Why do some people keep eating when they're already full?

1. They are undisciplined slobs.

2. They are gluttons.

3. They get signals from their brains telling them to.

A hormone called ghrelin sends us a signal motivating us to eat when our bellies are empty. It's part of a complex of hunger signals the body provides. This one associates pleasure with eating, and apparently it works just a little too well for some folks:

"What we show is that there may be situations where we are driven to seek out and eat very rewarding foods, even if we're full, for no other reason than our brain tells us to," said Dr. Jeffrey Zigman, assistant professor of internal medicine and psychiatry at UT Southwestern and co-senior author of the study appearing online and in a future edition of Biological Psychiatry.

Scientists previously have linked increased levels of ghrelin to intensifying the rewarding or pleasurable feelings one gets from cocaine or alcohol. Dr. Zigman said his team speculated that ghrelin might also increase specific rewarding aspects of eating.

Rewards, he said, generally can be defined as things that make us feel better.

"They give us sensory pleasure, and they motivate us to work to obtain them," he said. "They also help us reorganize our memory so that we remember how to get them."

Like aging, obesity is proving to be a highly complex set of interrelated factors. One reason that most diets (and other weight-loss programs) prove ineffective in the long run is that they address only one or two of these factors. If someone has spent years becoming hardwired to overeating, they have their work cut out for them. I don't know whether ghrelin is as addictive as, say heroine or cocaine, but I do know this -- it's a lot easier to create a heroine- or cocaine-free environment than it is a food-free environment.

And how, precisiely, does one get away from one's own brain?