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The Endurance Performance Nerd Alert.
Learn to train smart, run fast, and be strong with Thomas Solomon PhD 
March 2025
This page contains the latest research studies on exercise science and sports nutrition.
Use them to help level up your running performance or coaching practice.
The studies are divided into sub-topics:
training methods,
sports nutrition and hydration,
athlete health (inc. mental health), and
female athlete physiology,
plus my beer of the month to wash it all down.
I’ve also provided a deeper insight into my four favourite studies of the month. For everything else, you can click on the study title to plunge deep into the full paper and evaluate the authors’ findings. In doing so, aim to be skeptical, not cynical.
My favourite papers this month.
Evidence on sex differences in sports performance. Joyner et al. (2025 ) J Appl Physiol. (click the title to access the full article)
What type of study is this?
- This study is a
narrative review.A narrative review describes an entire body of evidence to summarise what is known on a topic. However, instead of using a systematic approach, a narrative review usually takes a subjective approach that allows the author(s) to express their opinion on the topic.
What is the authors’ hypothesis or research question?
- This review aimed to make evidence-informed statements about sex differences in sports performance. The authors use biological terms and sex chromosomes to describe males (XY) and females (XX) and generally refrain from using culturally specific terms to describe constructs of gender identity and gender expression.
What did the authors do to test the hypothesis or answer the research question?
- The article is a review of existing evidence. The authors synthesised data from observational studies, performance records, case studies, and systematic reviews to examine sex differences across various athletic disciplines.
- As a review article, it does not involve a new experimental study with a defined number of participants or subjects; rather, it compiles findings from numerous studies that include data on both males and females across a range of ages and sports.
What did the authors find?
- The authors reported that males consistently outperform females in sports events that depend on strength, speed, power, endurance, and body size, with performance gaps ranging from approximately 10% to 40% among elite adult athletes.
- They noted that even before puberty, a modest performance gap exists—around 3%–5% in running events and 5%–10% in jumping events—and that this gap widens substantially during and after puberty.
- A key finding is that the surge in endogenous testosterone during puberty is the principal driver of these differences, as it leads to increases in muscle mass, strength, and other physiological attributes that confer an athletic advantage.
- In the context of testosterone suppression (e.g., in transgender women athletes), the review highlights that while performance may decline modestly (by about 5%), much of the male-typical performance advantage is retained, suggesting that prior physiological adaptations are not fully reversible.
- The authors concluded that the inherent biological effects of testosterone and puberty underpin the observed sex differences in sports performance, and these findings should be considered in policies governing competitive sports.
What are the strengths of the review?
- The review is comprehensive, drawing on a wide range of studies and performance data across different age groups and sports disciplines.
- It clearly integrates biological, physiological, and performance-based evidence to provide a coherent framework for understanding the impact of sex-steroid hormones on athletic performance.
- The article offers a balanced discussion that is useful for informing policy decisions regarding eligibility criteria in competitive sports.
What are the weaknesses of the review?
- As a review article, it does not present new experimental data, and its conclusions depend on the quality and consistency of the underlying studies.
- The synthesis does not involve a formal meta-analysis with quantitative effect sizes, confidence intervals, or heterogeneity statistics, which may limit the precision of its conclusions.
- Some aspects, such as the long-term effects of hormonal interventions, remain controversial and warrant further experimental research.
How was the study funded, and are there any conflicts of interest that may influence the findings?
- The article does not provide specific details on funding sources. No conflicts of interest, financial or otherwise, were declared by the authors.
Are the findings useful in application to training and/or coaching practice?
- Yes → The findings are useful for endurance training and coaching as they clarify that while training strategies and performance-enhancing interventions (such as exogenous testosterone) can modify athletic performance, the underlying biological differences driven by testosterone and pubertal development persist. This information can help coaches better understand the physiological basis of performance differences and tailor training and competition policies to promote fairness.
What is my Rating of Perceived scientific Enjoyment?
-
RP(s)E = 9 out of 10. The review is a comprehensive synthesis of evidence with a balanced discussion and clear implications for sports policy.
The role of footwear in improving running economy: a systematic review with meta-analysis of controlled trials. Xu et al. (2025 ) Sci Rep. (click the title to access the full article)
What type of study is this?
