How Gut Microbes Respond to Physical Exercise

“The microbiota of lean individuals may be more responsive to an exercise intervention than that of overweight or obese individuals,” says Professor Woods.

Everyone can agree that the pillars of overall health include (but not limited to) physical exercise and gut health. But do these two pillars operate independently or cooperatively?

A healthy gut microbiota profile, in general, means one with (i) high microbial diversity, (ii) high abundance of gut microbes that produce beneficial metabolites like butyrate and other short-chain fatty acids (SCFAs), and (iii) low abundances of inflammatory gut microbes that produce toxins like lipopolysaccharides (LPS). More details here:

3 Ways Exercise Influences Gut Microbes

First, exercise directly modulates the gut-associated lymphoid tissue (GALT) — where 70% of the body’s immune cells reside. Exercise resolves inflammatory and oxidative reactions in the GALT system so that it can better contain microbes that leak from the gut barrier.

Put it simply, exercise improves GALT immunity to enable better control of the gut microbiota.

Second, exercise increases the core body temperature and diverts blood away from the gut. While this temporarily increases leaky gut, the accompanying activation of heat-shock proteins preserves the tight junctions’ integrity of the gut barrier. As Professor Woods explains,

“Exercise represents a hormetic stressor to the gut that stimulates beneficial adaptations and improves the long-term resilience of the gut barrier.”

Third, exercise stimulates the secretion of bile acids into the gut. Bile acids are one of the known factors in shaping the gut microbiota, owing to its antimicrobial properties that suppress harmful gut microbes. As follows, the absence or lack of bile acids are detrimental to gut health. As Jason Ridlon, an assistant professor at Virginia Commonwealth University who specializes in bile acids research writes,

“Decreasing levels of bile acids in the gut favor gram-negative members of the microbiome, some of which produce potent LPS, and include potential pathogens.”

What Animal Studies Show

In controlled animal experiments, physical exercise had consistently improved the gut microbiota profile — independent of other factors such as diet or body fat mass. There are 14 such studies to back this up, says Jeffy Woods, a professor of kinesiology at the University of Illinois Urbana-Champaign as a senior co-author in Exercise and Sport Sciences Reviews (2019).

The gut microbiota might even be responsible for the physiological benefits of exercise. Transferring the gut microbiota of individuals who exercised into mice mimicked the effects of exercise, such as increased insulin sensitivity and better control of blood sugar.

What Human Studies Show: Observational

Elite athletes had higher gut microbial diversity and abundance of beneficial gut microbes than lean sedentary people. Women aged 18–40 who exercised at least 3 hours/week had increased populations of butyrate-producing Faecalibacterium and Roseburia species and mucin-degrading Akkermansia muciniphila —all of which are markers of metabolic health.

Other studies have reported a correlation between cardiorespiratory fitness and more diverse gut microbiota. For example, one study calculated that cardiorespiratory fitness accounted for 22% of the variance of gut microbial diversity — independent of age and diet — in young adults.

What Human Studies Show: Interventional

One important takeaway research shows are that consistent exercise is key to friendly gut microbiota:

• In a 2018 study led by Professor Woods, sedentary adults underwent a 6-week endurance training program (30–60 mins for 3x/week) with strict diet control based on their previous diet. The exercise program increased several butyrate-producing bacteria and butyrate levels in their gut. After reverting to a sedentary lifestyle, however, the gut microbiota and butyrate levels also returned to baseline.
• A 2020 paper also shows that, in regular swimmers, a reduction in training volume led to a lower gut microbial diversity and lower abundances of butyrate producers, compared to when they were in their peak training period.

A second key point is that the gut microbiota of certain individuals might be more resistant to changes from exercise:

• A 2018 randomized trial with mainly overweight or obese adults found that an 8-week aerobic + resistance exercise (30–60 mins for 3x/week) did not significantly alter their gut microbiota profile. But the researchers also postulated that their diet may have skewed the results.
• A further 2018 study put sedentary overweight women into a 6-week cycling program (40–60 mins for 3x/week). Only about half of the participants’ gut microbiota responded positively to exercise — that was independent of age, body fat percentage, and diet.
• In a 2020 paper by Nature International Journal of Obesity, a 6-month randomized controlled trial showed that overweight/obese individuals in the exercise group had a 5% increase in gut microbial diversity vs. controls. “Exercise-induced subtle changes to the human gut microbiota,” the researchers concluded.
• Another 2020 clinical trial involving prediabetics showed that they are responders and non-responders in terms of exercise-induced alterations in gut microbiota. “The microbiome of responders exhibited an enhanced capacity for biosynthesis of short-chain fatty acids and catabolism of branched-chain amino acids,” the researchers explain.

“The microbiota of lean individuals may be more responsive to an exercise intervention than that of overweight or obese individuals,” agrees Professor Woods in 2019. Two additional studies from 2020 further support his proposition. While reasons for that remain unclear, it cannot be denied that research showed that exercise promotes a healthier gut microbiota profile characterized by increased diversity and butyrate-producers.

Although this may dishearten non-lean individuals, it also suggests that weight loss by means of proper diet and exercise may return compounding benefits. As you get leaner, the more likely your gut microbiota will respond positively to exercise and keep you healthier as a result.