When you diet, your body thinks it is being starved, burns calories more slowly and stores up fat. This physical reaction to calorie restriction has evolutionary benefits. In ancient times, when the food supply wasn't constant, it was the people whose bodies had large fat stores who survived. Dieting can cause feelings of deprivation and depression that often lead to overeating. Diets that eliminate red meat and vegetables may leave you tired, irritable and even anemic.
Strict vegetarians are prone to iron-deficiency anemia.
Failure to lose weight, or regaining the weight you just lost, can make you feel even worse about yourself than before. Diets can cause tension between children and parents. Overeating may not be the cause of excess weight. Very low-calorie diets can interfere with a girl's periods. Diets that don't provide adequate nutrients can weaken your immune system, so you are more likely to get sick.