- This 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. with meta-analysis.A meta-analysis quantifies the overall effect size of a treatment by compiling effect sizes from all studies of that treatment.
What is the authors’ hypothesis or research question?
- The authors’ research question was to determine whether different footwear types (shod, minimalist, and barefoot) and footwear characteristics (mass, cushioning, longitudinal bending stiffness, and comfort) significantly affect the running economy (RE) of long-distance runners.
What did the authors do to test the hypothesis or answer the research question?
- The study is a systematic review and meta-analysis that strictly included randomized controlled trials.
- A comprehensive literature search was conducted across five databases (Web of Science, PubMed, SPORTDiscous, SCOPUS, and CNKI) up to April 2024, and the review was registered on INPLASY (registration number INPLASY2024120032).
- The meta-analysis included 26 studies with a total of 483 participants (predominantly male, with some studies including female subjects) aged between 20 and 40 years.
- Methodological quality was evaluated using the Physiotherapy Evidence Database (PEDro) scale, and RE was measured via oxygen uptake (ml/kg/min) or power output (W/kg) during constant-submaximal treadmill running.
What did the authors find?
- When comparing barefoot running with shod running, the pooled standardized mean difference (SMD) was –0.28 (95% CI: –0.56 to –0.01, 104 participants; P = 0.04), indicating improved RE in barefoot conditions.
- For minimalist running versus shod running, the pooled SMD was –0.62 (95% CI: –0.83 to –0.42, 200 participants; P < 0.00001), further supporting the benefit of lighter footwear on RE
- The comparison between barefoot and minimalist running showed an SMD of 0.37 (95% CI: –0.07 to 0.81, 42 participants; P = 0.10), which was not statistically significant.
- Regarding footwear characteristics, increased longitudinal bending stiffness (SMD = –0.53; 59 participants; P = 0.005) and cushioning (SMD = –0.48; 103 participants; P = 0.0007) were associated with improved RE. This observation is synonymous with “supershoes”, which have a high degree of longitudinal bending stiffness and cushioning.
- The authors concluded that barefoot running or minimalist running, as well as shoes (“supershoes”) with high longitudinal bending stiffness and cushioning, may enhance running economy compared to conventional shod running.
What are the strengths of the study?
- The review employed a rigorous systematic search strategy across multiple databases and followed the PRISMA guidelines.
- Strict inclusion criteria ensured that only randomised controlled trials were considered, and methodological quality was assessed using the PEDro scale.
- The statistical analyses were comprehensive, including the evaluation of heterogeneity and publication bias.
What are the weaknesses of the study?
- Although the total sample size across studies was 483, some individual studies had small sample sizes, which may affect the robustness of specific comparisons. Furthermore, the meta-analytical comparisons only included approximately. 100 participants at best.
- The inability to blind participants and researchers to footwear conditions introduces potential expectancy bias.
- There was variability in measurement methods (e.g., oxygen uptake vs. power output) and short follow-up durations (mostly up to 15 minutes), limiting conclusions about long-term effects.
- The analysis does not explicitly assess the overall strength/certainty of evidence with a tool like GRADE.
How was the study funded, and are there any conflicts of interest that may influence the findings?
- The study received financial support from vivo Mobile Communication Co., Ltd. The funders had no role in the study design, data analysis, interpretation, or the decision to publish. The authors declared no competing interests.
Are the findings useful in application to training and/or coaching practice?
- Yes → The findings are useful for endurance training and coaching as they suggest that selecting footwear that promotes barefoot or minimalist running or footwear that optimises longitudinal bending stiffness and cushioning (e.g., “supershoes”) may improve running economy, which is a key factor in enhancing long-distance performance.
What is my Rating of Perceived scientific Enjoyment?
-
RP(s)E = 8 out of 10.
Important: Don’t make any major changes to your 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 certainty of evidence in a meta-analysis). What do other trials 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 systematic review and meta-analysis evaluating the entirety of the evidence in this field? (Follow the link to explore those reviews.) If so, what does the analysis show? What is the risk of bias or certainty of evidence of the included studies?
Pre-Season Energy Deficiency Predicts Poorer Performance During a Competitive Season in Collegiate Female Long-Distance Runners. Lundstrom et al. (2025 ) Eur J Sport Sci. (click the title to access the full article)
What type of study is this?
