Hungry or full, fat or thin: it is mostly a matter of hormones, dozens of them, carrying messages between the digestive tract, the fat cells and the brain. Eat. Don't eat. Burn calories. Store fat. Researchers at Stanford University are reporting that they have found a previously unknown member of this chemical cascade, a hormone with a much coveted power: it sharply reduces the desire to eat. The new substance, which the scientists named obestatin (OHB-statin), is made in the stomach and small intestine, and it seems to prompt the brain to send out a signal that says "eat less."