The Endurance Performance Nerd Alert.
Learn to train smart, run fast, and be strong with Thomas Solomon, PhD 
September 2025
Use this Nerd Alert of the latest exercise science and sports nutrition research to improve your running performance or coaching practice.
The research studies are divided into subtopics —
training methods,
sports nutrition,
supplements,
athlete health,
injuries and rehab, and
female athlete physiology
— but I’ve also provided a deeper dive into 1 study:
And, there’s my beer of the month to wash it all down.
All the interesting papers I found this month are immediately below.
Dig in and evaluate the authors’ findings by clicking on the titles to access the full papers.
Evaluate each paper thoughtfully—be sceptical, not cynical. To guide you, consider using the framework I applied when doing my deep dives. This approach will help you assess the quality of a study while also appreciating the complexity and nuance of scientific research.
Share this nerd alert with your people:
General training methods:
Strength Training Improves Running Economy Durability and Fatigued High-Intensity Performance in Well-Trained Male Runners: A Randomized Control Trial. Zanini et al. (2025) Med Sci Sports Exerc. An empirical model for world record running speeds with distance, age, and sex: anaerobic and aerobic contributions to performance. Roy et al. (2025) J Appl Physiol. Inspiratory muscle warm up improves 400 m performance in elite male runners. Yilmaz et al. (2025) Sci Rep. Sport-specific variability in the energy cost of constant speed running: Implications for metabolic power estimations. Venzke et al. (2025) PLoS One. 
Sports nutrition and hydration:
General and sport-specific nutrition knowledge and behaviors of adolescent athletes. Gibbs et al. (2025) J Int Soc Sports Nutr. Ketone ester ingestion increases exogenous carbohydrate storage and lowers glycemia during post-exercise recovery: a randomised crossover trial. Clarke et al. (2025) Eur J Nutr. Strategic carbohydrate feeding improves performance in ketogenic trained athletes. Carpenter et al. (2025) Clin Nutr. Sports supplements.
Ergogenic effects of supplement combinations on endurance performance: a systematic review and meta-analysis of randomized controlled trials. Zart et al. (2025) J Int Soc Sports Nutr. Athlete health:
Bone mineral density varies throughout the skeleton of athletes dependent on their sport: Allometric modelling identifies the "effective" forces associated with body mass. Nevill et al. (2025) J Sci Med Sport. The role of mental toughness, sport imagery and anxiety in athletic performance: structural equation modelling analysis. Demir et al. (2025) BMC Psychol. Nutritional strategies for minimizing gastrointestinal symptoms during endurance exercise: systematic review of the literature. Mlinaric et al. (2025) J Int Soc Sports Nutr. Injury and rehab:
Practitioner Perspectives on the Association Between Mental Fatigue and Injury Risk in High-Performance Sport: A Mixed Methods Study. Fazackerley et al. (2025) Eur J Sport Sci. Female athlete physiology and sex differences:
Sex differences in resting skeletal muscle and the acute and long-term response to endurance exercise in individuals with overweight and obesity. Dreher et al. (2025) Mol Metab.
My deep dives:
Nutritional strategies for minimizing gastrointestinal symptoms during endurance exercise: systematic review of the literature.
Mlinaric et al. (2025) J Int Soc Sports Nutr.
What type of study is this?
This study is a systematic reviewA systematic review answers a specific research question by systematically collating all known experimental evidence, which is collected according to pre-specified eligibility criteria. A systematic review helps inform decisions, guidelines, and policy..
What was the authors’ hypothesis or research question?
The authors aimed to examine the nutritional causes of gastrointestinal (GI) symptoms in endurance athletes and identify dietary strategies that may reduce these symptoms and improve performance.
What did the authors do to test the hypothesis or answer the research question?
