It's a pretty common lament, the idea that you just can't eat what you used to. But why is that so? And is it avoidable? There are a number of reasons why we put on the pounds as years go by, but take heart: There are ways to fight back — and win!

There are some particular biological changes that happen as we age. For one, aging muscles actually contribute to the increase in the amount of fat we store in our bodies, says Cheryl Phillips, president of the American Geriatrics Society.