Around 1980, scientists began uncovering how rich, sweet tastes make the brain go wild.
Researchers in many laboratories found that giving rats morphine made them eat more fatty and sugary food.
Later experiments would show that injecting opiates directly into one of the brain's main pleasure centers, the nucleus accumbens, prompted rats to eat up to six times as much sweetened lard as they normally would.
Reversing the binges was simple. Scientists gave the rats opiate-blocking drugs such as naloxone, used in people to counteract heroin overdoses.
In each case, blocking the brain's ability to use opiates dulled the appeal of fat and sugar, while giving opiates magnified the rewards of such foods.
That led to a startling conclusion: The same sort of opiates that create the high of drugs such as heroin also shape how the brain gets pleasure from food, especially those high in fat and sugar. Eating-related pleasure seemed to come from chemicals known as endogenous opioids, produced within the brain itself.
Putting rats on a fat binge was one thing. The challenge was to see whether sweet snacks had similar opiatelike effects in humans.
In the 1990s, Adam Drewnowski, now director of the University of Washington's nutritional science program, led a University of Michigan study showing that Oreos and other sweet snacks act on the same brain pleasure centers that respond to addictive drugs.
Drewnowski said he got the idea from a line in the 1986 Bob Hoskins' movie "Mona Lisa," in which a heroin junkie talks of craving ice cream. The notion that a sweet taste could quench an addict's longing sounded right to Drewnowski's colleagues, Dean Krahn and Blake Gosnell, who had studied opiates in rat brains with Levine at Minnesota.
The pattern they had found in rats also applied to people. Bulimia patients in the 1995 study who were injected with opiate blockers ate less of the sugary foods they liked to binge on--including Oreos, Snickers, M&Ms and chocolate chip cookies, Drewnowski said.
Such work didn't show that snacks were addictive; the effect in Drewnowski's study was strong only for binge eaters. But it proved that the allure of such food goes beyond being tasty.
Other scientists found that blocking opiates in the brain changed even healthy people's basic perceptions of sugar.
"They said they could taste that it was sweet, but it wasn't quite as interesting," said Levine, who led some of the research.
In a brain-scan study last year, scientists found that the thought and sight of ice cream set off the same neurological pleasure centers in healthy subjects as the images of crack pipes did for drug addicts.
Food companies and many nutritionists note that such research doesn't negate the need for consumers to take responsibility for what they eat.
Oreo fans agree, saying they choose the cookie simply because it has a taste they can't find anywhere else.
"They're obviously not a health food, and they don't market themselves as such," said Ryan MacMichael, 29, an Internet specialist from northern Virginia who said he eats about four Oreos a day.
Yet MacMichael and other consumers compared the cookie's appeal to that of a drug. "It's some kind of rush that once you get a taste of it, it's hard to not eat it," he said.
Harder for some than for others. Just as many people can stop at one glass of wine and others become alcoholics, genetics and family dining habits make certain individuals more vulnerable to overeating, according to new research.
"I think what's going to be coming out is that food is like alcohol," said Marcia Pelchat, who studies food cravings at Monell Chemical Senses Center in Philadelphia. "There are some people, who for genetic or environmental reasons, can't handle it very well. But the vast majority of people aren't like that."
Most biologists believe the tendency to put on fat in times of plenty helped humans thousands of years ago, when famine was a constant threat. Sweets were so rare and alluring that Australian aborigines would tie small leaves to bees and chase their flight to the honey-filled hive.
Now that overindulgence is easy, that biological heritage has become a millstone. Americans today have greater access to calorie-rich, intensely craved foods than any people in history, putting willpower to the test.
"There's an illusion that you have complete control over how much you weigh--in contrast to how tall you are, or what color your eyes are, or all the other things we have to accept," said Jeffrey Friedman, a researcher on the genetics of obesity at New York's Rockefeller University.
"The notion that there might be a biological system that evades our conscious control is not attractive to a lot of people."
Kraft's taste for brain research
To understand food's effects, Kraft studies the brain. At times the company has shared expertise with nicotine researchers.
The implications of brain science are of great interest to food companies such as Kraft.