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Iron deficiency is a common health issue in active and athlete populations. Accordingly, research into iron status, regulation, absorption, and iron deficiency treatment strategies is increasing at a rapid rate. However, despite the increase in the quantity of research, various methodological issues need to be addressed as we progress our knowledge in this area. The purpose of this review is to highlight specific considerations for conducting iron-related research in active and athlete populations. First, we discuss the methodological importance of assessment and interpretation of iron status, with reference to blood collection protocols, participant screening procedures, and biomarker selection. Next, we consider numerous variables that should be accounted for in the design of iron-related research studies, such as the iron regulatory hormone hepcidin and its interaction with exercise, in addition to an examination of female physiology and its impact on iron metabolism. Subsequently, we explore dietary iron and nutrient interactions that impact iron regulation and absorption, with recommendations made for optimal methodological control. Consideration is then given to key features of long-term study designs, such as the monitoring of training load, oral iron supplementation, dietary analysis, and general lifestyle factors. Finally, we conclude our recommendations with an exploration of stable iron isotope tracers as a methodology to measure iron absorption. Ultimately, it is our intention that this review can be used as a guide to improve study design, biomarker analysis, and reporting of findings, to maximize the quality of future research outputs in iron-related research focused on active and athlete populations.

This study aimed to investigate the effects of different exercise training programs on fasting plasma levels of oxylipins, endocannabinoids (eCBs), and eCBs-like molecules in middle-aged sedentary adults. A 12-week randomized controlled trial was conducted using a parallel group design. Sixty-five middle-aged adults (40–65 years old) were randomly assigned to: (a) no exercise (control group), (b) concurrent training based on international physical activity recommendations (PAR group), (c) high-intensity interval training (HIIT group), and (d) HIIT together with whole-body electromyostimulation (HIIT + EMS group). Plasma levels of oxylipins, eCBs, and eCBs-like molecules were determined in plasma samples before and after the intervention using targeted lipidomics. Body composition was assessed through dual-energy X-ray absorptiometry, and dietary intake through a food frequency questionnaire and three nonconsecutive 24-hr recalls. The physical activity recommendations, HIIT, and HIIT-EMS groups showed decreased plasma levels of omega-6 and omega-3-derived oxylipins, and eCBs and eCBs-like molecules after 12 weeks (all Δ ≤ −0.12; all p < .05). Importantly, after Bonferroni post hoc corrections, the differences in plasma levels of omega-6 and omega-3 oxylipins were not statistically significant compared with the control group (all p > .05). However, after post hoc corrections, plasma levels of anandamide and oleoylethanolamide were increased in the physical activity recommendations group compared with the control group (anandamide: Δ = 0.05 vs. −0.09; oleoylethanolamide: Δ = −0.12 vs. 0.013, all p ≤ .049). In conclusion, this study reports that a 12-week exercise training intervention, independent of the modality applied, does not modify fasting plasma levels of omega-6 and omega-3 oxylipins, eCBs, and eCBs-like molecules in middle-aged sedentary adults.

This study sought to (a) estimate how the duration of running influences sweat losses and contributes to the daily fluid requirement, and (b) empirically estimate the drinking rates required to prevent significant dehydration (≥2% body weight as body water). Individual sweating data and running duration were obtained from male (n = 83) and female (n = 36) runners (n = 146 total observations) performing under highly heterogeneous conditions and over a range of exercise durations (33–280 min). Running <60 min/day increased daily fluid needs by a factor of 1.3, whereas running >60 min/day increased the daily fluid need by a factor of 1.9–2.3. Running <60 min/day generally required no fluid intake to prevent significant dehydration before run completion (31/35 runners). In contrast, running >60 min/day required more than 50% replacement of sweating rates to prevent the same (65/111 runners). Overall sweat losses ranged from ∼0.2 to ∼5.0 L/day, whereas the drinking rates required to prevent significant dehydration ranged from 0 to 1.4 L/hr. The characterization of sweat losses, sweat rate, and required drinking among runners in this study indicate wide individual variability that warrants personalized hydration practices, particularly when running is prolonged (>60 min) and performance is important. This study may serve as a useful guidepost for sports dietitians when planning and communicating fluid needs to athletes, as well as complement guidance related to both personalized programmed and thirst-driven drinking strategies.

