HCG (Human Chorionic Gonapotropin) is a hormone present in both men and women that is typically used to increase fertility. HCG levels are especially high in pregnant women, and subsequently it's been dubbed the "pregnancy hormone." The HCG hormone is thought to trigger our bodies to burn off more stored fat than we usually would, a belief that has lead to the foundation of the HCG Diet.

The HCG diet is fairly controversial for a number of reasons. Firstly, it restricts dieters to 500 calories a day -- well below the recommended daily amount -- is believed by virtually all qualified health professionals to be extremely unhealthy. Secondly, the FDA has only approved use of HCG for fertility treatments, and there are no comprehensive studies showing that the HCG hormone either works for weight loss or is safe in the long run. Finally, one of the diet's main promoters, Kevin Trudeau, is well-known in the weight-loss industry for being a convicted fraudster.

The first component of the HCG Diet is daily administration of the HCG hormones, either through an oral supplement or an injection. The second component of the diet is a strict eating plan that allows dieters to consume only 500 calories a day. Breakfast consists of only coffee or tea without sugar but with a tablespoon of skim milk if desired. Lunch and dinner on the HCG diet each consist of 100g of lean protein with one vegetable, one cracker and a small rationing of fruit. Regular doses of the HCG hormone are thought to reduce the side effects of extreme hunger, including nausea, headaches and hunger pains. There are no exercise requirements on the HCG diet.