HCG is naturally produced in pregnant women and helps provide the fetus with the calories it needs to grow by using up the stored fats in the mother's body. This attribute of using up stored fats is what lies behind the hormone's use as a weight-loss treatment. However, the fat-busting power of hCG was actually discovered when using the hormone to treat delayed genital development in boys. It was noted that the boys ate very little, didn't seem to be hungry, and were losing weight. Doctor A. T. W. Simeons saw the potential to develop a diet using hCG.
Dieters receive a daily injection of hCG for between 23 and 40 days. After the third day (once the hCG is well established in the body), the dieters are supposed to reduce calorie intake to just 500 calories a day. Some versions of the diet also give the patients an appetite suppressant to ward off any hunger pangs, although the food that they are allowed is intended to stop them from being hungry. These foods should be high in protein and low in fats, carbohydrates and starches. Since 500 calories is not really enough to sustain a person throughout a day (even with minimal physical activity), the hCG prompts the body to start using up the old fat deposits to get the energy it needs. In this way, dieters can shed their excess pounds quite quickly. Treatment stops after no more than 40 days as the body seems to develop immunity to hCG after this time. Additionally, once all the stored fat is used up, the body will no longer be able to function on so few calories. It takes about three days for the hCG to cycle out of the body before the dieter can go back to a normal pattern of eating. It is intended that the days of low-calorie intake will also serve to break old eating habits, so that the dieter can maintain his or her new weight.