This review included a total of 29 studies, encompassing randomised controlled trialsThe “gold standard” approach for determining whether a treatment has a causal effect on an outcome of interest. In such a study, a sample of people representing the population of interest is randomised to receive the treatment or a no-treatment placebo (control), and the outcome of interest is measured before and after the exposure to treatment/control., crossover trialsCrossover means that all subjects completed all interventions (control and treatment) usually with a wash-out period in between., and case reportsA report of an interesting and unexpected outcome (a side effect, usually an adverse outcome) in a person who has received a specific treatment. A case study shows what is possible in one person, not what is probable in all people.. The authors grouped the research into five categories: gut training, carbohydrate solutions, low-FODMAP diets, hydrogel carbohydrate products, and probiotic supplementation. Outcomes focused on the incidence and severity of GI symptoms during endurance exercise. Risk of biasRisk of bias in meta-analysis refers to the potential for systematic errors in the studies included in the analysis, which can lead to misleading or invalid results. Assessing this risk is crucial to ensure the conclusions drawn from the combined data are reliable. was assessed using the Cochrane risk of bias (RoB2) toolThe Cochrane Risk of Bias 2 (RoB 2) tool is a standardized instrument developed by Cochrane for assessing the risk of bias in randomized controlled trials (RCTs). It is widely used to evaluate the internal validity of results from studies in a systematic review by examining bias arising from the randomization process, deviations from intended interventions, missing outcome data, the measurement of the outcome, and the selection of the reported result., and most studies were judged to have a low risk of bias.
What did the authors find?
Gut training protocols showed promise, with some studies reporting significant reductions in GI symptoms after two weeks of high carb ingestion during exercise, particularly using 2:1 glucose-to-fructose blends. Carb intake tailored to exercise intensity and type generally resulted in fewer GI symptoms—though not universally. Studies comparing different carb types (e.g., glucose-only vs. glucose-fructose mixes) showed modest or unclear differences. Low-FODMAP diets helped reduce bloating, flatulence, and discomfort in sensitive individuals, but the data came from very small samples, mostly case studies. Hydrogel-based carbohydrate solutions, despite being a trendy innovation, did not offer consistent GI benefits over regular carb products. Finally, evidence on probiotic use was mixed: some studies reported reduced GI symptoms while others showed no effect. The authors emphasized that GI distress is highly individual, influenced by training status, nutrition habits, exercise intensity, and gut microbiota—and that one-size-fits-all strategies are unlikely to work. The authors concluded that managing GI symptoms in endurance athletes requires a personalized approach, with several nutritional strategies showing potential, but none are universally effective.
What were the strengths?
This review adhered to PRISMA guidelines and conducted a clear risk-of-bias assessment using a validated tool. The study grouped research meaningfully, which helped organize a messy and heterogeneous topic. The authors made a solid attempt to synthesize findings despite differences in study designs. Including crossover and case studies allowed them to reflect real-world practice as well as controlled settings. They searched a reputable database (PubMed), and their inclusion/exclusion criteria were clearly defined.
What were the limitations?
There was no mention of protocol pre-registration, which weakens transparency and reproducibility. Only one database was searched, so relevant studies may have been missed. Several included studies had small sample sizes (some less than 10 participants), particularly in the low-FODMAP and probiotic sections, increasing the risk of false negatives (type II errors)Failing to detect an effect or difference when there actually is one. I.e, “a missed detection.” or overinterpretation. And, because a statistical synthesis (like a meta-analysis with effect sizeAn effect size is a quantitative measure of the magnitude of a relationship or difference between groups in a study. Unlike p-values, effect sizes show how large or meaningful the effect is. Common effect size measures include standardised mean difference (SMD), Cohen’s d, Hedges’ g, eta-squared, and correlation coefficients. and heterogeneity/variabilityHeterogeneity shows how much study results in a meta-analysis truly differ from each other. It is measured using I2, which is the percent of variation beyond chance: roughly 25% low heterogeneity (good), 50% moderate, 75% high heterogeneity (bad). High heterogeneity means more variability in effects between studies and, therefore, a less precise overall effect estimate. calculations) was not attempted, we don’t get a clear sense of overall effect sizes or certainty of evidenceCertainty of evidence tells us how confident we are that the results reflect the true effect. It’s based on factors like study design, risk of bias, consistency, directness, and precision. High certainty means strong, consistent research. Low certainty means more doubt and that new studies could easily change the conclusions. (GRADEGRADE, which stands for Grading of Recommendations Assessment, Development and Evaluation, is a system used to assess the quality of evidence and the strength of recommendations in healthcare, including in meta-analyses. It provides a structured approach to evaluate how confident researchers can be in the results of studies and the recommendations that follow from them.). So, while the authors’ conclusions were cautious, some interpretations leaned on shaky ground given the limited and varied data.
