Thin people might be able to gather more mental defenses that obese at the time to resist the temptation to eat foods high in calories, according to a study in the United States.
Brain scans of people looked at pictures of thin high-calorie foods showed increased activity in a brain region used to control the impulse, while obese men showed little activity in this brain area, researchers said.
"I think it would essentially biological reasons why people can not control your desire for food," said Robert Sherwin, Faculty of Medicine at Yale University in Connecticut, who worked on the study published in the Journal of Clinical Investigation .
The study is part of an attempt to understand the underlying biological processes that contribute to obesity, which affects more than one third of adults and nearly 17 percent of U.S. children and is becoming an epidemic world.
Experts from Yale and the University of Southern California imagery used functional magnetic resonance imaging (fMRI) to examine which brain areas were activated when people viewed images of high calorie foods, healthy foods like fruits and vegetables, and other things non-food.
The study included 14 healthy volunteers - nine lean and five obese - who underwent brain scans two hours after eating. The researchers manipulated the levels of blood sugar, when evaluating the subjects had normal amounts and low glucose.
The team found that when blood sugar levels were low, regions of the brain called insula and striatum - linked to the rewards - were active, signaling a desire to eat.
The prefrontal cortex, which normally discourages the desire to eat, was less able to curb the signals generated from the striatum to feed.
That was especially in the obese participants who were shown pictures of energy dense foods.
But when blood sugar levels were normal, lean subjects showed increased activity in the prefrontal cortex and that reduced activity in brain regions associated with rewards.
"This is a driver, a higher function that controls the reward centers. This driver is deficient in people with obesity. They is not activated that system," said Sherwin.
He noted that larger studies are needed to confirm the results, but added that research suggests that obese people would be less able to clear areas of the brain that lead to the temptation of food. "That probably contributes to obesity," he said.