Juleen Zierath and Romain Barrès at the Karolinska Institute in Stockholm, Sweden, and colleagues looked for epigenetic changes – the addition of a methyl group to genes – in muscle cells during strenuous exercise. To do so, the team collected biopsies from the thigh muscles of eight men who led relatively sedentary lives, both before and after an hour of exercise.

Several genes involved in fat metabolism that were methylated before the exercise lost their methyl group. Such demethylation allows genes to more easily make proteins, which suggests that more proteins involved in the breakdown of fat are being made after exercise, says Zierath.

The group was surprised to see these effects happen so quickly. They think calcium, produced in muscle cells during exercise, may be involved since subjecting the same biopsies to caffeine – which also increases calcium in muscles – caused the same demethylation.