WASHINGTON (Reuters) - The very sight of food causes the brain to react with pleasure, scientists said on Wednesday in a report they say shows why so many people are fat.

The reaction looks very different from the way the brain lights up when people actually eat--and explains the phenomenal success of advertisements for junk food and snacks, the team at the US Department of Energy's Brookhaven National Laboratory found.

"This shows us why all the advertisements about food are so powerful and why we are having problems with obesity in this country--because we are constantly being bombarded with food stimuli," Dr. Nora Volkow, a psychiatrist who led the study, said in a telephone interview.

The good news is that if people are aware of what is going on they may be able consciously to block the effects of advertising, the smell of bread in the supermarket and other stimuli that make people want to eat when they know they should not.

Volkow and colleagues used positron emission tomography (PET) brain scans to measure dopamine levels in 10 hungry volunteers. Dopamine is a key neurotransmitter, or message-carrying chemical, associated with pleasure.

"They were all of normal weight," Volkow said. "We asked them what their favorite foods were. Then first we studied them under a condition where there was no stimulation of food --we just asked them to please tell us about their family genealogy."

While this was going on, each volunteer underwent a PET scan. Writing in the June 1 issue of the journal Synapse, Volkow and colleagues said this gave them a baseline measurement for the next step.

'WE WERE BEING A LITTLE MEAN'

"Then we exposed them to the food they said they liked. But we told them they wouldn't be able to eat it," she said. "We were being a little mean for the good of science."

The dopamine levels shot up, she said.

This is perfectly normal, Volkow added.

"It is obviously a mechanism by which nature ensures that we actually consume food when food is available," she said.

"We never know when food is going to be available. Well, now we do because we have (convenience stores). But when we were evolving we didn't, and when there was food accessible you had to eat it."

The system worked well for millennia.

"Unfortunately, we really have created a system where we are flooded with information about food," Volkow said.

The result is that 60% of Americans are overweight and a quarter are obese, and at high risk of diabetes, heart disease and cancer. People in other industrialized countries such as Britain are rapidly catching up.

A second part of the study surprised the researchers.

Trying to make their results easier to measure, they gave their volunteers Ritalin, known generically as methylphenidate. Best known for its use in children with attention deficit hyperactivity disorder, it can amplify the effects of dopamine in the brain.

It amplified the responses of the volunteers to food--something Volkow did not expect.

"Methylphenidate is known to take away your appetite," she said. "It's a problem because children don't want to eat while on methylphenidate."

But she thinks she has figured it out. The mechanism involved means that methylphenidate increases appetite, but only when food is presented under unusual circumstances.

"So if you want your child to eat while on Ritalin, try to give him the food in non-regular way. Don't sit him down but present him with food that he is not expecting to have and then entice him to eat. Give him a novelty," she said.

Reference Source 89

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