Drastically cutting the number of calories you eat may sound like a fast way to lose a few pounds, and it can be, but in the long run, it's bad news. In fact, there's a high likelihood that crash dieters who restrict the number of calories they consume to 800 or fewer per day will, in addition to being hungry, regain the weight they've lost within the first six months after giving up the diet.
Eating too few calories, less than 1,200 per day, doesn't just cause fat loss; it also causes muscle loss. And in the long term, severe calorie restriction can cause anemia, dizziness, headaches, low blood sugar, liver and kidney problems, and a host of other complications. It also triggers the body to conserve its potential energy (that means it hoards calories and uses them slowly). Crash dieters often find that their body fat percentage ratio is worse after dieting than before they started cutting calories [sources: Hensrud, Zelman].