Well, the original rationale for the hcg diet was created by a doctor in the 50's who was injecting real hcg into patients for weight loss. the idea is that when a woman is pregnant, the hcg is responsible for freeing up all the calories and nutrients to be used by the baby. This is why a pregnant woman can eat total crap food and the baby is not harmed, but the mother is. He theorized that by freeing up the energy easier, a person can burn more calories. Of course, it's really difficult to tease out the difference between the injections and the low calorie diet. anyone is going to lose weight on a low calorie diet. Placebos can have a profound effect on motivation as well.
The hcg that is being sold is not really hcg, it's a serum that they drink, which dr. scally claims is impossible to be absorbed through our stomachs. It's a gimmick that motivates people to diet and exercise.