While preliminary, the findings indicate that those who shed pounds -- and keep them off -- tap into regions of the brain related to control over urges.
"It may be that they actually recruit new brain regions to help with their weight loss," said study author Jeanne McCaffery, an assistant professor of psychiatry and human behavior at Brown Medical School, in Providence, R.I.
McCaffery and her colleagues wanted to understand how people react to "food cues" -- in this case, photos of food. "People make decisions about whether or not they're going to eat food, and that decision-making usually comes when they first smell or see the food," she said.
The researchers recruited several groups of participants: 18 people of normal weight, 16 fat people and 17 people who had successfully shed weight -- at least 30 pounds from their maximum weight -- and kept it off for at least three years.
The participants underwent brain scans as they looked at pictures of high-calorie and low-calorie foods. The MRI scans revealed that those who had successfully lost weight showed more activity in the parts of the brain that are associated with inhibition and in dealing with complex tasks, McCaffery said.
Those of normal weight didn't show this pattern. This may be because "they've been of normal weight all of their lifetime. The successful weight losers have to put in more effort to avoid eating foods or to control their response to food."
The findings appear in the October issue of the American Journal of Clinical Nutrition.
Ian McDonald, a professor of metabolic physiology who wrote a commentary accompanying the study, said questions remain.
For one, did the people who lost weight begin to have this brain response when they started shedding pounds or later? "Similarly, are the obese different from the non-obese because of an intrinsic difference or as a result of the inappropriate eating which has led to their obesity?" asked McDonald, a researcher at the University of Nottingham Medical School in England.
In other words, does inappropriate eating by heavy people lead to differences in the way their brains work when they look at food?
Also, McDonald said, future research needs to figure out what the differences in brain activity mean for the choices people make. "Similar measurements need to be made before, during and after weight loss," he noted.
For now, McCaffery said the researchers would like to understand better how the brain works in people who have lost weight successfully.
In the future, she said, it's possible that "we'll be able to teach other people how to do that."