Binge eating changes the way the body responds to Leptin, the key appetite-suppressing hormone, say researchers in New York. Their experiments with rats could explain why people who overeat continue to crave food despite ginning weight.

As we eat, Leptin levels in our blood gradually rise and suppress the appetite until more energy is needed. Leptin also speeds up the metabolism, so that less food is converted to fat. "The object is to take you back to the original point," says Luciano Rossetti, a diabetes researcher at the Albert Einstein College of Medicine.

But a popular theory holds that if Leptin worked too well, it would defeat the purpose of fat, which is to store energy for lean times. Animals that can build up fat reserves when meals abound have a better chance of survival than their skinny companions should famine strike.

To understand how Leptin keeps fat in check without preventing it from accumulating vital reserves, Rossetti and his colleagues created feast and famine conditions for two groups of rats. Over three days, members of one group could eat all they wanted, while the other rats were kept on a restricted diet.

Normally, Leptin moderates its own production slightly by reducing the activity of the Leptin gene in fat cells. In the rats with a skimpy diet, however, an injection of Leptin into the bloodstream hardly reduced the activity of the gene in fat tissue at all.

By contrast, in the overeaters the injection cut Leptin gene activity in half. That came as a surprise, says Rossetti, as it means the overeaters would counteract an appetite-suppressing boost in Leptin, such as that caused by a meal, more quickly than the dieting rats. One possible outcome is that the guzzlers would be ready to eat again sooner.

Similar mechanisms in humans may have given them an evolutionary advantage in prehistoric times, when food was probably scarce. But today it may lead to a spiral of overeating, Rossetti says, with each binge making a person more prone to the next; "People go on numerous circles of increased feeding." Since the tendency toward obesity varies from person to person, Rossetti plans to look for differences in the Leptin binge response in lean and fat strains of rats.

Jeffrey Flier, a Leptin researcher at Beth Israel Hospital in Boston, says it would also be important to check whether moderately overweight people fail to increase the activity of their Leptin gene when they overeat. Without such experiments, he says, it's hard to conclude that Rossetti has discovered why inflated waist lines are so common. "At this stage, it's still purely hypothetical."