Obesity on the Brain

Low Number of Dopamine Receptors May Be One Reason for Obesity

by Dr. Barry Starr, Stanford University

October 30, 2008

In 2004, over 30% of Americans, or 90 million people, were considered obese. The Surgeon General has said that this level of obesity is responsible for over 300,000 deaths per year.

This is unacceptable. Scientists need to figure out why people are getting fatter and fatter. Perhaps by figuring this out, they can find ways to help people stay thinner. And healthier.

A new study links a certain DNA change with being obese. This change in the DRD2 gene, called A1, may cause people to experience less pleasure from food.

Brains and Food

The brain plays an
important role in
weight gain.

A big part of eating actually takes part in the brain. The brain tells us when we're full, when we're hungry again, and even how much pleasure we can get from food. Various studies have shown that differences in all of these can affect a person's weight.

Some people need to eat more to feel full. Others get hungry sooner. And still others need to eat more to enjoy their food. Each one of these can lead to obesity.

The new study looks at how enjoying food can affect weight. And how genes can affect the level of enjoyment someone gets from a meal.

Eating is necessary to live but can also, given the right foods, be a pleasurable thing to do. When someone eats something he or she likes, the brain releases dopamine. This dopamine meets up with dopamine receptors and the brain then registers that the chocolate milkshake or ginormous burger was a good thing.

People with the A1 version of the DRD2 gene have fewer dopamine receptors. So the theory is that these people need to eat more to get a jolt from their food. The new study set out to show if this is true by looking directly at women's brains.

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Less Pleasure, More Weight Gain

Obese people may find
food less pleasurable.

The study looked at the brains of women as they drank either a chocolate milkshake or a glass of water. The researchers were specifically looking at how blood flow changed in a part of the brain called the dorsal striatum. More blood flow should mean more pleasure from the chocolate milkshake.

The researchers did two separate studies. The first looked at 43 college students with a body mass index (BMI) that ranged from 23.8 to 33.2. The second looked at 33 adolescent women with a BMI range of 17.5-38.9. (A BMI from 18.5-24.9 is considered normal, 25-29.9 overweight and 30 or more obese.)

What they found was that obese women tended to have less blood flow with a chocolate milkshake than did women with normal body weight. They next looked at the women's DNA.

They found that women with the A1 version of the DRD2 gene had less blood flow to the dorsal striatum than did women with the other version, A2. When they combined these two sets of data, they found that the biggest difference in blood flow was between obese women with the A1 version and women in the normal BMI range who had the A2 version.

So it definitely looks like having the A1 version correlates with being obese. And that women with this version of the DRD2 gene tend to have lower blood flow to the dorsal striatum of their brains when they eat tasty foods.

More Work To Be Done

The researchers have not nailed down the why though. People with the A1 version of the DRD2 gene have fewer dopamine receptors. And dopamine is a big part of the enjoyment people feel from food. But the researchers did not show a direct correlation between the two. It is the most likely explanation but is not yet proven.

Also, the study was done on just women and so the authors caution against extrapolating it to men. A previous study had shown that both men and women with the A1 version of the DRD2 gene tended to need more food to get the same enjoyment as people with the A2 version. But this study looked at behaviors and not directly at the brain.

The next step will be to confirm that the reason for the decreased enjoyment is fewer receptors. Then scientists can get started on finding ways to help people who have the A1 version of DRD2.

Perhaps researchers can come up with a way to get these people to make more dopamine when they eat. Or maybe find a way to make the receptors they do have more sensitive.

In the meantime, it might simply help for people to know whether they have the A1 version or not. Perhaps by understanding why they overeat, they may be able to better control their eating.

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Content provided by the Department of Genetics, Stanford University.