In the 1950s, Dr. A.T.W. Simeons noticed in a study that boys with underdeveloped gonads who were being treated with hCG (human chorionic gonadotropin, a substance found in pregnant women) lost weight by eating less than usual, and that they weren't hungry. Although Simeons' initial study claimed the hCG diet was effective, subsequent studies disagree.
Most independent studies of the hCG diet show that subjects on a low-calorie diet who received a placebo lost just as much weight as those who received hCG injections. In one study, both groups reported severe hunger pangs.
The hCG diet involves daily injections of hCG, and a daily calorie intake of just 500 calories. The hCG helps the body release stored fat deposits. The burning of this fat combined with the 500 calories is supposed to be enough to sustain the body without intense hunger. But keep in mind that this model does not factor in exercise. If a dieter is exercising, he will need more than 500 calories!
Proponents of the hCG diet claim that while on the diet (which lasts no more than 43 days at a time), dieters break their abnormal eating patterns and food addictions, modifying their behavior for the long term while helping them lose weight in the short term. But if you do gain the weight back, the doctor or clinic you're working with may just put you on the diet again, in which case you'll never learn to eat right or exercise appropriately; you'll just continue to fill your body up with chemicals and give up on learning to live a healthy life.
So the real question is: Will the hCG diet actually change your eating habits and the way you look at food? This is the bottom line, because while diet fads come and go, the only thing we know for sure is that you have to eat right and exercise regularly if you want to maintain a healthy weight.