WEDNESDAY, Oct. 17, 2007 (HealthDay News) -- Chronic overeaters may have their DNA to blame, research suggests.
Scientists from the University at Buffalo say people with genetically lower levels of dopamine, a brain chemical that helps make eating and other behaviors more rewarding, may be driven to consume more food.
"We weren't studying obesity, per se, but motivation to eat. We wanted to understand how the brain regulates motivation to eat," explained study co-author Jennifer Temple, a research assistant professor of pediatrics.
Reporting in the October issue of Behavioral Neuroscience, Temple's team looked at genes associated with differences in brain activity, in particular the influence of a genetic variation linked to a lower number of dopamine D2 receptors on cells. About half the population has this variation, called the Taq1A1 allele.
According to the researchers, people with fewer of the dopamine receptors need to take in more of a rewarding substance -- such as food or drugs -- to get an effect that other people get with less.
Investigating further, the Buffalo team studied 29 obese and 45 non-obese men and women, aged 18 to 40. The researchers took DNA samples from inside each person's cheek to see if they carried the Taq1A1 variation.
"They came to the lab twice," Temple said. "The first visit, we gave them a large portion of six snack foods and told them it was a taste test. They rated the food on taste characteristics. We left the food in the room while they were completing the rating." Participants were told they could eat as much as they wanted, and the researchers took note of their intake.
On the second visit, the researchers evaluated each participant's motivation to eat. To earn a food reward, each person had to click a computer mouse 20 times. "To get more food, they had to click 40 times," Temple said. "We were looking to see how hard they were willing to work for food." The participants could choose food or the chance to read a newspaper as a reward.
"The combination of being very motivated to get food and having the genotype made people eat the most," Temple said. "We had people very high in motivation to get food who didn't have the genotype," she added, but those people still "ate less than people who were both motivated and had the [Taq1A1] genotype."
The bottom line: "A combination of [having] this genotype with being very motivated to consume food or with being obese seems to make people more prone to overeat," Temple said.
The study results do not imply that your genes doom you to obesity, however. "People who had the genotype were heavier, but there were certainly people who had the genotype who were not obese," Temple stressed.
While other research has turned up similar findings, Temple said her team looked at behaviors associated with the genotype. "Others have found that differences in the density of dopamine are associated with obesity," she noted.
In their future work, the team will use brain scans to reveal more about the relationship between the genotype and the drive toward eating.
Eventually, Temple said, the dopamine system may become a target for weight-loss therapies. For instance, drugs that affect the dopamine system, such as drugs now used for attention deficit hyperactivity disorder (ADHD), might help with weight-loss efforts, she theorized.
Another expert, Dr. Samuel Klein, director of the Center for Human Nutrition at Washington University School of Medicine in St. Louis, said the study "addresses an important and relatively overlooked area in obesity -- the contribution of reward addiction in the regulation of food intake."
Dr. Julio Licinio, professor and chairman of psychiatry and behavioral sciences at the University of Miami Miller School of Medicine, agreed. "This really is like another piece of the puzzle, showing there is a genetic component and that those with this genotype are likely to have different weights because of the food reinforcement."
Licinio published similar research last year, in which he found that people with a particular genotype for a receptor for the brain chemical serotonin were more likely to eat red meat than those who lacked it.
To learn more about nutrition, visit the American Dietetic Associaton.