Skinny on Obese Teens: It may be Brain Size, Not Lack of Willpower

A brain region in charge of controlling impulsively is smaller in obese teens than in lean ones, according to a new study.

The results suggest the bad eating habits that lead to obesity aren't simply due to a lack of self-restraint, researchers said. Rather, the smaller size of this impulse-control region in the brain might predispose certain children to gain weight . Or the obesity itself may even influence brain size, which in turn fuels uninhibited eating.

"It's not as simple as kids who have weight problems simply don’t have enough willpower to actually lose weight," said study researcher Dr. Antonio Convit, of the New York University School of Medicine and the Nathan S. Kline Institute for Psychiatric Research in NY.

"It could be that their brains are either wired differently, or damaged in a particular way that leads them to gain weight from overeating," he said. Convit and his colleagues administered questionnaires on eating behaviors to 91 adolescents. The teens were on average 17 years old; 37 were normal weight and 54 were obese. The obese teens had an average body mass index, or BMI, of 39, and weighed 300 pounds on average.

The researchers scanned the teens' brains using magnetic resonance imaging (MRI), and the teens completed several cognitive tests designed to see how well a particular brain region, known as the frontal lobe, was working. The frontal lobe is essential for planning, self-monitoring and modifying behavior. People who have damage to this area are less likely to have good impulse control, Convit said.

As expected, the obese teens were more impulsive in their eating behaviors. They also had a smaller orbitofrontal cortex, part of the frontal lobe, and performed worse on the cognitive tests than lean adolescents did. In addition, the adolescents with a smaller orbitofrontal cortex were more likely to eat impulsively, known in psychology as "disinhibited" eating.

Because the children came from families with the similar income levels , it's unlikely that their socioeconomic class played a role in their performance on the cognitive tests. And the results held even after the researchers took into account the participants' blood pressure, which may influence frontal lobe function, Convit said.

The researchers noted the study only shows an association, and not a cause-effect link. And they don't know whether it's obesity that may lead to a smaller frontal lobe, or the small frontal lobe that may contribute to obesity.

But there is a plausible explanation for how obesity changes the brain.

Obesity is known to cause changes to the immune system, boosting inflammation in the body. This increased inflammation may impact the brain and "lead to a vicious cycle, where the obesity leads to inflammation, which damages certain parts of the brain, which in turn leads to more disinhibited eating and more obesity," Convit told MyHealthNewsDaily.

And while the obese adolescents were not diabetic , they had elevated fasting insulin levels , which may also harm the brain. Insulin is a hormone that allows cells to take up sugar, or glucose, from the blood. Obese individuals, because they have insulin resistance, often need to produce more of it to keep their blood sugar levels normal.

The study was presented today (Oct. 28) at the American Academy of Child and Adolescent Psychiatry Meeting in New York City.

Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.