The notion that our physical exercise routines could potentially enrich the vast community of microbes within our intestines—encompassing bacteria and viruses crucial for immunity, metabolism, digestion, and more—might not be immediately apparent. This connection isn’t as straightforward as the link between our diet and these gut microbes, known as the gut microbiome. However, research increasingly suggests that aerobic activities like running can enhance the well-being of these gut microbes, which, in turn, bolsters our overall health. Preliminary findings also suggest that a robust gut microbiome could boost one’s ability to exercise.

“Typically, when people think of the gut, their minds go straight to diet and probiotics,” notes Sara Campbell, an exercise physiologist at Rutgers University with expertise in gut microbiota. She adds that the scientific community is gradually recognizing that “exercise also offers significant benefits for gut health.”

A “healthy” microbiome is often characterized by a rich and diverse population of gut bacteria; exercise impacts both these aspects. The gut microbes in elite athletes show greater diversity compared to those in non-athletes or casual sports enthusiasts. However, Jacob Allen, an exercise physiologist at the University of Illinois Urbana-Champaign, points out that the crucial factor is understanding “the specific actions of these microbes.”

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Aerobic activities stimulate bacteria that generate short-chain fatty acids, crucial for various bodily functions.

A key discovery is that aerobic exercise promotes the activity of bacteria that produce short-chain fatty acids, vital for numerous physiological functions. Unlike the typical fatty acids, which are composed of 16 to 18 carbons, short-chain fatty acids have only one to six carbons. Among these, butyrate stands out as particularly significant in linking exercise with gut health. It not only provides energy to various tissues, including the gut lining cells, but also reduces inflammation and enhances insulin absorption. While our bodies produce some butyrate naturally, most is produced by microbial activity, which aerobic exercise can increase. It’s worth noting that studies connecting resistance training with butyrate production are scarce and have not shown similar results.

This connection between exercise and gut health was not on the radar about 15 years ago when exercise immunologist Marc Cook was a graduate student at the Urbana-Champaign campus. He was aware that exercise alleviated symptoms of inflammatory bowel diseases, such as ulcerative colitis, but the underlying reasons were unclear. His research with mice demonstrated that exercise protected them against a type of colitis and resulted in a sevenfold increase in beneficial bacteria in their colons.

In 2018, Allen, Cook (now at North Carolina A&T State University), and their team conducted the first human study on exercise and gut health. They trained sedentary individuals, both lean and obese, on treadmills or bikes, starting with moderate exercise three times a week and escalating to an hour of intense activity. After six weeks, all participants showed increased levels of butyrate and two other short-chain fatty acids, acetate and propionate, alongside expected exercise benefits like reduced body fat and enhanced cardiorespiratory fitness. However, these benefits were more pronounced in lean individuals—a phenomenon researchers are still trying to understand. Once the participants stopped exercising for another six weeks, their gut microbes and health benefits reverted to baseline levels.

While it’s challenging to isolate the effects of exercise on microbiota from other exercise-induced changes, it’s evident that the gut environment is altered. Allen explains, “Exercise slightly redirects blood flow towards the muscles and away from the gut, reducing oxygen levels in gut tissues.” This, along with changes in pH and temperature within the gastrointestinal tract, might influence microbial survival.

Human studies are complicated by the vast diversity in individual microbiomes. Researchers like Campbell are now exploring how responses vary by gender, while Cook investigates how gut bacteria that produce short-chain fatty acids affect Black individuals, who have higher rates of hypertension. In preliminary studies, they’ve identified bacteria linked to high blood pressure in Black athletes and are exploring potential interventions.

Although most of the evidence on microbiota’s effects on exercise capacity comes from animal studies, it’s clear that mice with depleted microbiomes due to antibiotics show reduced exercise capacity and quicker fatigue. Studies also suggest that a healthy gut microbiome contributes to better muscle development.

This growing body of research doesn’t alter the standard exercise guidelines, which recommend at least 150 minutes of moderate exercise weekly. However, it reinforces the benefits of such activity and might soon help explain why individuals respond differently to exercise. Future strategies might even enhance the microbiome to better support gym efforts. Meanwhile, this research adds a new layer of meaning to powering through your workouts.

This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.