metabolism is the set of chemical reactions in our cells and the body which convert what we eat into all the energy we need for daily living. The main reason why, you actually, scientists think that we do n put on waight as we get older is simply because we're consuming more calories,. So we're eating more highly processed, calory heavy foods. But if you're leading a very sedentary life with no exercise at all, then things like stress are more going to be used for other processes within your body such as aging.