeatability (Gaskell et al., 2023; Gaskell, Rauch, Parr, Costa, 2021). This study found that 7 days of a repetitive feeding-challenge with fat had no effect on OCTT at rest (i.e., test–retest) and when compared against a repetitive feeding-challenge with CHO. Several individuals in the current study had no breath H2 (or CH4) turning point, likely due to delayed transit time or having non-H2/CH4 producing bacteria (Keller et al., 2018). A main limitation is that OCTT during exercise was not measured due to lactulose interference with primary outcomes. How OCTT at rest relates to OCTT during exercise, and its impact on small intestinal transit, remains unclear. Specific or sensitive methods for assessing gastrointestinal motility (e.g., fluoroscopic techniques, electrogastrography, and ingestible gas-sensing capsule), and measuring OCTT during exercise for athletes with Ex-GIS should be considered (Costa et al., 2022; Gaskell, Rauch, Parr, Costa, 2021).
CHO malabsorption—indicated by breath H2 rise in the postexercise recovery period—can cause GIS, especially if above clinical threshold (Miall et al., 2018). This study showed low incidence of clinically significant CHO malabsorption before and after the repetitive feeding-challenge intervention, suggesting good tolerance of the CHO dose/blend. The repetitive feeding-challenge reduced CHO malabsorption among responders, with peak breath H2 lower in FFC than CFC in T2. Excessive CHO intake during exercise saturates intestinal transporters (i.e., GLUT5 and/or SGLT1) resulting in malabsorption and ileal break activation (Costa et al., 2017; Miall et al., 2018). The findings indicate that in response to low fructose CHO gel ingestion, a repetitive feeding-challenge with CHO and fat improves CHO malabsorption, likely contributing to Ex-GIS severity reduction, but increased nutrient load does not enhance this effect. Previous studies (Costa et al., 2017; Miall et al., 2018) showed higher CHO malabsorption rates at baseline (60%), and greater reductions (45%–54%) after 2 weeks of gut-training with CHO compared with placebo, but this was not observed by King et al. (2022) in elite athletes. CHO blend of the supplement was a key difference between the current study and the previous gut-training studies (Costa et al., 2017; Miall et al., 2018), and the authors postulate that a longer duration protocol (i.e., 2 weeks) primarily impacts upregulation of GLUT5, reducing CHO malabsorption (Ferraris & Diamond, 1997). Although clinically significant CHO malabsorption was not evident on a cohort level in the current study, the repetitive feeding-challenge with fat and CHO benefitted individuals who had malabsorption. Collectively, these outcomes highlight individual assessment and intervention needed in professional practice and subsequent tailored sports supplement production (Martinez et al., 2024).
Skeletal muscles require a stable supply of glucose during exercise, relying on CHO and fat oxidative metabolism for energy in prolonged efforts (Hargreaves & Spriet, 2020). The current study showed a steady rise in blood glucose levels during exercise with no trial or group differences. Postexercise blood glucose concentration was 14% higher in FFC after the repetitive feeding-challenge, likely due to increased gluconeogenesis resulting from faster running and/or better absorption, but overall glucose availability during the exercise period was similar between groups. This contrasts with the results of Costa et al. (2017), where blood glucose increased during and after exercise following 2 weeks of gut-training with CHO, associated with improved intestinal absorption. Use of nonmetabolizable glucose analogues (e.g., D-xylose and 3-O-methul-D-glucose) in future studies can help elucidate effects on both passive and active intestinal absorption (Costa et al., 2022).
Glucose availability and subsequent skeletal muscle uptake influence performance (Stellingwerff & Cox, 2014). This study found that 7 days of a repetitive feeding-challenge with fat when compared with CHO had no impact on whole-body CHO oxidation. Whole-body fat oxidation during steady-state running was reduced in CFC (19%), likely due to CHO ingestion during exercise (Bosch et al., 1994). Increased exogenous CHO oxidation was previously observed in athletes after a high-CHO diet (8.5 g·kg−1·day−1) for 28 days, without performance improvements (Cox et al., 2010). Therefore, substantial dietary changes for longer periods are possibly required to alter fuel substrate contribution to energy production, as short-term gut-training, or feeding-challenge protocols alone per se do not impact whole-body oxidation rates (Martinez et al., 2023). The current findings are limited by the lack of substrate utilization measurement at the point of muscle glycogen depletion (>3 hr), which would provide further insight on changes in whole-body substrate oxidation and is recommended for future studies.
