A very low-calorie diet (less than 1,000 calories a day) that causes rapid weight loss negatively affects your body in several ways:

• You lose lean body tissues (muscle and organ tissue). During starvation or when you eat a very low-calorie diet, about half the weight you lose is fat and the other half is lean tissue. On a more moderate diet, the loss is about 75% fat and 25% lean tissue. It is important to preserve lean tissue, since it increases your resting metabolic rate. This is one reason it is so easy to regain weight when you lose weight quickly.
• Your metabolism slows to conserve energy because the body thinks it is starving. A slower metabolism burns fewer calories.
• You may develop mineral and electrolyte imbalances, which can be life-threatening. This is the reason very low-calorie diets must be used under a doctor's supervision.
• You lose bone mass. This is more risky for women, because they are the ones who diet the most often, and they are also at higher risk for developing osteoporosis.
• Finally, regaining weight is almost certain on these very low-calorie diets. This is damaging both physically and psychologically.

Rapid weight-loss diets can be harmful to your health. For example, losing weight quickly can cause gallstones. The most common time for a gallbladder attack is after rapid weight loss. Don't use such a diet without talking with your doctor first. If you need to lose weight, it is better to lose weight slowly. You will be more likely to lose the weight safely and keep it off.