A team of scientists led by researchers at the Mayo Clinic recently discovered that forcing mice to exercise led to exacerbated inflammation, whereas allowing the mice to exercise voluntarily attenuated inflammation. They hypothesized that the gut microbiome may be responsible for this rather peculiar phenomenon, so they repeated the study, this time measuring changes in the microbiome. They published their results in the Journal of Applied Physiology.

The scientists separated 29 mice into three groups, a group that was forced to exercise for 40 minutes per day, a group that voluntarily exercised, and a sedentary group that did not exercise. They conducted their study over 6 weeks, after which the mice were euthanized and their microbiomes sampled. The researchers discovered that even though each of the groups gained approximately 10% body weight over the course of the study, their microbiomes deviated. Interestingly, the mice that voluntarily exercised, those same mice that attenuated inflammation, had the lowest gut microbiome diversity (a trait normally related to dysbiosis and illness). On the genus level, Mollicutes, which has been associated with ulcerative colitis, and Nauatilia were elevated in the feces of mice forced to exercise. In addition, Turicibacter, which is related to various organisms associated with IBD, was actually decreased in the mice that voluntarily exercised. Beyond these loose associations though, the exercise’s impact on the microbiome was not clear.

Exercise can change the microbiome, and the authors suggest that the total amount of exercise between the voluntary and forced exercise groups may have been important, as it was not controlled in this study. Strangely, in humans, excessive exercise and no exercise at all have both been linked to gastrointestinal issues like diarrhea and constipation. Unfortunately, the authors did not study whether the mouse version of Jillian Michaels trainers constituted forced, or voluntary exercise.