Studies have shown that people who lose weight and keep it off tend to watch what they eat, whereas those who pack the pounds back on are less meticulous. A new study, albeit a small one, suggests brain differences are at work.

When people who had lost weight and kept it off for years were shown photos of food, they were more likely to engage the areas of the brain associated with behavioral control, compared with obese and normal weight participants.

"Our findings shed some light on the biological factors that may contribute to weight loss maintenance," said the study's lead author, Jeanne McCaffery of the Miriam Hospital. "They also provide an intriguing complement to previous behavioral studies that suggest people who have maintained a long-term weight loss monitor their food intake closely and exhibit restraint in their food choices."

The research, funded by the National Institutes of Health, is detailed in the American Journal of Clinical Nutrition.

Participants in behavioral weight loss programs lose an average of 8 to 10 percent of their weight during the first six months of treatment and will maintain approximately two-thirds of their weight loss after one year, McCaffery and colleague say. However, despite intensive efforts, weight regain appears to continue for the next several years, with most patients returning to their baseline weight after five years.

A separate study earlier this year found that when healthy eaters choose, say, vegetables over a candy bar, they use a small region in their brains that indulgers don't use. The region is thought to be associated with willpower, scientists say.

Three groups of volunteers were involved in the new study: 18 people of normal weight, 16 obese people, and 17 people who had lost at least 30 pounds and kept it off at least 3 years.

After a 4-hour fast, to ensure participants would be hungry, they were shown pictures of food items, including low-calorie foods (such as whole grain cereals, salads, fresh vegetables and fruit); high-calorie foods (including cheeseburgers, hot dogs, French fries, ice cream, cake and cookies), and nonfood objects with similar visual complexity, texture and color (rocks, shrubs, bricks, trees and flowers).

Brain scans documented their reactions.

Those in the successful weight loss maintenance group, when looking at pictures of food, exhibited strong signals in the left superior frontal region and right middle temporal region of the brain — a pattern consistent with greater inhibitory control in response to food images and greater visual attention to food cues, the researchers explained in a statement today.

"It is possible that these brain responses may lead to preventive or corrective behaviors — particularly greater regulation of eating — that promote long-term weight control," said McCaffery, who is also an assistant professor of psychiatry and human behavior (research) at The Warren Alpert Medical School of Brown University. "However, future research is needed to determine whether these responses are inherent within an individual or if they can be changed."