The performance-enhancing aspect of gut-training, at least in the short term, appears to be mainly linked with gastrointestinal functional and symptomatic improvements (Costa et al., 2017; Miall et al., 2018). This study showed that while FFC covered greater distance in T2 than T1, this was not significantly different from CFC. The reduction in total (31%) and upper GIS (33%) at the beginning of T2 DT versus T1 likely contributed to better performance in FFC. This translated in athletes feeling better and pacing faster during T2, consistent with previous findings (Costa et al., 2017). CFC covered more distance at T2, but did not reach statistical significance, despite significant reductions in severity of total (51%) and upper GIS (50%) during exercise after the intervention. The lack of difference in exercise performance between groups can potentially be linked to the fact that magnitude of symptom severity improvement was similar between groups. This shows the multifactorial nature of exercise performance and that Ex-GIS reduction can support performance, but is not the sole determinant. From a mechanistic perspective, the authors postulate that FFC enhanced gastric emptying (Clegg & Shafat, 2011; Cunningham et al., 1991), along with hypersensitivity, and perception of improvement in gut comfort, thus aiding performance (Farmer & Aziz, 2013). Therefore, managing Ex-GIS by challenging the gut with greater volume/dose/nutrient density leading into a race should be standard practice for symptomatic athletes.
During exercise, compromised gastrointestinal status can lead to Ex-GIS, worsened by feeding and drinking, especially in longer events (Costa et al., 2016). This study found substantial and similar reductions in total and upper GIS severity during exercise in both groups. Severity of lower GIS during exercise were reduced in both groups after the repetitive feeding-challenge but only reached statistical significance in FFC, with no group difference in magnitude of improvement. Common symptoms reported can be linked to increased intragastric pressure from high fluid and/or CHO intake exceeding gastric emptying capacity. Potential gastric emptying and/or gastric accommodation enhancement from training with a high gastric load improved upper GIS in FFC; however, this was not confirmed, given the study design limitation on OCTT measurement. In CFC, improved CHO absorption may have dampened the ileal break, reducing upper GIS (Miall et al., 2018). Similar total and upper GIS improvement in both groups can be linked to overall desensitization to gastric distention and decrease in perceived gut discomfort resulting from the repetitive feeding-challenge, independent of macronutrient composition and density (Costa et al., 2017; Lambert et al., 2008; Miall et al., 2018; Noakes et al., 1991). Lower GIS improvement in FFC can be due to a shorter mouth-to-cecum transit time (Cunningham et al., 1991), with a compensatory effect in intestinal absorption from increased nutrient delivery. Previous studies (Costa et al., 2017; Miall et al., 2018) showed greater improvements in Ex-GIS severity, likely due intervention duration and/or the CHO blend used. Postexercise, gut discomfort (60%), and total (56%), and upper GIS (63%) severity were higher in FFC than CFC in T2, indicating a potential delayed impact of exercise stress and feeding to the gut. Despite this, feeding tolerance variables (i.e., interest/tolerance to food and appetite) remained high during recovery in FFC. Taken together, a repetitive feeding-challenge with fat similarly reduces Ex-GIS during exercise when compared with CHO, with the magnitude of GIS improvement potentially influenced by the specifics of the protocol (i.e., frequency, duration, and CHO dose/blend). The current study also highlights the flexibility in the exercise modality within a repetitive feeding-challenge protocol, with a combination of running and cycling sessions of similar duration and intensity, resulting in positive symptomatic outcomes during a running gut-challenge. The authors acknowledge that the exercise performance results in the current study are limited by the lack of a familiarization session and should be considered in future studies, especially if the recruited study cohort includes nonendurance trained athletes. Overall, findings of the current study highlight the importance of a comprehensive assessment of GIS and feeding tolerance to inform effective gut-training or feeding-challenge protocol design.