A sufficient omega-3 index may enhance cardiovascular function, enhance performance, and decrease inflammation. However, most collegiate athletes are deficient in omega-3s, requiring supplementation. A new type of omega-3 (N3) supplement, Enhanced Recovery™ (ER), claims to improve N3 index while addressing the current issues with traditional supplementation. The purpose of this study was to determine if ER improves N3 status and enhances compliance compared with the current standard in collegiate Division I Track and Field and cross-country athletes during a competitive season. Twenty-five (male = 15 and female = 10) athletes completed this longitudinal, randomized controlled crossover trial. Measurements of N3 status were collected at baseline prior to supplementation, and every 2 weeks for 6 weeks with a 33- to 36-day washout period before crossing over. Supplement compliance and dietary intake of N3 rich foods were collected throughout. Visual analog scales and an exit survey asked questions regarding each treatment. Results showed that N3 index increases within 6 weeks (p < .001) for ER (+37.5%) and control (CON; +55.1%), with small differences between treatments at Weeks 4 (ER = 7.3 ± 1.0; CON = 7.7 ± 1.1; p = .043) and 6 (ER = 7.4 ± 1.2; CON = 7.9 ± 1.2; p = .043). Dietary intake of N3-rich foods and supplement compliance were significant drivers of improvements in N3 status (p < .050). Compliance was not different between treatments but was affected by sex (males = 90.0 ± 17.0% and females = 76.5 ± 21.0%; p = .040), likability (p = .001; r = .77, p < .001), ease (p = .023; r = .53, p = .006), and supplement preference (p = .004), which appeared to favor ER. We conclude that consumption of N3-rich foods and consistent supplementation should be implemented for improvements in N3 status in collegiate athletes, but taste tests/trial periods with ER or CON may help determine preference and improve compliance.

Athletes from weight-sensitive sports are reported to consume low-fiber diets (LOW) to induce acute reductions in body mass (BM). However, evidence supporting their efficacy is anecdotal. Therefore, we aimed to determine the effect of a LOW on acute changes in BM. Nineteen healthy males (32 ± 10 years, 1.79 ± 0.07 m, 77.5 ± 8.1 kg) consumed their habitual diet (∼30 g fiber/day) for 7 consecutive days followed by 4 days of a LOW (<10 g fiber/day) that was matched for energy and macronutrient content. Participants also matched their daily exercise load during LOW to that completed during habitual diet (p = .669, average 257 ± 141 arbitrary units). BM was significantly reduced in LOW versus habitual diet after 4 days (Δ = 0.40 ± 0.77 kg or 0.49% ± 0.91%, p < .05, effect size [ES] [95% confidence interval] = −0.53 [−1.17, 0.12]) and on the morning of Day 5 (Δ = 0.58 ± 0.83 kg or 0.74% ± 0.99%, p < .01, ES = −0.69 [−1.34, −0.03]). LOW resulted in moderately higher hunger (Δ = 5 ± 9 mm, p = .015, ES = 0.55 [−0.09, 1.20]), a decline in stool frequency from 2 ± 0 to 1 ± 0 bowel movements per day (p = .012, ES = 0.64 [−0.02, 1.29]) and stool softness decrease (p = .005). Nonetheless, participants reported the diet to be tolerable (n = 18/19) and were willing to repeat it (n = 16/19). Data demonstrate for the first time that consumption of a short-term LOW induces reductions in BM.

Dietary intake and physical activity impact performance and adaptation during training. The aims of this study were to compare energy and macronutrient intake during British Army Officer Cadet training with dietary guidelines and describe daily distribution of energy and macronutrient intake and estimated energy expenditure. Thirteen participants (seven women) were monitored during three discrete periods of military training for 9 days on-camp, 5 days of field exercise, and 9 days of a mixture of the two. Dietary intake was measured using researcher-led food weighing and food diaries, and energy expenditure was estimated from wrist-worn accelerometers. Energy intake was below guidelines for men (4,600 kcal/day) and women (3,500 kcal/day) during on-camp training (men = −16% and women = −9%), field exercise (men = −33% and women = −42%), and combined camp and field training (men and women both −34%). Carbohydrate intake of men and women were below guidelines (6 g·kg⁻¹·day⁻¹) during field exercise (men = −18% and women = −37%) and combined camp and field training (men = −33% and women = −39%), respectively. Protein intake was above guidelines (1.2 kcal·kg⁻¹·day⁻¹) for men and women during on-camp training (men = 48% and women = 39%) and was below guidelines during field exercise for women only (−27%). Energy and macronutrient intake during on-camp training centered around mealtimes with a discernible sleep/wake cycle for energy expenditure. During field exercise, energy and macronutrient intake were individually variable, and energy expenditure was high throughout the day and night. These findings could be used to inform evidenced-based interventions to change the amount and timing of energy and macronutrient intake around physical activity to optimize performance and adaptations during military training.

This study investigated the effects of 7 days of 600 mg/day anthocyanin-rich blackcurrant extract intake on small intestinal permeability, enterocyte damage, microbial translocation, and inflammation following exertional heat stress. Twelve recreationally active men (maximal aerobic capacity = 55.6 ± 6.0 ml·kg⁻¹·min⁻¹) ran (70% VO₂max) for 60 min in an environmental chamber (34 °C, 40% relative humidity) on two occasions (placebo/blackcurrant, randomized double-blind crossover). Permeability was assessed from a 4-hr urinary excretion of lactulose and rhamnose and expressed as a ratio of lactulose/rhamnose. Venous blood samples were taken at rest and 20, 60, and 240 min after exercise to measure enterocyte damage (intestinal fatty acid-binding protein); microbial translocation (soluble CD14, lipopolysaccharide-binding protein); and interleukins 6, interleukins 10, and interleukins 1 receptor antagonist. Exercise increased rectal temperature (by ∼2.8 °C) and heart rate (by ∼123 beats/min) in each condition. Blackcurrant supplementation led to a ∼12% reduction in lactulose/rhamnose ratio (p < .0034) and enterocyte damage (∼40% reduction in intestinal fatty acid-binding protein area under the curve; p < .0001) relative to placebo. No between-condition differences were observed immediately after exercise for lipopolysaccharide-binding protein (mean, 95% confidence interval [CI]; +80%, 95% CI [+61%, +99%]); soluble CD14 (+37%, 95% CI [+22%, +51%]); interleukins 6 (+494%, 95% CI [+394%, +690%]); interleukins 10 (+288%, 95% CI [+105%, +470%]); or interleukins 1 receptor antagonist (+47%, 95% CI [+13%, +80%]; all time main effects). No between-condition differences for these markers were observed after 60 or 240 min of recovery. Blackcurrant extract preserves the GI barrier; however, at subclinical levels, this had no effect on microbial translocation and downstream inflammatory processes.

