Between sport and microbiota: the gut, an organ too often underestimated

In brief

• The gut microbiota is a dynamic element that can be modified by physical activity, diet, and lifestyle.
• Differences in microbial composition and diversity are often observed between athletes and less active individuals, but not all changes are necessarily favorable for health or performance.
• Intense and prolonged activity can increase intestinal permeability; this is physiologically relevant for gastrointestinal symptoms and training-related inflammatory responses.
• Microbial products such as short-chain fatty acids (SCFAs, particularly butyrate) are plausibly useful for mucosal integrity and immunoregulation, but direct clinical evidence on performance effects is limited.
• Nutritional interventions (fiber-rich diet, various dietary strategies) and, in specific cases, some probiotics can reduce symptoms or modulate immune responses, but interpretive caution remains necessary.

Abstract: what does science say?

The gut hosts a complex microbial community that interacts with metabolism, the mucosal barrier, and the immune system. Observational studies and experimental interventions indicate that physical activity can modify the composition and function of the microbiota, often increasing diversity and certain taxa associated with the production of beneficial metabolites. However, prolonged or very intense training can induce gastrointestinal stress and increase intestinal permeability, with possible translocation of microbial products into the systemic circulation. Microbial metabolites such as short-chain fatty acids (e.g., butyrate) support mucosal homeostasis and immune modulation; diet remains the primary determinant of the availability of these metabolites. Some randomized studies on probiotics show effects on symptoms or immunological markers in athletes, but the evidence is heterogeneous in terms of strain, dose, and duration. Therefore, the relationship between microbiota, performance, and athlete health is plausible and supported by emerging data, but it is not yet possible to draw definitive causal conclusions or generalize recommendations without contextualizing age, discipline, training load, and individual nutrition.

Why the gut matters in sports

The gastrointestinal tract is not just a digestive system: it is an ecosystem where bacteria, yeasts, and viruses exchange metabolites with the host and regulate immune and metabolic functions. In sports, clinical observations and comparative studies have shown systematic differences in microbial composition between elite athletes and sedentary individuals, with greater richness and presence of specific genera in some athletes [1]. However, these differences do not automatically translate into improved health or performance: the relationship is complex, conditioned by diet, type and load of training, and personal factors. Some experimental works have isolated plausible mechanisms: for example, strains capable of metabolizing lactate produced during exercise can generate propionate, a metabolite that has shown an effect on physical endurance in animal models [2]. In parallel, controlled exercise interventions in sedentary volunteers have recorded variations in the production of some bacterial metabolites (including SCFAs) and in taxa associated with butyrate formation, suggesting that training can influence microbial functions relevant to intestinal homeostasis [3].

Microbiota and performance: what we know

Group comparison studies indicate that elite athletes tend to have higher microbial diversity compared to controls, but this is often accompanied by significant dietary differences that confound interpretation [1]. A multidisciplinary study documented the post-race over-representation of certain genera (e.g., Veillonella) and demonstrated in experimental models that metabolites derived from lactate metabolism can improve endurance; however, this mechanism still needs to be fully validated in humans and does not imply that introducing specific strains is automatically "performance-enhancing" for all athletes [2]. Intervention studies in humans show variable results: some training routines increase fecal SCFA production in some groups, but responses are individual and depend on the nutritional context [3].

Exercise, intestinal stress, and permeability

A well-established chapter concerns the effect of exercise on the intestinal barrier. Intense or prolonged training, especially in hot conditions or with dehydration, can reduce splanchnic blood flow and temporarily damage the epithelium, increasing markers of intestinal permeability measurable in controlled studies [4]. Classic research on volunteers has shown that increased permeability correlates with exercise intensity; subsequent analyses confirmed the role of thermoregulation and intestinal ischemia in explaining part of the phenomenon [5]. This framework helps to understand why some athletes report gastrointestinal symptoms during prolonged competitions or training and why hydration and temperature management are relevant for mucosal protection.

Short-chain fatty acids: butyrate and the barrier

Among microbial products, short-chain fatty acids (SCFAs) — particularly butyrate, acetate, and propionate — play key functions in the colonic epithelium: they are an energy source for colonocytes, modulate mucus production, and influence local immune signals. Numerous reviews indicate that butyrate contributes to barrier stability and the modulation of intestinal inflammation in experimental and clinical models [6]. However, fecal or plasma levels of SCFAs depend heavily on diet (quantity and quality of fermentable fibers), microbial composition, and intestinal transit, making generalizations and immediate translations into integration or supplementation strategies complex.

Factors influencing the microbiota in athletes

The composition and function of the microbiota are conditioned by multiple factors: diet, use of medications (antibiotics, anti-inflammatories), age, genetics, recovery time, and, of course, exposure to exercise. Experimental studies show that drastic dietary changes can reshape the microbiota in a few days, confirming the primary role of diet as a modulating factor [7]. In athletes, this translates into a complex interaction: high loads and specific nutritional strategies (e.g., high-protein diets or very low-fiber diets) can favor different microbial profiles, with possible beneficial or undesirable effects.

