Obesity and the Brain: Is There a Gene that Makes Us Fat?

A new study just published in the October issue of Behavioral Neuroscience suggests there may be a genetic component to why some people tend to eat more than others. The experiment tested 29 obese and 45 normal people for their genotype, motivation to eat, and energy consumption. The genotype that was studied has to do with the dopamine system in the brain, specifically the gene determining the amount of dopamine receptors. One of the two alleles for this gene, called the Taq1 A1 allele, “has been associated with a 30%–40% reduction in the density of the dopamine D2 receptor (DRD2).” About half the world's population carries the A1 allele. Most A1 carriers are heterozygous, meaning they carry one A1 allele and one A2 allele. The other half is homozygous for the A2 allele, meaning they have a higher number of dopamine receptors. So what does dopamine (the molecule shown on the left) have to do with eating? According to the study, “The reinforcing value of food is related to activity of the dopaminergic system. Food consumption increases brain dopamine levels in animals and humans.” It should be noted that the “reinforcing value of food” is measured by how hard someone will work to get food not how much pleasure one derives from eating food.

The study involved two behavioral experiments. In the first, the participants were given various foods and told to rate them in order of taste and personal preference. However, the rating was designed to mask the real purpose of the task, which was to see how much the subjects ate when food was freely available. The second experiment had the subjects press buttons on two different computers. Pressing buttons on one computer earned them points to eat a favorite food, whereas pressing buttons on the other earned them points to read a newspaper. This task measured how hard the participants were willing to work to get food. Based on these data, the researchers were able to divide the subjects into two groups, one low and one high on “food reinforcement.” The latter group, the ones who were willing to work hardest to earn food, consumed the most calories. In addition, the high reinforcers that were carriers of the A1 allele consumed even more calories. Yet some people with the A1 allele were also low in food reinforcement, and there was no difference among the two genotypes in that group. Instead, the A1 allele combined with high levels of food reinforcement was what made people in this group consume the most. The researchers speculated that this powerful combination could be a major risk factor for obesity.

The reason the A1 allele and its corresponding reduction in the number of dopamine receptors may increase the reinforcing value of food is similar to the way drugs act on our brains. Indeed, the study mentioned that, “Food is a powerful reinforcer that can be as reinforcing as drugs of abuse.” When dopamine receptors are stimulated (picture on the right), they produce a feeling of reward for whatever actions were taken to stimulate them. Both drugs and food can increase levels of dopamine and thus increase the feeling of reward. Having more dopamine receptors makes it easier to experience the reward, and thus less of the rewarding substance is needed to get the feeling. In contrast, to those with fewer receptors, more of the rewarding substance is needed to get the same feeling, and in some people, that substance may be food. There are many drugs that act on the dopamine system, including drugs for Parkinson’s disease and schizophrenia as well as illegal drugs like cocaine. This study leaves open the possibility that a future treatment for obesity and overeating may come from drugs that affect the dopaminergic system, especially for those individuals that have a genetic propensity for higher food reinforcement.