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.

When people eat way less food and exercise a lot for a few days, their body lowers the hormones that help build muscle and raises the stress hormone, which may explain why their muscles stop growing even if they eat protein.

Obese women on a low-calorie diet for 10 weeks show reduced activity of the leptin gene in their fat tissue, which is linked to hunger regulation.

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.