- This study is an
observational (prospective cohort) study.Observational studies can have different types of study designs, including cross-sectional, case-control, retrospective cohort, and prospective cohort studies. Typically, an observational study examines a specific cohort of people to characterise them and learn about them; there are no intentional interventions (i.e., people are not prescribed a diet, drug, or exercise). Sometimes the cohort of people is followed over time to determine whether there is a relationship between the natural exposure to a specific variable (e.g. exposure to dietary saturated fat intake) and a specific outcome of interest (e.g. cardiovascular disease).
What is the authors’ hypothesis or research question?
- The authors hypothesised that baseline energy status—as defined by the measured‐to‐predicted resting metabolic rate ratio (RMRratio) and total triiodothyronine (TT3) concentration—would be associated with differences in running performance, body composition, and other energetic measures.
What did the authors do to test the hypothesis or answer the research question?
- The study enrolled collegiate female long‐distance runners aged 18–25 years. A total of 42 runners initially consented; 38 completed the pre‐season testing and body composition assessment, while 21 completed both pre‐ and post‐season performance evaluations. All participants were on university‐affiliated competitive teams and met strict eligibility criteria (healthy, nonsmoker, etc.).
- Methods included measurement of resting metabolic rate (RMR) via indirect calorimetry following standardised fasting and rest conditions, body composition via DXA scanning, and performance assessed by a competitive 5 km time trial. In addition, VO2max was measured and blood samples were taken to assess TT3 levels.
- RMR was compared to predicted values (using the Cunningham equation) to calculate the RMRratio, with a cutoff of 0.92 used to indicate metabolic suppression (energy deficiency). NOTE: RMR ratio (i.e., measured RMR / predicted RMR) was used to define “energetic status” and identify individuals who exhibit energetic adaptation to a chronic energy deficit. The authors previously demonstrated that an RMR < 0.92 using the Cunningham equation (1991) may indicate metabolic suppression secondary to energy deficiency in a population of exercising women. This concept has been supported in the literature and associated with other proxy indicators of energy deficiency, specifically, low total triiodothyronine (TT3).
What did the authors find?
- The authors found that 32% of participants were classified as energy deficient (RMRratio < 0.92).
- In terms of performance, the metabolically suppressed group (SUP) recorded slower 5 km race times than the energy repleted group, with mean times of 22.4 minutes versus 20.4 minutes, respectively (p = 0.04).
- Additionally, SUP runners exhibited significantly lower RMR, lower TT3 concentrations (p = 0.013), and lower energy availability compared to energy-repleted runners.
- Linear regression analysis—controlling for post‐season VO2max—revealed that pre‐season TT3 was a significant predictor of post‐season performance (R² = 0.614, p = 0.001) and of the change in running performance across the season.
- The authors concluded that pre‐season energy deficiency, as measured by RMRratio and TT3, is associated with poorer running performance across a competitive season. They suggest that early detection of metabolic compensation may be key to optimising training adaptations and competitive outcomes.
What are the strengths of the study?
- The study is prospective and conducted under real‐life competitive conditions, using a standardised 5 km time trial to assess actual performance rather than solely laboratory-based proxies.
- It employs objective, laboratory‐based measures (RMR, DXA for body composition, TT3 via immunoassay) to assess energy status, reducing reliance on self-reported data.
- The design included detailed statistical analyses with regression models to control for confounding factors such as VO2max.
What are the weaknesses of the study?
- The sample size is modest—only 38 participants in pre‐season and 21 completing both time points—which may limit generalizability and statistical power.
- The study did not assess carbohydrate availability specifically, which might be a confounding factor when evaluating endurance performance.
- There is potential inaccuracy in the self-reported energy intake, despite the use of objective measures for RMR and TT3.
- Performance was not compared with participants’ personal bests from previous seasons, which could have provided additional context for performance changes.
How was the study funded, and are there any conflicts of interest that may influence the findings?
- The authors reported that they received no specific funding for this work. They also declared no conflicts of interest, which suggests that there are no apparent biases related to funding or personal interests that might influence the finding.
Are the findings useful in application to training and/or coaching practice?
- Yes → The findings are useful for endurance coaches and practitioners because they highlight the importance of monitoring energy status in female long‐distance runners. By using objective measures like RMR and TT3, coaches may be able to identify energy deficiency early, allowing them to adjust nutritional and training regimens to avoid performance decrements and reduce the risk of adverse health outcomes associated with the female athlete triad.
