Researchers have known for a while that the microbiomes of endurance athletes are different from the microbiomes of the rest of us, but they haven’t known if exercise if responsible for that. Now they do. The New York Times reports on a study by researchers at the University of Illinois at Urbana-Champaign, that tracked changes in the guts of people who started a new exercise routine. The researchers took blood and fecal samples and tested the aerobic fitness of 32 people, half of whom were obese. Then they had the men and women begin supervised workouts, during which their efforts increased to about an hour of vigorous jogging or pedaling three times a week. Blood and fecal samples collected after two six-week periods showed that the bacteria did change, with some increasing and some decreasing, and everyone’s changing in unique ways. The Times reports that one consistent change was an increase in microbes that can help to produce short-chain fatty acids, which appear to reduce inflammation and bolster metabolisms. The bad news? Most changes in people’s guts dissipated after six weeks of not exercising, and the gut bacteria reverted to what they had been at the study’s start.