For some unfortunate humans, sweaty workouts don’t bring sweeping health benefits. Now research suggests a reason: a person’s unique microbiome influences their body’s response to exercise.

Aimin Xu at the University of Hong Kong and his colleagues studied 39 men with insulin resistance, a sluggish response to insulin that signals a heightened risk for diabetes. Twenty men took part in a 12-week exercise regimen, which improved insulin resistance and other health measures in 14 ‘responders’ but not in 6 ‘non-responders’.

Analysis showed that certain bacterial genes were more abundant in responders’ guts after the exercise programme than before it. One such gene helps to break down amino acids that promote insulin resistance. But in non-responders, a different set of genes became more abundant post-exercise — among them a gene implicated in breaking down substances that promote insulin sensitivity.

When the team transplanted the participants’ gut microbes into mice, blood-sugar levels dropped in animals that received post-exercise microbiomes from responders. But animals that received bacteria from non-responders showed no such improvement.