Thin people may be able to summon more mental defenses to resist tempting, high-calorie foods than obese people, U.S. researchers said on Monday.

Brain scans of thin people who looked at pictures of high-calorie foods showed increased activity in a region of the brain used for impulse control, but obese people showed little activity in this region, the researchers found.

I think there essentially may be biological reasons why people can't necessarily control their desire for food, said Robert Sherwin of Yale University School of Medicine in Connecticut, who worked on the study published in the Journal of Clinical Investigation.

U.S. researchers have found the brains of normal-weight and obese people react differently to pictures of energy-rich foods, with obese people having less activation in a brain area that can inhibit cravings.

The study is part of a push to understand an underlying biological processes that contribute to obesity, which affects more than one third of adults and nearly 17 percent of children in the United States.

The study involved 14 healthy people -nine thin, non-obese and five obese volunteers- who underwent brain scans two hours after eating.

The researchers manipulated blood sugar levels, testing the subjects when they had normal and low blood sugar levels.

Two hours later, they lay in a functional magnetic resonance imaging (fMRI) machine that scanned their brains as they looked at pictures of either food- or non-food-related objects.

The study, which was carried out by Kathleen Page and Dongju Seo of Yale University along with their colleagues, investigated which areas of people's brains are most active when blood glucose levels are either normal or slightly low.