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November 13, 2003
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Has an ad for a big juicy burger ever made your mouth water? The smell of a bakery ever made you feel hungry?

As this ScienCentral News video reports, scientists are finding that your nose and eyes could be tricking you into eating more than you want to.

Food: You just can’t live without it

Scientists at the U.S. Department of Energy’s Brookhaven National Labs found that the mere display of food causes a significant elevation in brain dopamine, a neurotransmitter associated with feelings of pleasure and reward. The results of the study were published in the June issue of the journal Synapse.

The Brookhaven scientists had previously done extensive research showing that while addictive drugs increase the levels of dopamine in the brain, addicts have fewer dopamine receptors than non-addicts. They also studied the relationship of the dopamine system to obesity, and found that obese people also had fewer dopamine receptors than normal control subjects.

In this new study the scientists were trying to measure the role of dopamine in the food intake of healthy, normal, non-obese individuals. Dr. Nora Volkow, the study’s lead investigator, used positron emission tomography (PET), a brain-scanning technique, to measure dopamine levels in ten volunteers who were asked not to eat for several hours prior to the PET scan. Each volunteer was given an injection containing a radiotracer, a radioactive chemical tag which is designed to bind with the dopamine receptors in the brain in order to help the researchers locate them.

Then the volunteers’ favorite foods were warmed to enhance the smell, and the subjects were allowed to see it, smell it, as well as taste a small portion placed on their tongues with a cotton swab. But they were not allowed to eat it. Study subjects’ brains were scanned four times over a two-day period, with and without food stimulation. (When food stimulation was not used, subjects were asked to describe in as much detail as possible their family genealogy.)

All the study participants were also asked to describe, on a scale of 1 to 10, whether they felt hungry or desired food. This was done prior to food stimulation and then at five-minute intervals for a total of 40 minutes.

The PET scans revealed that even though they were not able to eat the food, volunteers’ brains released more dopamine during food stimulation than without food stimulation. According to Volkow, the implication of this finding is "if you’re exposed visually or by smell to food, even though you cannot eat it, your brain is going to automatically respond by liberating dopamine, and that liberation of dopamine is going to be associated with a desire to eat the food."

But to their surprise the researchers found this happening in an unusual location in the brain. The dopamine "did not go up in the pleasure centers as we had expected", says Volkow, "but in an area that is actually on top of those pleasure centers."

A survival trait

This particular finding was surprising because traditionally it had been believed that the motivation for eating food was regulated by dopamine which was directly associated with the pleasurable responses of actually eating food. But since the dopamine in this study was being released elsewhere, it seems that simply seeing or smelling food gives us a unique pleasure response that is not associated with eating. The brain might make us want to eat even when we may not be hungry.

So if we have a motivation to eat that is separate from the motivation caused by hunger, Volkow says it may be a more basic, instinctual drive. This may have been a useful survival trait in prehistoric days when we didn’t have refrigerators and fast food restaurants. In those days, Volkow says, "When you saw food you had to eat it because you didn’t know when there was going to be food again". But in this day and age when food is plenty, at least in most countries, this role of the brain might be detrimental.

Volkow also pointed out that genetics and environment interplay, and that some people may have a genetic predisposition to be more sensitive to these pleasurable responses of desiring food. That may put certain people at a higher risk to use food for gratification and therefore over-eat. "Stress makes us eat too," she says, "and this society puts us in stressful situations."

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