What is my Rating of Perceived scientific Enjoyment?
-
RP(s)E = 7 out of 10. Despite the limitations of a small sample size and the lack of carbohydrate-specific data, the study used rigorous methodology, stated a clear hypothesis, and used detailed performance outcomes and appropriate statistical analyses.
- Important: Don’t make any major changes to your 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 certainty of evidence in a meta-analysis). What do other trials 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 systematic review and meta-analysis evaluating the entirety of the evidence in this field? (Follow the link to explore those reviews.) If so, what does the analysis show? What is the risk of bias or certainty of evidence of the included studies?
Carbohydrate ingestion eliminates hypoglycemia and improves endurance exercise performance in triathletes adapted to very low- and high-carbohydrate isocaloric diets. Prins et al. (2025 ) Am J Physiol Cell Physiol. (click the title to access the full article)
What type of study is this?
- This study is a
randomised controlled trial.The “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. with crossover.Crossover means that all subjects completed all interventions (control and treatment) usually with a wash-out period in between.
What is the authors’ hypothesis or research question?
- The authors hypothesised that a 6‑week adaptation to a very-low-carbohydrate high‑fat (LCHF) diet would impair time-to-exhaustion performance due to reduced pre-exercise muscle and liver glycogen, and that minimal carbohydrate supplementation during exercise would prevent exercise-induced hypoglycemia (EIH) and improve performance.
What did the authors do to test the hypothesis or answer the research question?
- The study used a randomized crossover design with 10 competitive, trained male triathletes.
- Each participant underwent two 42‑day isocaloric dietary interventions: one with a very-low-carbohydrate high‑fat (LCHF; 40 g/day) diet and one with a high‑carbohydrate low‑fat (HCLF; 380 g/day) diet, with a 2‑week washout period between conditions.
- During each intervention, subjects performed time-to‑exhaustion (TTE) tests at 70% V̇O₂max on a cycle ergometer. In each test, they ingested either a carbohydrate solution (providing approximately 10 g CHO/h) or a placebo.
- Multiple metabolic parameters were continuously monitored, including substrate oxidation rates, blood glucose, blood ketone concentrations, and perceptual measures.
What did the authors find?
- Time-to‑exhaustion performance was equivalent between the LCHF and HCLF diets when no carbohydrate was ingested during exercise.
- Ingestion of a minimal carbohydrate dose (10 g/h) during exercise prevented EIH and increased TTE performance by approximately 22% (an improvement of about 19 ± 7 minutes) in both dietary conditions.
- The LCHF diet led to significantly lower 24‑hour glucose concentrations; however, after about 4 weeks of adherence, blood glucose levels normalised concurrently with the normalisation of blood ketone (R‑b‑hydroxybutyrate) levels.
- The authors concluded that a well‑adapted LCHF diet does not impair prolonged endurance performance and that even minimal carbohydrate supplementation during exercise is effective at preventing hypoglycemia and enhancing performance.
What are the strengths of the study?
- The study employed a rigorous randomised crossover design with controlled isocaloric dietary interventions, which minimises inter-individual variability.
- Continuous metabolic monitoring and comprehensive physiological measurements provided detailed insights into substrate oxidation, glycemic control, and performance outcomes.
- The protocol maintained constant training load and fat-free mass, reducing potential confounding factors.
What are the weaknesses of the study?
- The sample size was small (n = 10), which may limit the statistical power and generalizability of the findings.
- The study included only male triathletes, limiting the applicability of the results to other populations, such as females or athletes from different endurance disciplines.
- Some methodological details (e.g., intention-to-treat analysis, complete reporting of blinding procedures beyond single-blind design) were not fully described.
How was the study funded, and are there any conflicts of interest that may influence the findings?
- The article did not explicitly provide details regarding the funding source or disclose conflicts of interest. Therefore, it is not possible to assess how these factors might have influenced the study findings.
Are the findings useful in application to training and/or coaching practice?
- Yes → The findings are useful for endurance training and coaching as they demonstrate that with proper adaptation (a minimum of 4 weeks), athletes on very-low-carbohydrate diets can maintain endurance performance. Moreover, the data support that minimal carbohydrate supplementation during exercise can effectively prevent hypoglycemia and enhance performance, offering a potential strategy for nutritional periodisation in endurance athletes.
