Exercise promotes health, reducing most people’s risks of developing diabetes and growing obese. But just how, at a cellular level, exercise performs this beneficial magic — what physiological steps are involved and in what order — remains mysterious to a surprising degree. Several striking new studies, however, provide some clarity by showing that exercise seems able to drastically alter how genes operate. Genes are, of course, not static. They turn on or off, depending on what biochemical signals they receive from elsewhere in the body. When they are turned on, genes express various proteins that, in turn, prompt a range...