Mar 5, 2012
In general, "we know that if you exercise your arms, you do not lose fat from your arms, but from all over," said exercise physiologist William J. Evans , director of the Nutrition, Metabolism, and Exercise Laboratory at the University of Arkansas for Medical Sciences . Exercise stimulates production of an enzyme called hormone sensitive lipase, which acts on fat stores all over the body.
But there is a wrinkle to this, albeit one you can't control, Evans added in an e-mail. The extent to which lipases are "turned on" appears to vary to some degree from one part of the body to another, and from person to person. "So, some women have a very difficult time losing fat from their legs, but no problem losing fat from their breasts or arms," he said.
In other words, you can lose weight, by using up more calories than you consume, but you have no control over where you lose it. And you can't turn fat into muscle, because the processes of losing fat and building muscle are separate.