The hormone leptin tells the body it’s full, and to stop eating

Obesity is a growing problem around the world. This trend worries health care experts because obese people are much more likely to develop chronic diseases, such as diabetes or heart disease, later in their lives. So researchers have been looking for some way to help people control their appetites.

A few years ago, scientists identified a hormone called leptin. Fat cells normally produce leptin, which sends signals to the brain telling it that the stomach is full. Unfortunately, says Dr. Edythe London from the University of California at Los Angeles, just giving people injections of leptin didn't keep them from eating too much.

"We think that in obesity there's a problem with sensitivity of the receptors to leptin," she explains. "So it's as if, you know, someone's shouting, but the shouting is falling on deaf ears. The leptin receptors just weren't turning on to the leptin." But London says that by studying leptin and how people respond to it, scientists have been able to learn about the ways in which the brain craves food.

To do that, London has studied members of a family who have a genetic mutation that makes them unable to produce leptin. They were all very obese. But when they were given leptin, they all lost weight easily.

London took brain scans of the family members both when they were receiving leptin and when they weren't, to see what was happening in their brains. "And what we did was show them pictures of food while they were in the scanner and we showed them pictures of high calorie food and low calorie food," she says. "When they were off leptin, we got a very large response to these food images in a part of the brain that is very important in craving for drugs of abuse… It's called the insula."

The insula also gets "turned on" when people addicted to cocaine see images related to the drug, or when smokers see pictures of cigarettes. When London gave her subjects leptin again, the activity in the insula went away, and she and her colleagues saw activity in different parts of the brain instead. "The activation was in regions that were linked to self-control and also the feeling of [satisfaction]. So it appears that leptin is modulating eating through these circuits that are turned on and turned off by the hormone."

London says these observations won't solve the obesity problem right away. But they do show scientists how the brain produces feelings of craving and satisfaction — and offer clues about future treatments for obesity.

London's research is published online in the Proceedings of the National Academy of Science.