The term “food first” has been widely accepted as the preferred strategy within sport nutrition, although there is no agreed definition of this and often limited consideration of the implications. We propose that food first should mean “where practically possible, nutrient provision should come from whole foods and drinks rather than from isolated food components or dietary supplements.” There are many reasons to commend a food first strategy, including the risk of supplement contamination resulting in anti-doping violations. However, a few supplements can enhance health and/or performance, and therefore a food only approach could be inappropriate. We propose six reasons why a food only approach may not always be optimal for athletes: (a) some nutrients are difficult to obtain in sufficient quantities in the diet, or may require excessive energy intake and/or consumption of other nutrients; (b) some nutrients are abundant only in foods athletes do not eat/like; (c) the nutrient content of some foods with established ergogenic benefits is highly variable; (d) concentrated doses of some nutrients are required to correct deficiencies and/or promote immune tolerance; (e) some foods may be difficult to consume immediately before, during or immediately after exercise; and (f) tested supplements could help where there are concerns about food hygiene or contamination. In these situations, it is acceptable for the athlete to consider sports supplements providing that a comprehensive risk minimization strategy is implemented. As a consequence, it is important to stress that the correct terminology should be “food first but not always food only.”

Physical fitness (PF) is a cornerstone of metabolic health. However, its role in maternal–fetal metabolism during pregnancy is poorly understood. The present work investigates: (i) the association of PF with maternal and fetal cardiometabolic markers, and with clustered cardiometabolic risk during pregnancy, and (ii) whether being fit counteracts cardiometabolic abnormalities associated with overweight/obesity. Several PF components (flexibility, lower and upper body strength, and cardiorespiratory fitness [CRF]) were objectively assessed in 151 pregnant women at gestational weeks 16 and 33, and an overall PF cluster score calculated. At the same times, maternal glycemic and lipid markers, cortisol, and C-reactive protein were assessed with standard biochemical methods, along with blood pressure and a proxy for insulin resistance, and a cardiometabolic risk cluster score determined. These analytes were also measured in maternal and umbilical cord arterial and venous blood collected at delivery. PF was found to be associated with several maternal and a small number of fetal cardiometabolic markers (p < .05). Lower and upper body muscle strength, CRF, overall PF (week 16), and CRF changes (weeks 16–33) were inversely associated with clustered cardiometabolic risk (p < .05). Normal weight fit women had lower values for insulin level, insulin resistance, triglycerides, low-density lipoprotein cholesterol, C-reactive protein, and diastolic blood pressure than did overweight/obese unfit women at week 16 (p < .05). In conclusion, greater PF, especially muscle strength and CRF in early–middle pregnancy, appears to be associated with a better metabolic phenotype, and may protect against maternal cardiometabolic risk. “Keep yourself fit and normal weight before and during early pregnancy” should be a key public health message.

This systematic review analyzed whether carbohydrate source (food vs. supplement) influenced performance and gastrointestinal (GI) symptoms during endurance exercise. Medline, SPORTDiscus, and citations were searched from inception to July 2021. Inclusion criteria were healthy, active males and females aged >18 years, investigating endurance performance, and GI symptoms after ingestion of carbohydrate from a food or supplement, <60 min before or during endurance exercise. The van Rosendale scale was used to determine risk of bias, with seven studies having low risk of bias. A total of 151 participants from 15 studies were included in the review. Three studies provided 0.6–1 g carbohydrate/kg body mass during 5–45 min precycling exercise (duration 60–70 min) while 12 studies provided 24–80 g/hr carbohydrate during exercise (60–330 min). Except one study that suggested a likely harmful effect (magnitude-based inferences) of a bar compared to a gel consumed during exercise on cycling performance, there were no differences in running (n = 1) or cycling (n = 13) performance/capacity between food and supplemental sources. Greater GI symptoms were reported with food compared with supplemental sources. Highly heterogenous study designs for carbohydrate dose and timing, as well as exercise protocol and duration, make it difficult to compare findings between studies. A further limitation results from only one study assessing running performance. Food choices of carbohydrate consumed immediately before and during endurance exercise result in similar exercise performance/capacity responses to supplemental carbohydrate sources, but may slightly increase GI symptoms in some athletes, particularly with exercise >2 hr.