The gut microbiome and sport
The human intestine is inhabited by trillions of microorganisms, and over the past two decades it has become clear that they affect metabolism, immunity and even behaviour. Sports science has also become interested in the microbiome: whether training changes it, whether bacteria can increase endurance and whether athletes should take probiotics. The editorial team examined what is already known and what still remains a hypothesis.
What the gut microbiome is
The microbiome is the term for the totality of microorganisms — bacteria, archaea, fungi and viruses — and their genes that live in a particular environment. The most microorganisms in the human body are concentrated in the large intestine, where they digest what the person's own enzymes cannot handle, primarily dietary fibre.
The products of bacterial fermentation of fibre are short-chain fatty acids: acetate, propionate and butyrate. Butyrate is an important source of energy for the cells of the mucous membrane of the large intestine, while the other short-chain fatty acids participate in the regulation of glucose metabolism, appetite and immune responses.
The composition of the microbiome is unique in each person and is formed under the influence of genetics, the manner of birth, nutrition, medications (especially antibiotics), place of residence and lifestyle. However, there is as yet no single standard for assessing the "health" of the microbiome.
A measure of diversity is often used: a greater number of different species of bacteria is usually associated with better metabolic health. However, this is an association, not a proven cause, and a single "ideal" composition of the microbiome does not exist.
How physical exertion changes the microbiome
One of the first studies to draw attention to the topic was the work of Clarke and colleagues (2014), published in the journal Gut. Professional rugby players had significantly greater diversity of gut bacteria than men in the comparison group. However, the athletes also differed substantially in nutrition, in particular they consumed significantly more protein.
This is an important caveat: in cross-sectional studies it is hard to separate the effect of training from the effect of nutrition and other lifestyle features. To test a causal link, intervention studies are needed.
One such study was conducted by Allen and colleagues (2018): people who had not previously trained performed endurance exercises over 6 weeks under controlled nutrition. The composition of the microbiome and the concentration of short-chain fatty acids changed, and differently in lean people and people with obesity. After training stopped, the changes largely disappeared.
In addition, very intense and prolonged exertion can temporarily disrupt the barrier function of the intestine because of a decrease in its blood supply. It is with this that gastrointestinal symptoms in marathon runners and triathletes are linked — nausea, cramps, diarrhoea.
Can bacteria improve performance
The most media attention was received by the study of Scheiman and colleagues (2019) in Nature Medicine. The authors found that in participants of the Boston Marathon, after the race the number of bacteria of the genus Veillonella increased, which are able to use lactate as a source of energy, converting it into propionate.
When the researchers colonised the intestines of mice with these bacteria, the animals ran on a treadmill longer than the controls. The authors suggested that lactate from the muscles can enter the lumen of the intestine, where the bacteria process it into propionate, which further affects performance.
However, these results were obtained in mice and in a small group of people. There are as yet no studies showing that taking Veillonella or other bacteria increases sporting results in humans. Products with "marathon runners' bacteria" are already appearing on the market, but their effectiveness is not proven.
In general, the field of "the microbiome for performance" is currently at the stage of hypotheses. The review by Mohr and colleagues (2020) sums up that the link between the microbiome and physical performance looks plausible, but for practical recommendations there is a lack of quality clinical studies.
The athlete's nutrition and the microbiome
The most powerful known factor influencing the microbiome is nutrition, primarily fibre. Different dietary fibres feed different groups of bacteria, so the diversity of plant food — vegetables, fruit, legumes, whole grains, nuts, seeds — is linked to the diversity of the microbiome.
In sport there is a conflict of interests: before competitions and intense training, athletes are often advised to reduce fibre in order to avoid discomfort in the stomach. In addition, diets with a very high content of protein and simple carbohydrates and a small amount of plant food can impoverish the microbiome.
| Factor | Possible effect on the microbiome | Practical advice |
|---|---|---|
| Fibre and the diversity of plants | Supports diversity and the production of short-chain fatty acids | Various vegetables, fruit, legumes daily outside the pre-start days |
| Fermented products | Can increase the diversity of the microbiome | Kefir, yogurt, sauerkraut, kimchi |
| A very high-protein diet with a small amount of plants | Can shift the microbiome towards protein fermentation | Combine protein with plant food |
| Antibiotics | Substantially and lastingly change the composition of the microbiome | Take only as prescribed by a doctor |
| Excessive consumption of alcohol | Disrupts the barrier function of the intestine | Limit, especially during the recovery period |
A practical compromise for the athlete is to maintain the diversity of plant food in the overall diet, and to reduce fibre only temporarily, on the pre-start days and before individual intense sessions.
Fermented products — kefir, yogurt, sauerkraut — contain live microorganisms and fermentation products. They fit into a healthy athlete's diet, although their effect on sporting results separately is not proven.
Probiotics: where the evidence is
By the definition of the International Scientific Association for Probiotics and Prebiotics (Hill et al., 2014), probiotics are live microorganisms that, when taken in a sufficient amount, bring a benefit to health. Important: the effects of probiotics are specific to a particular strain, so the results of one product cannot be transferred to another.
The ISSN position stand on probiotics (Jäger et al., 2019) notes that individual strains can reduce the frequency or duration of upper respiratory tract infections in athletes, as well as some gastrointestinal symptoms. The data on a direct effect on strength or endurance are limited and contradictory.
- Choose products with a specified particular strain and number of colony-forming units.
- Assess the effect over at least a few weeks and test the product outside the competitive period.
- Remember that probiotics do not compensate for a diet poor in plant food.
- People with a weakened immune system should agree the intake of probiotics with a doctor.
During or after a course of antibiotics, some probiotics can reduce the risk of antibiotic-associated diarrhoea, but the decision about this should be made together with a doctor. Taking antibiotics on your own without a prescription harms both the microbiome and health in general.
Editorial conclusions
Physical training affects the gut microbiome, but these changes are moderate, depend on the person and partly disappear after training stops. Nutrition remains the main factor shaping the microbiome.
The idea of bacteria that increase endurance is interesting, but so far confirmed mostly in animals. Products with "athletes' bacteria" do not have proven effectiveness.
The best strategy is varied plant food, fermented products, limiting alcohol and the sensible use of antibiotics. Probiotics can be useful for particular goals, in particular reducing the frequency of respiratory infections, but the effect depends on the strain.
The editorial team also recommends familiarising yourself with our materials about nutrition in the heat, about the glycemic index for the athlete and about probiotics in sports nutrition.
References
- Clarke SF, Murphy EF, O'Sullivan O, et al. Exercise and associated dietary extremes impact on gut microbial diversity. Gut. 2014;63(12):1913–1920.
- Allen JM, Mailing LJ, Niemiro GM, et al. Exercise alters gut microbiota composition and function in lean and obese humans. Med Sci Sports Exerc. 2018;50(4):747–757.
- 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. 2019;25(7):1104–1109.
- Mohr AE, Jäger R, Carpenter KC, et al. The athletic gut microbiota. J Int Soc Sports Nutr. 2020;17(1):24.
- Hill C, Guarner F, Reid G, et al. Expert consensus document: the International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol. 2014;11(8):506–514.
- Jäger R, Mohr AE, Carpenter KC, et al. International Society of Sports Nutrition position stand: probiotics. J Int Soc Sports Nutr. 2019;16(1):62.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
