Your muscles stay the same size over time because your body constantly builds and breaks them down. Eating tips the balance toward building, while not eating tips it toward breaking down — and over...

As we get older or if we're inactive or sick, our muscles don't build new protein as well after eating — and that's a big reason why we lose muscle, not because of changes in how our muscles work when we're not eating.

When young, active people eat more protein after working out, their bodies don’t just steadily build more muscle — instead, the way their body handles protein changes in a more complicated, up-and-down way involving both making and breaking down protein.