When a person suddenly stops taking in the amount of calories to which they are accustomed their body responds by slowing their metabolism. The body will act as if it is starving, which sometimes it is.
Metabolism is the rate at which a person's body burns the calories needed to function normally.
It is for this reason that rapid weight loss programs don't work in the long run. The body responds to the plunge in calorie intake by slowing the metabolism so that it can continue to operate on fewer calories. The dieter will hit a plateau in weight loss, which for some people stimulates a frustrating pattern of weight loss and gain, known as yo-yo dieting. Repeated attempts at rapid weight loss can permanently slow a person's metabolism, making weight loss harder with each diet. Generally, diet plans that are well balanced and do not fall below 1200 calories a day are manageable.