How was the study funded, and are there any conflicts of interest that may influence the findings?
The study was supported by the Slovenian Research and Innovation Agency (ARIS) Research Programme P5–0443 (Javna agencija za znanstvenoraziskovalno in inovacijsko dejavnost Republike Slovenije (ARIS) [P5–0443]). No potential conflict of interest was reported by the authors.
How can you apply these findings to your training or coaching practice?
For endurance athletes and coaches, this review is a good starting point but not the final word. It confirms what many already suspect: the gut matters, and what you eat before and during training can make or break your performance. It reinforces the value of gut training with higher CHO intakes and the potential of low-FODMAP strategies in sensitive individuals. However, it also reveals how uncertain and person-specific the science is — what helps one athlete may wreck another. It’s basically a call to experiment methodically and track symptoms closely. Coaches, in particular, might find the gut-training protocols useful for structuring pre-competition blocks.
What is my Rating of Perceived scientific Enjoyment?
RP(s)E = 6 out of 10. My Rating of Perceived Scientific Enjoyment was moderate because although the paper followed a systematic structure, grouped its findings sensibly, and did a decent risk-of-bias check, it loses marks for not searching more than one database (PubMed), not pre-registering its protocol, skipping any formal evaluation of evidence strength (e.g., GRADE). The writing is solid, the scope is practical—but the methods don't quite rise to the level needed for top-tier confidence.
Important: Don’t make any major changes to your daily habits based on the findings of one study, especially if the study is small (e.g., less than 30 participants in a randomised controlled trial or less than 5 studies in a meta-analysis) or poor quality (e.g., high risk of bias or low GRADEGRADE, which stands for Grading of Recommendations Assessment, Development and Evaluation, is a system used to assess the quality of evidence and the strength of recommendations in healthcare, including in meta-analyses. It provides a structured approach to evaluate how confident researchers can be in the results of studies and the recommendations that follow from them. certainty of evidence in a meta-analysis). What do other studies in this field show? (Follow the link to explore those trials.) Do they confirm the findings of this study or have mixed outcomes? Is there a high-quality meta-analysis evaluating the entirety of the evidence in this field? If so, what does the analysis show? What is the risk of biasRisk of bias in meta-analysis refers to the potential for systematic errors in the studies included in the analysis, which can lead to misleading or invalid results. Assessing this risk is crucial to ensure the conclusions drawn from the combined data are reliable. or quality of evidence of the included studies?
To help you wash down the latest evidence, here's a snifter from my recent indulgence:
My beer of the month.
Double Dragon.
Brewed by Basqueland Brewing (Spain).
DIPA.
8% ABV.
A double IPA that's nowhere near as good as a night in with Billy and Jimmy Lee. Double Dragon; nostalgic but needs some work to become a black belt among beers.
RP(be)E(r)(Rating of Perceived beer Enjoyment)
7 out of 10
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Disclaimer I occasionally mention brands and products, but it is important to know that I don't sell recovery products, supplements, or ad space, and I'm not affiliated with / sponsored by / an ambassador for / receiving advertisement royalties from any brands. I have conducted biomedical research for which I’ve received research money from publicly-funded national research councils and medical charities, and also from private companies, including Novo Nordisk Foundation, AstraZeneca, Amylin, the A.P. Møller Foundation, and the Augustinus Foundation. I’ve also consulted for Boost Treadmills and Gu Energy on R&D grant applications, and I provide research and scientific writing services for Examine.com. Some of my articles contain links to information provided by Examine.com but I do not receive any royalties or bonuses from those links. Importantly, none of the companies described above have had any control over the research design, data analysis, or publication outcomes of my work. I research and write my content using state-of-the-art, consensus, peer reviewed, and published scientific evidence combined with my empirical evidence observed in practice and feedback from athletes. My advice is, and always will be, based on my own views and opinions shaped by the scientific evidence available. The information I provide is not medical advice. Before making any changes to your habits of daily living based on any information I provide, always ensure it is safe for you to do so and consult your doctor if you are unsure.