This is something that you'll hear repeated a lot, and it sounds logical. After all, isn't it risky to lose a lot of weight fast? Many people have tried really extreme measures − so-called "crash diets" where you might eat just 800 calories a day, combined sometimes with ingesting odd stuff such as cayenne pepper. Doctors worry, and rightfully so, about dieters eating so little that they deprive their bodies of vital nutrients such as potassium, magnesium and copper, which can cause heartbeat irregularities and other scary side effects [source: Miller].
Even when aggressive diets are safe, health experts have said they usually are counterproductive. The reasoning was that rapid weight loss slowed down your metabolism, so over the longer term, you'd stop losing weight and even start gaining it again [source: Miller].
But research doesn't back that argument up. According to a 2013 New England Journal of Medicine article, experimental trials showed that people who achieve rapid weight loss actually tended to weigh less at the end of longer-term follow-ups as well. "A recommendation to lose weight more slowly might interfere with the ultimate success of weight-loss efforts," the researchers concluded [source: Casazza et al.].