Conclusions
A repetitive feeding-challenge for 7 days using a high-fat supplement reduced Ex-GIS severity similar to a repetitive feeding-challenge with CHO, with no difference between fat and CHO interventions observed. Improvement in running performance was observed after a repetitive feeding-challenge with fat but did not significantly differ from the repetitive feeding-challenge with CHO. The 7-day repetitive feeding-challenge with fat did not differ compared with CHO in terms of OCTT at rest, as well as feeding tolerance, and substrate oxidation during endurance running. Overall, repetitive feeding-challenge with fat does not enhance nor worsen gastrointestinal and fueling outcomes compared with CHO repetitive feeding-challenge.
Acknowledgments
The authors would like to sincerely thank the individuals who volunteered to participate in this study, as well as the members of the Monash University Sports Dietetics and Extremes Physiology group for assisting in the study. Special thanks are also due to Michael Houghton and Gary Williamson, our collaborators in developing the CHO gel, as well as Raeana Connell and Andrius Ramonas of Roam for the nut butter products used in the study. This study was supported by Monash University, Department of Nutrition, Dietetics, and Food. Biesiekierski is supported by a National Health and Medical Research Council Emerging Leadership Fellowship (APP2025943). Author Contributions: Conceptualization: Martinez, Costa, Biesiekierski. Methodology: Martinez, Costa, Biesiekierski. Project administration: Martinez, Biesiekierski, Costa. Resources: Costa. Investigation: Martinez, Biesiekierski, Rauch, Costa. Data curation and formal analysis: Martinez, Costa, Biesiekierski. Supervision and validation: Costa, Biesiekierski. Writing original draft: Martinez, Costa, Biesiekierski. Writing, review, and editing: Martinez, Costa, Biesiekierski, Rauch.
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Nontechnical Summary
Exercise-associated gastrointestinal symptoms (Ex-GIS) are commonly experienced by athletes, particularly those engaging in endurance and ultraendurance events (>2 hr). In these events, nutrition during exercise is crucial for exercise performance. However, consuming more than what an individual can tolerate may further worsen Ex-GIS and cause an athlete to slow down or even drop out of a race.
Repetitively challenging the gut on consecutive days with large amounts of carbohydrates (CHO) or large fluid volumes before or during exercise has been shown to improve how well an athlete tolerates feeding during exercise, reduces gut discomfort, and lessen the severity of Ex-GIS. Incorporating nutrients (e.g., fat) that are moved and digested slower along the gastrointestinal tract, compared with CHOs, may further challenge the gut and potentially help it adapt better.
The study aimed to investigate the effects of a 7-day repetitive feeding-challenge using a high-fat versus a high-CHO product on markers of gastrointestinal function, glucose availability, substrate oxidation, and subsequent performance when consuming a high-CHO intake (87 g/hr) during exercise. Endurance athletes completed a preintervention gut-challenge trial, which involved a 2-hr steady-state run while consuming a low fructose CHO gel every 20 min, followed by 1 hr self-paced distance test run with as much water as desired. Participants were then randomly assigned to either a fat (FFC; 20 g nut butter: 124 kcal, 11 g fat, 3 g protein, 3 g CHO) or CHO feeding-challenge group (CFC; 47 g CHO gel: 123 kcal, 29 g CHO). This involved daily 1 hr easy exercise (running or cycling) and supplement intake at 0, 20, and 40 min with water for seven consecutive days. After this period, the participants completed a follow-up postintervention gut-challenge trial.
The results showed that the 7-days FFC did not differ compared with CFC in terms of the time it took for food to move through the gut at rest, how well athletes could tolerate food/fluid during exercise, how much CHO can be absorbed in the gut, the level of glucose in the blood, and the use of fat or CHO as fuel during exercise during the gut-challenge trial. The repetitive feeding-challenge with fat reduced severity of total and upper Ex-GIS similar to a repetitive feeding-challenge with CHO. Exercise performance improved after FFC, but this was not significantly different from CFC. Overall, a repetitive feeding-challenge with fat does not enhance nor worsen gastrointestinal function, substrate oxidation, endurance exercise performance, and Ex-GIS when compared with a CHO repetitive feeding-challenge.
