I've read that in cases such as in anorexia or a lack of food, the body slows down the metabolism to preserve the little food it does get. If it can do this, though, why doesn't it keep your metabolism slowed, preserving energy? Are there effects to slowing down your metabolism?
Edit: I read it in various realistic fiction books about anorexia, which claimed to have accurate medical information. One was Wintergirls, by Laurie Halse Anderson, where the girl with anorexia says she has a slow metabolism, and her doctor mother says something to the effect of "your body's slowed down your metabolism because it thinks you're in a famine."