The diet requires that you eat only 500 calories a day, supposedly as part of an effort to help reset your metabolism and change your abnormal eating patterns, as the FDA explainsin its warning against the diet. (1) The HCGdiet also requires you to take a daily dose of the hormone human chorionic gonadotropin (HCG). Available in injections, pellets, sprays, oral drops, and pills, HCGis the hormone produced by the placenta during pregnancy.

In 1954, British physician A.T.W. Simeons theorized that HCGallows mothers-to-be to access fat reserves to feed their fetuses. He published a book,Pounds and Inches: A New Approach to Obesity,in which he suggested that HCGcould help people access their fat reserves and achieve weight loss.

The idea might have been that, if a woman had morning sickness or some other condition that prevented her from taking in ample nutrition at a certain point in her pregnancy, her baby would still have a reserve of energy to draw on, and the hormone HCGcould help facilitate access to that supply, Weinandy says. But, she notes, this really oversimplifies the numerous systems at work in pregnancy: A number of hormones are active, and the way they interact can sometimes even promote fat gain for the mother.

In fact, Weinandy says, this kind of speculation about HCGs role in weight loss can be really dangerous, and its sending a bad message to people because we dont know for certain how HCGworks. Another worry? Many chronic diseases, including breast cancer and prostate cancer, have been linked to an imbalance of hormones, so we cant predict what effects manipulating our hormones might have, Weinandy cautions.

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The HCG Diet: What Is It and Does It Work? - Everyday Health