A new branch of medicine is quickly emerging from the growing evidence of the link between the gut microbiota – the complex set of microbial populations living within our gastrointestinal tract- and human health.
In a recent blog, I discussed how nutrition shapes bacterial communities in the gut, and how dysbiosis -the loss of bacterial biodiversity- can trigger or facilitate common health problems. Here, I’ll review evidence gathered over the last few years that shows that exercise can modify the gut microbiota and influence host physiology with beneficial effects on metabolism, immunity, cognition, and behavior. I’ll focus on human studies, which have largely confirmed more in-depth investigations carried out on rodents (1–5).
Impact of Exercise on Microbiota Balance and Function
In recent years, bioinformatics analyses of data generated by high-throughput DNA and RNA sequencing techniques have allowed detailed characterizations of the bacterial populations that reside in the gut. By incorporating biochemical analyses of bacterial metabolites present in feces and plasma, shifts in gut microbiota composition and activity can now be accurately assessed following changes in diet, physical activity, or other factors such as disease or therapies. Relying on these tools, a number of studies have shown that physical activity and exercise positively modulate gut bacteria, with beneficial effects on metabolism, immunity, organ function, and overall health (6, 7). Several studies showed that cardiorespiratory fitness, independent of diet, correlated positively to bacterial diversity and richness in the gut.
A 2016 study published in Microbiome assessed 39 healthy participants with similar age, BMI, and diets, but different cardiorespiratory fitness levels. Analysis of gut bacteria composition revealed that cardiorespiratory fitness, estimated through VO2max (maximum oxygen uptake, a measure of aerobic capacity) is responsible for more than 20 % of the variation in taxonomic richness, after accounting for all other factors, including diet. This study proposed that exercise could be used as a therapeutic support in the treatment of dysbiosis-associated diseases (8).
Another investigation in Medicine & Science in Sports & Exercise compared the effects of a supervised, 6-week endurance-based exercise program on gut microbiota composition and metabolic output in 18 lean and 14 obese sedentary individuals. Molecular taxonomy analyses showed that gut bacteria composition in lean and obese subjects was different at baseline, but became homogeneous after exercise. Exercise training also improved body composition parameters (increased total lean body mass, decreased body fat percentage, and increased bone mineral density) and VO2max in both lean and obese subjects. However, mimicking results reported in mice fed a high-fat diet, some beneficial changes induced by exercise were absent or reduced by obesity. These included production of butyrate, a short chain fatty acid (SCFA) generated by bacterial fermentation of dietary fiber in the colon, that showed an increment only in lean individuals. Butyrate is a primary fuel source for colonocytes, contributes to maintaining gut barrier integrity, and reduces inflammation by modulating gut-resident immune cells. Notably, these changes were reversed after stopping exercising and returning for 6 weeks to a sedentary lifestyle (9, 10).
Similarly, another study appearing in Frontiers in Microbiology found that following a 6-week endurance exercise regimen overweight women showed improved cardiovascular performance and a beneficial shift in gut bacteria composition. However, the latter was not associated with improved metabolism, which suggested that excess weight can preclude -at least in the short-term- the benefits of exercise on gut bacteria (11).
A positive correlation between cardiorespiratory fitness and beneficial gut microbiota composition was also found in 71 premenopausal women. The study, reported in the journal Nutrients, showed that participants with low aerobic fitness had higher representation of potentially pathogenic Firmicutes (Eubacterium rectale-Clostridium coccoides) and Proteobacteria (Enterobacteria), independent of age and carbohydrate or fat intake (12).
Other findings shed light on the link between exercise and microbiota:
- An article published in Gut in 2014 found that professional rugby players had higher bacterial diversity, higher proportions of beneficial bacteria, lower levels of inflammatory cytokines (IL-6, TNF-α, IL-1), and higher anti-inflammatory cytokines (IL-10, IL-8) than size- and age-matched controls. This profile also correlated to increased protein consumption among the athletes (13).
- Physical activity based on long-distance running significantly increased the number of bacteria involved in the metabolism of bile salts and steroids, and the activation of dietary polyphenols, which aid the growth of beneficial bacteria. This Frontiers in Microbiology study identified the family Coriobacteriaceae, member of Actinobacteria, one of the major gut bacteria phyla in humans, as a potential biomarker that links exercise with health improvement (14).
- A 2018 PLoS One report showed that women who regularly performed the minimum dose of exercise recommended by the WHO had increased numbers of beneficial gut bacteria (15).
While there is no doubt that exercise has many health benefits, among them promoting a healthy gut microbiota, the aphorism “too much of a good thing can be bad for you” rings true here as well. Just like eating too much fiber will cause unpleasant side-effects, prolonged strenuous exercise may also be detrimental to gut health, as it can lead to leaky gut, diarrhea, gastrointestinal bleeding, and bacterial translocation into the bloodstream (16, 17).
How Does Exercise Change Gut Microbiota?
This question is still being researched, but there is already some information. Indirect mechanisms include the immune-boosting effect of exercise, which helps stave off outgrowth of harmful bacteria and prevent infections (18, 19). More directly, by increasing the mixing of intestinal contents, exercise could promote bacterial fermentation of dietary fiber, leading to higher SCFA production, decreased hunger, and glycemic control (20). Moreover, exercise reduces gut transit time and changes stool consistency, leading to faster turn-over of beneficial bacteria and quicker elimination of potentially harmful ones (21).
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Nutrition + Exercise = Happy Gut Microbes
Our nutritional needs are dictated by the energy spent on metabolic reactions, which basically serve to maintain cellular and organ homeostasis, needed for mental work and movement. This means that proper metabolism and adequate body functioning require that energy intake be balanced by energy expenditure. And this is where the link between nutrition and physical activity becomes most obvious: physical activity and exercise are fundamental to optimize the utilization of the energy that nutrition provides.
As discussed in much detail in my blogs and podcasts, poor nutrition and physical inactivity underly the most common health issues and non-communicable diseases across ages -acne, allergies, GI disorders, obesity, bronchitis, arthritis, hypertension, and diabetes, among others. It is now clear that combining good nutrition habits and physical activity can help build a diverse and healthy gut microbiota, necessary to avoid inflammation, maintain good health, and fend off disease.
Exercise plus nutrition modifies the gut microbiota, alleviates chronic inflammation and has positive health effects. Exercise therapy plus nutrition helps many lifestyle related diseases that contribute to chronic pain.
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REFERENCES:
1- Matsumoto, M., Inoue, R., Tsukahara, T., Ushida, K., Chiji, H., Matsubara, N., & Hara, H. (2008). Voluntary running exercise alters microbiota composition and increases n-butyrate concentration in the rat cecum. Bioscience, biotechnology, and biochemistry, 72(2), 572-576.
2- Allen, J. M., Mailing, L. J., Cohrs, J., Salmonson, C., Fryer, J. D., Nehra, V., … & Woods, J. A. (2018). Exercise training-induced modification of the gut microbiota persists after microbiota colonization and attenuates the response to chemically-induced colitis in gnotobiotic mice. Gut Microbes, 9(2), 115-130.
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21-Roager, H. M., Hansen, L. B., Bahl, M. I., Frandsen, H. L., Carvalho, V., Gøbel, R. J., … & Hansen, T. (2016). Colonic transit time is related to bacterial metabolism and mucosal turnover in the gut. Nature microbiology, 1(9), 16093.