Regarding integrative strategies, the literature on probiotics and supplements shows positive but heterogeneous signals. Some randomized studies in athletes report a reduction in the incidence or duration of respiratory tract infections or a modulation of immune markers following supplementation with selected strains, while others show no clear effects on athletic performance [8][9]. Even for targeted nutrients (e.g., glutamine, bovine colostrum, polyphenols), the evidence is partial and depends on dosages, administration times, and the population studied [10].

What it means in practice

For athletes, coaches, and healthcare professionals, the operational message must be cautious and contextualized. Several practical elements emerge from the synthesis of evidence: favoring a varied diet, rich in plant sources and fermentable fibers, is the most robust strategy to support microbial production of useful metabolites (SCFAs) and promote mucosal integrity. Avoiding sudden changes in meal composition in the phases close to competitions can reduce the risk of gastrointestinal symptoms. In conditions of intense training or heat exposure, proper hydration and "gut-training" strategies (accustoming the gut to tolerate carbohydrate loads during exercise) can decrease the risk of permeability-related disorders.

As for probiotics, some specific products have shown benefits on symptoms or immune markers in controlled trials: the choice of strain, dose, and duration are crucial; there is no "universal probiotic" valid for all disciplines or for all people. For this reason, the adoption of supplementation should be evaluated with a professional, taking into account objectives, clinical history, and any ongoing therapies. Finally, the prolonged and unjustified use of antibiotics or non-steroidal anti-inflammatory drugs can compromise the stability of the microbiota and favor episodes of dysbiosis: limiting their use to clinical indications is a simple and effective measure for protecting intestinal health.

Key takeaways

• Physical activity and microbiota are correlated, but the direction and quality of the effect depend on diet, intensity, and individual context.
• Prolonged or very intense training can increase intestinal permeability; prevention strategies include hydration, thermoregulation management, and 'gut-training'.
• SCFAs like butyrate support mucosal integrity and immune modulation, but their production essentially depends on diet.
• Some probiotics have shown benefits on symptoms or immune markers in athletes; however, the effects are strain-specific and not generalizable.
• Personalized interventions, based on clinical and nutritional evaluation, are preferable to standardized and decontextualized solutions.

Limitations of Evidence

It is essential to distinguish between observational data and causal evidence: many studies show associations between microbial composition and training status, but observing a link does not prove that a specific microbial community causes performance improvement. Interventional studies in humans are relatively few, often small in size, with heterogeneous populations and different protocols; for this reason, results are not always replicable. Much research on the exercise-microbiota axis comes from animal models or experiments with microbiota transfer in animals, which offer mechanistic explanations but do not always fully translate to human complexity [2].

The biological measures used in trials (e.g., fecal composition, fecal SCFA concentrations, indirect markers of permeability) have interpretation limitations: fecal levels do not necessarily reflect metabolic activity in the mucosa, while individual and dietary variability complicates the extraction of standard recommendations. Finally, the heterogeneity in the definition of "athlete" (training level, discipline, sex, age) makes it difficult to apply study results to different populations.

Editorial Conclusion

Research on the gut microbiota and sport is a rapidly evolving field: there are solid elements linking diet, physical activity, microbial metabolite production, and intestinal mucosal integrity. However, it is equally clear that the relationship is multifactorial and contextual. For those working in sports, the priority remains protecting the athlete's health through dietary choices based on whole foods and careful management of training, hydration, and medication use. The use of probiotics or supplements can be considered in targeted cases, but requires professional evaluation and a choice guided by the strain, dose, and available evidence. The scientific community has made significant progress, but larger, replicated, and discipline-specific clinical studies are needed to move from promising mechanistic hypotheses to reliable operational recommendations.

Scientific research

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2. Scheiman J, Luber JM, Chavkin TA, et al. Meta‑omics analysis of elite athletes identifies a performance‑enhancing microbe that functions via lactate metabolism. Nat Med. https://doi.org/10.1038/s41591-019-0485-4. [2]
3. Allen JM, Mailing LJ, Niemiro GM, et al. Exercise Alters Gut Microbiota Composition and Function in Lean and Obese Humans. mSystems. https://doi.org/10.1128/mSystems.00044-18. [3]
4. Pals KL, Chang RT, Ryan AJ, Gisolfi CV. Effect of running intensity on intestinal permeability. J Appl Physiol. https://doi.org/10.1152/jappl.1997.82.2.571. [4]
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7. Venegas DP, De La Fuente MK, Landskron G, et al. Short chain fatty acids (SCFAs) and their role in gut health: a review. Nutrients. https://doi.org/10.3390/nu9121348. [7]
8. de Sire A, Giustino V, Pesce M, et al. Effect of Multi‑Strain Probiotic Supplementation on URTI Symptoms and Cytokine Production by Monocytes after a Marathon Race: A Randomized, Double‑Blind, Placebo Study. Nutrients. https://doi.org/10.3390/nu13051478. [8]
9. de Oliveira EP, Burini RC, Jeukendrup AE. Probiotic supplementation in marathonists and its impact on lymphocyte population and function after a marathon: a randomized placebo‑controlled double‑blind study. Sci Rep. https://doi.org/10.1038/s41598-020-75464-0. [9]
10. Belkaid Y, Hand TW. Role of the microbiota in immunity and inflammation. Annu Rev Immunol. https://doi.org/10.1146/annurev-immunol-042718-041841. [10]