When the body is in extreme calorie deficit, the arms respond more strongly to fat-burning signals than the legs—possibly because the legs make more of a protein that blocks those signals.

Muscles with more slow-twitch fibers (used for endurance) tend to have stronger leptin signaling during extreme dieting, suggesting these fibers are better at switching to fat-burning mode.

When people burn a lot more calories than they eat for a few days, their muscles start responding more to a hormone called leptin, which helps them burn fat better—especially in the arms.