What is my Rating of Perceived scientific Enjoyment?
-
RP(s)E = 7 out of 10. Despite a small sample size and a lack of some methodological details, the study uses a randomised design, inclusion of control measures, clear inclusion/exclusion criteria, and pre-determined sample size.
- Important: Don’t make any major changes to your 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 certainty of evidence in a meta-analysis). What do other trials in this field show? Do they confirm the findings of this study or have mixed outcomes? Is there a high-quality systematic review and meta-analysis evaluating the entirety of the evidence in this field? If so, what does the analysis show? What is the risk of bias or certainty of evidence of the included studies?
- Important: I’ve written a deep-dive on this topic; check it out here.
All the other interesting papers I found this month are below.
You can dig in and evaluate the authors’ findings by clicking on the titles to access the full papers.
Learn to critically evaluate each paper using the framework I used for my favourite papers. 
General training methods.
The role of footwear in improving running economy: a systematic review with meta-analysis of controlled trials. Xu et al. (2025 ) Sci Rep. Continuous Glucose Monitoring Underreports Blood Glucose During a Simulated Ultraendurance Run in Eumenorrheic Female Runners. Dole et al. (2025 ) Int J Sports Physiol Perform. Sports nutrition and hydration.
Carbohydrate ingestion eliminates hypoglycemia and improves endurance exercise performance in triathletes adapted to very low- and high-carbohydrate isocaloric diets. Prins et al. (2025 ) Am J Physiol Cell Physiol. Boston Marathon athlete performance outcomes and intra-event medical encounter risk associated with low energy availability indicators. Whitney et al. (2025 ) Br J Sports Med. Athlete health (including mental health).
Pre-Season Energy Deficiency Predicts Poorer Performance During a Competitive Season in Collegiate Female Long-Distance Runners. Lundstrom et al. (2025 ) Eur J Sport Sci. Effects of low energy availability on performance in male athletes: A scoping review. Hänisch et al. (2025 ) J Sci Med Sport. Female athlete physiology and sex differences.
Evidence on sex differences in sports performance. Joyner et al. (2025 ) J Appl Physiol. Risk of Low Energy Availability, Disordered Eating, and Menstrual Dysfunction in Female Collegiate Runners. Dambacher et al. (2025 ) J Athl Train. No sex differences in performance and perceived fatigability during a self-paced endurance exercise performed under moderate hypoxia. Hasegawa et al. (2025 ) Am J Physiol Regul Integr Comp Physiol.
And, to help you wash down the latest evidence, here's a snifter from my recent indulgence...
My beer of the month.
King Crush.
Brewed by BrewDog (Ellon, Scotland).
Double Milkshake IPA.
8.4% ABV.
Sweet and foamy, like a fire extinguisher that fires honey.
RP(be)E(r)(Rating of Perceived beer Enjoyment)
7 out of 10
Access to education is a right, not a privilege:
Equality in education, health, and sustainability is important to me. I was lucky to be born into a social welfare system where higher education was free. Sadly, that is no longer true. Consequently, to provide access to exercise science and sports nutrition education to folks from all walks of life, I publish freely accessible high-quality exercise science and nutritional science content. This nerd alert newsletter is part of that offering. You can find more free educational resources from me, Thomas Solomon PhD, at veohtu.com.
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Disclaimer: I occasionally mention brands and products but it is important to know that I am not affiliated with, sponsored by, an ambassador for, or receiving advertisement royalties from any brands. I have conducted biomedical research for which I have received research money from publicly-funded national research councils and medical charities, and also from private companies, including Novo Nordisk Foundation, AstraZeneca, Amylin, A.P. Møller Foundation, and Augustinus Foundation. I’ve also consulted for Boost Treadmills and Gu Energy on their research and innovation grant applications and I’ve provided research and science writing services for Examine — some of my articles contain links to information provided by Examine but I do not receive any royalties or bonuses from those links. These companies had no control over the research design, data analysis, or publication outcomes of my work. Any recommendations I make are, and always will be, based on my own views and opinions shaped by the evidence available. My recommendations have never and will never be influenced by affiliations, sponsorships, advertisement royalties, etc. 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.