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New evidence in the Brain's role in Obesity
People are obese because they love to eat, right? Not according to a recent brain study. Researchers found that obese people actually enjoy the food they eat less than lean people do, and therein lies the problem. In order to compensate for the missing pleasure, obese people eat more high-calorie food. In turn, overeating further dulls the enjoyment and begins a vicious cycle. They also found that people who carry a variant gene are more likely to gain weight.
To better understand the biology of obesity, Eric Stice, PhD, of the Oregon Research Institute and colleagues from Yale University and the University of Texas at Austin recruited volunteers, 43 female college students aged from 18 to 22 and 33 adolescent girls, ages 14 to 18. The young women ranged from very skinny to obese, according to body mass index calculations. Each participant was also tested for a gene variant known as Taq1A1, which is linked with a lower number of dopamine receptors in the brain. Dopamine is the primary neurotransmitter involved in the brain’s reward pathways and is released in response to eating. The amount released depends on the degree of pleasure the food brings.
First, the researchers showed both groups a picture of a chocolate milkshake and a picture of a glass of water. Their brain responses were monitored using a technique called functional Magnetic Resonance Imaging (fMRI). The heaviest women had the most activity in the dorsal striatum region of the brain, which is dopamine rich. But when the women actually tasted a chocolate milkshake or a tasteless liquid, the heavier women had less activity in their brains’ pleasure centers. The women with the gene variant had the lowest pleasure response when tasting the milkshake. They had to consume more of the shake to get the same pleasure response. Over the following year, these women were more likely to gain weight than the women with a stronger response. “The research reveals obese people may have fewer dopamine receptors, so they overeat to compensate for this reward deficit,” said Dr. Stice, who has studied eating disorders and obesity for almost twenty years.
Emmanuel Pothos, Ph.D., a neuroscientist at Tufts University has seen the same thing in mouse studies. He was not involved in the Stice study. “Obesity is not only a function of brain systems that regulate body weight, but a function of brain systems that regulate eating for pleasure,” Pothos says. “In mice, the central dopamine system—the system that underlies pleasure from eating—is defective. The animals have a very low response to stimuli that release dopamine and food is one of those stimuli.”
Past research has shown that biological factors play a major part in obesity, but this study is one of the first to identify factors that increase people’s weight gain risk in the future. Dr. Stice says that while this study may not provide the “magic bullet” to cure obesity, the results are key for understanding weight gain and to helping at-risk individuals. He says if doctors could determine who carries the at-risk gene, children especially could be steered toward “recreational sports or other things that give them satisfaction and pleasure and dopamine that aren’t food…and not get their brains used to having crappy food. You want to change people’s behaviors before they become entrenched.”
Dr. Stice is now looking at whether the pleasure circuitry in obese people can be reset by switching to a healthy diet. He has found that when obese people stop eating energy-dense foods, their craving for those foods goes down, not up. “If we can get obese people to improve the quality of their diets and stay the course for a long time, eventually they do much better in craving and their pleasure circuits should go back to their old balance,” he says.
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