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The idea that our workouts could benefit the trillions of microbes that live in our guts—bacteria and viruses that help our immune systems, metabolism, digestion, and other key bodily functions—isn’t obvious. At least it’s not as obvious as the connection between diet and the gut microbiome, as these microbes are called. But evidence is growing that an aerobic workout, such as jogging, can improve the health of the gut microbes, which in turn improves overall physical health. There are early indications that the relationship works the other way, too: a healthy gut microbiome seems to increase exercise capacity.

“When people think about the gut, they default to diet and probiotics,” says Sara Campbell, an exercise physiologist at Rutgers University who specializes in gut microbiota. But now many scientists are “moving toward the reality that exercise can be beneficial for the intestines,” she says.

A “healthy” microbiome usually means gut bacteria are abundant and diverse; exercise appears to affect both these qualities. The gut microbes of an elite athlete are more diverse than those of nonathletes or recreational athletes. But a more pertinent issue for health, says Jacob Allen, an exercise physiologist at the University of Illinois Urbana-Champaign, is “what the microbe is actually doing.”

Aerobic exercise encourages activity in bacteria that produce short-chain fatty acids, which provide essential support for physiological processes.

One important finding is that aerobic exercise encourages activity in bacteria that produce short-chain fatty acids, which provide essential support for physiological processes. Most fatty acid molecules consist of 16 or 18 carbons, but—as the name suggests—short-chain fatty acids range from just one to six.

Of these smaller molecules, butyrate has emerged as an especially important link between exercise and the gut. It supplies energy for a variety of tissues, including the epithelial cells lining the gut, and it can reduce inflammation and improve the ability of cells to take in insulin. Our bodies naturally make a little bit of butyrate, but most is produced by microbes, and its output is boosted by aerobic exercise. (Very few studies have looked at the connection between strength training and butyrate levels, and those that have didn’t find the same effect.)

This link between exercise and the gut was barely a glimmer in scientists’ eyes some 15 years ago, when exercise immunologist Marc Cook was a graduate student at the Urbana-Champaign campus. He knew exercise improved symptoms of inflammatory bowel disease, particularly the type called ulcerative colitis. But scientists didn’t understand why. Cook turned to mice to investigate and found that if they ran on a wheel, they were protected against a mouse version of colitis. In addition, there was a sevenfold increase in beneficial bacteria in the lining of the rodents’ colons.

In a 2018 study, Allen, Cook (who is now at North Carolina A&T State University), and others tested a gut-health exercise intervention in humans for the first time. They trained both lean and obese people, all of whom were sedentary, to exercise on a treadmill or bike. Everyone started at moderate intensity three days a week and increased to one hour of high-intensity exercise per session.

After six weeks, all participants showed increases in butyrate and two other short-chain fatty acids, acetate and propionate. They also got the expected benefits of exercise, such as reductions in fat mass and improvements in cardiorespiratory fitness. (All the effects were greater in lean people, a finding that the researchers don’t yet understand.) After a further six weeks in which everyone stopped exercising, microbes in the gut returned to baseline levels, and health benefits decreased.

Researchers haven’t fully teased out which effects of exercise can be directly attributed to microbiota versus the other changes brought on by physical activity, but there is a clear difference in gut environment. “We know there’s a slight shunting of blood toward the muscles and away from the gastrointestinal tract during exercise,” Allen says. That causes a small decrease in oxygen in gut tissue. There are changes in pH and temperature within the GI tract as well. Each of these shifts could affect which microbes survive.

Studies in humans are complicated by the enormous diversity of microbiomes

from person to person and from group to group. Researchers are now trying to account for differences in response. Campbell is investigating variations by sex. Cook is studying the effects of short-chain-fatty-acid-producing bacteria in Black people, who have a high rate of hypertension. In a pilot study, he and his colleagues identified bacteria associated with high blood pressure in Black athletes, and they hope to identify a target for intervention.

As for the effects of microbiota on exercise capacity, most of that evidence comes from mice. Animals dosed with antibiotics to kill off their microbiomes exercise less than mice with healthy microbiomes and reach exhaustion faster. Research has also shown that an intact gut microbiota contributes to more muscle development.

This evolving research doesn’t change the standard recommendation for human exercise, which is to engage in at least 150 minutes of moderate physical activity a week. But it adds strength to the arguments for doing such activity and may ultimately help explain why people respond to exercise differently. Someday there may even be a way boost the microbiome so that it responds better to time in the gym. Already, though, the science gives new meaning to the idea of gutting out your workout.

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Jay Bendt

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