The Hormone That Says "Stop Eating!"

When it was first discovered, scientists dubbed peptide YY3-36 (PYY) the fullness hormone. That's because PYY — a protein produced by the gut — functions as a powerful appetite suppressant, at least in thin people. But would it work for those who need it most, the overweight and obese?

The answer, according to a study in the New England Journal of Medicine, is yes. When 24 volunteers were injected with PYY or a placebo and set loose on a buffet lunch, all the subjects who got PYY — fat and thin — packed away a third fewer calories. The same study revealed that obese people make a third less PYY than their thinner counterparts. No wonder they overeat!

But even if you could buy PYY in pill form, it might not work. The appetite is a complex system with many subtle triggers and controls. Remember leptin, the wonder hormone that seemed to make mice thin? A drug company paid tens of millions of dollars for the development rights in the mid-1990s only to discover that obese people were immune to it.