The human gut microbiota comprises more than 1,000 different bacteria species, representing 3 million unique genes. In total, the microbiota can weigh up to 2 kg and plays a key role in host digestion, metabolic function, and infection resistance. The sum of the microbial genes—the microbiome—contributes in multifarious ways to host physiology and pathophysiology. Researchers have expressed much interest in manipulating the microbiome to influence health and disease. For some time, diet has been recognized as a key factor in influencing the microbiome, but, recently, the role of exercise is also being elucidated. In a review published in Exercise and Sport Sciences Reviews, researchers took a deep dive into the impact of exercise on gut health. Cross-sectional data Numerous human cross-sectional studies have demonstrated the importance of the gut microbiome in health, with evidence that exercise has a role in shaping the gut microbiota, noted the review authors. Compared with sedentary controls, women who exercised at least 3 hours per week harbored higher levels of the bacteria Akkermansia muciniphila, which is linked to lean body mass index (BMI) and enhanced metabolic health. These women also hosted higher levels of butyrate-producing bacteria. Butyrate is a short-chain fatty acid (SCFA) produced from the bacterial fermentation of dietary fiber, and is the primary fuel for colonocytes. SCFAs boost epithelial cell proliferation and gut barrier integrity, as well as moderate host immunity, according to the authors. Results from a small study published in the BMC suggested that among healthy individuals, those with higher peak oxygen uptake (VO2peak)—the gold standard measure of cardiorespiratory fitness—had greater taxonomic richness, after taking into account diet and other covariates. These heart-healthy volunteers also produced more butyrate. “Reduced microbial diversity in human intestines has been implicated in various conditions such as diabetes, colorectal cancer, and inflammatory bowel disease,” the authors wrote. “The role of physical fitness in the context of human intestinal microbiota is currently not known. We used high-throughput sequencing to analyze fecal microbiota of 39 healthy participants with similar age, BMI, and diets but with varying cardiorespiratory fitness levels. Fecal short-chain fatty acids were analyzed using gas chromatography.” Longitudinal studies The authors of the aforementioned Exercise and Sport Sciences Reviews article highlighted findings from a novel, controlled longitudinal study, which assessed the impacts of exercise on the gut microbiome in 32 sedentary adults who took part in a 6-week supervised endurance exercise program with diet control. “Several taxa were differentially altered by exercise depending on BMI status,” they wrote. “For instance, exercise increased Faecalibacterium species in lean subjects but reduced its abundance in obese subjects; Bacteroides species decreased in the lean subjects and increased in the obese subjects. Six weeks of exercise also increased the abundance of butyrate=producing taxa and fecal acetate and butyrate concentrations, but only in lean subjects.” Furthermore, the majority of the bacterial taxa and SCFAs that increased with exercise took a dip during the subsequent 6-week sedentary washout period, suggesting that the effects of exercise on the microbiota were both transient as well as reversible. In a separate longitudinal study published in Frontiers in Microbiology, researchers examined whether 6 weeks of endurance exercise influenced gut metagenome and systemic metabolites in previously sedentary overweight women—even without diet changes. “Exercise training modified the gut microbiome without greatly affecting systemic metabolites or body composition,” the authors concluded. “Based on our data and existing literature, we propose that especially Akkermansia and Proteobacteria are exercise-responsive taxa. Our results warrant the need for further studies in larger cohorts to determine whether exercise types other than endurance exercise also modify the gut metagenome.” Mechanisms Authors of the article in Exercise and Sport Sciences Reviews hypothesized that exercise-induced changes in the gut yield health benefits. In addition to increased butyrate and levels of butyrate-producing bacteria, exercise could increase bacterial translocation, which refers to the movement of viable bacteria from the gastrointestinal tract to extraintestinal sites, such as the mesenteric lymph node complex, liver, spleen, and blood. Exercise could also decrease levels of pro-inflammatory cytokines, as well as possibly bolstering gut barrier function. These physiological changes could decrease the risk of colon cancer, inflammatory bowel disease, and diverticulosis. They could also increase the blood-brain barrier function and lower depression/anxiety. With respect to metabolic disease, exercise-induced to the microbiome could increase energy expenditure, insulin sensitivity, and decrease inflammation. Source
