Crunches may give you strong abs, but a new study hints that all exercise might have a deeper effect on your abdominal region—all the way down to your microbiome (the collective of microscopic organisms largely housed in your digestive system that govern your immune system and overall health).

Scientists from the University of Illinois at Urbana-Champaign rounded up 32 sedentary men and women to embark on a six-week endurance-exercise program without altering their usual diets. The workout intensity started out low and ramped up as the weeks progressed. Researchers checked gut flora at the beginning and end of the six weeks. They found that although each individual’s microbiome responded a bit differently, exercise did indeed seem to positively influence everyone’s gut bacteria. Specifically, it increased levels of microorganisms that help form short-chain fatty acids (SCFAs), which keep the colon healthy, and are linked to a lower risk of diabetes, cardiovascular disease, and inflammation-related diseases.

And now the fine print. It seems cosmically unfair, but lean people stand to gain more (microbiome-ly speaking) from exercise than do obese people. About half of the study participants were obese, while the other half were of normal weight. And those of normal weight boosted their SCFA levels much more than those who were obese. (Researchers aren’t sure why.) Of course, if obese people keep exercising, they might join the ranks of the lean and get those good benefits, so don’t despair if you’re on the bigger side.

Which brings us to the topic of not giving up. After the study, participants were asked to stop exercising for six weeks, at which point researchers retested their gut flora. Sadly, their microbiome-makeup had reverted nearly to its pre-study status.

In other words, if you want the gut-level benefits of exercise, you have to, well, exercise. I can just hear my microbiota asking, in a Janet Jackson-y sort of way: “What have you done for me lately?”

Kellee Katagi is one of those strange souls who actually enjoys working out for the sake of working out. She’s spent most of her 20-plus-year writing and editing career covering fitness, nutrition and travel, as well as outdoor sports ranging from skiing to spelunking to street luge (yes, that’s a thing).