Everyone has a certain number of muscle fibers, but the thickness is different. If you exercise regularly, the blood flow will be accelerated, and the more energy the muscle fibers get, the thicker they will naturally grow. On the contrary, it will shrink due to insufficient nutrition supply.

When people exercise, they usually rely on blood sugar to provide energy. If they don't eat and their blood sugar is insufficient, they will break down fat and produce glucose. If they don't have enough fat, they have to break down protein in muscles, which is similar to muscle atrophy.