Regular and adequate levels of physical exercise help protect against several diseases and all-cause mortality. One of the mechanisms that has been gaining increasing attention in relation to how exercise impacts health outcomes is the favorable modification of the human gut microbiota. Although research in this field is still scarce, previous animal and human research findings showed that exercise can affect the gut microbiota regardless of diet.
A new review, led by Dr. Ileana Terruzzi from the Diabetes Research Institute at San Raffaele Scientific Institute in Milan (Italy), explores mechanisms involved in gut microbiome modulation through exercise.
Gut microbiota functions interacting with metabolism, the immune system and the hypothalamic-pituitary-adrenal-axis are all interconnected and when dysregulated may contribute to a dysbiotic state related to malabsorption, metabolic syndrome, inflammation, autoimmunity and neurological disorders. Within the anaerobic conditions of the large intestine, commensal bacteria digest carbohydrates and proteins to produce short-chain fatty acids (SCFAs) that are involved in multiple signals, which may mediate some of effects of physical exercise on gut microbiota composition and functional diversity. For instance, gut microbiota directly and indirectly stimulates the immune system and SCFAs (butyrate, acetate, propionate). This promotes the release of serotonin (5-hydroxytryptamine, 5-HT), which is a neurotransmitter involved in the gut-brain axis and which acts as a key regulator of gastrointestinal motility and secretion.
The review focused on the effect of exercise on gut microbiota in different animal models and (mainly cross-sectional) human studies. The authors mentioned several mice studies, showing the role of manipulating gut microbial communities in improving the pro-inflammatory status in the gut and protecting against a high-calorie westernized diet through increasing fatty acid oxidation in muscle cells. The mechanisms involved in the effects of exercise on the gut microbiome include an increase in fecal bile acids, an elevated production of SCFAs, a suppression of toll-like receptor signaling in the liver, muscle, and adipose tissue by reducing lipopolysaccharide serum levels, an increase of immunoglobulin A (IgA), and a reduction in intestinal transit time.
Few controlled studies in humans have been conducted to confirm the findings of the animal studies, which have been carried out in greater numbers. The authors recognized that in studies of athletes’ gut microbiome it remains to be explained how dietary interactions contribute to exercise-related changes in both the composition and function of the gut microbiota. A recent study that supports this has found that 40 metabolites and selected bacterial species changes in people’s intestine after they ran a half marathon might have been the shared outcome of both running and diet. Besides this, weight loss and improved sleep quality are other exercise-inducible factors that may be responsible for particular changes in gut microbiota, making it difficult to conclude whether weight loss or the amount and quality of sleep time causes gut microbiota modifications or vice versa.
Of the health-promoting bacteria found at higher levels following exercise programs in human studies, Roseburia hominis and Akkermansia muciniphila are highlighted. Regarding types of exercise, although moderate-intensity exercise could be more beneficial than strenuous and very intense exercise on promoting anti-inflammatory immune responses and better overall health status in the intestine, the authors recognized that mechanisms mediating the relationship between exercise, the host immune system and the microbiota are poorly understood. Furthermore, a dose-response analysis between exercise levels and their beneficial alterations in gut microbiota composition deserves future studies.
Finally, the authors covered how exercise-based interventions could be therapeutically exploited to promote health via the gut microbiota. Low gut microbiota diversity and gut barrier dysfunction have previously been suggested to play a role in the development of autoimmunity for type 1 diabetes (T1D). Specific microbial signatures in pre-diabetic and diabetic people include high levels of Bacteroides and a reduced abundance of Prevotella, together with a decreased fraction of butyrate-producing species. Most studies focusing on the role of gut microbiota in autoimmune T1D development have been carried out in mice and suggest that exercise might exert a beneficial immune-modulation of systemic functions to both T1D and inflammation. In humans, observational studies have found that exercise may have positive effects on our immunometabolism and autoimmunity, showing that the gut microbiota of T1D patients with good glycemic control and high physical fitness levels was similar to matched-people without diabetes. According to the authors, however, “to date, physical exercise has been hardly considered as a therapeutic regulator of gut microbiota composition on the roadmap toward T1D-healing.”
In conclusion, the review focuses on the recent findings on gut microbiota modification through exercise and suggests several mechanisms by which physical exercise may cause changes in gut microbiota. Although these mechanisms are not fully understood, current data suggest that gut microbiota modification could be another piece in the puzzle of benefits that physical exercise has on human health.
Codella R, Luzi L, Terruzzi I. Exercise has the guts: How physical activity may positively modulate gut microbiota in chronic and immune-based diseases. Dig Liver Dis. 2018; 50(4):331-41. doi: 10.1016/j.dld.2017.11.016.
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