In brain-imaging studies of human subjects, Wang and Nora Volkow, director of the National Institute on Drug Abuse, have shown that the mere sight and smell of barbecued chicken, hamburgers, and pizza release dopamine in the striatum. The amount released correlates with the strength of the subjects' yearning to eat: the subjective impression I want that. "This is how our brains control our desire," says Wang.
That desire can easily get out of hand. The dopamine reward system is implicated in compulsive gambling and drug abuse, as well as in eating and sex. Consequently, Wang and other researchers have begun to suspect that obesity, eating disorders, and even the ordinary urges of appetite might resemble addiction. Bart Hoebel's studies of rat junkies show that every drop of sugar syrup they swallow causes a surge in their dopamine levels—a benchmark of desire and a biochemical marker of substance abuse.
"Boosting dopamine time after time is what drugs of abuse do," Hoebel says. "That makes you wonder whether food might have addictive properties."
Actually, it's the other way around: Drugs have addictive properties because they tap into appetite's pleasure network. Food, you might say, is the original addiction. Edibles high in fat and sugar are known to cause release of the group of feel-good chemicals known as opioids, which mask pain and promote euphoric sensations. The same brain receptors that bind the opioids released by Double Stuf Oreos also respond to morphine and heroin, with more pronounced results.
"Food gives you a modest physiological response via those pathways; drugs give you a tremendous response," says psychiatrist Walter H. Kaye, director of the eating-disorders program at the University of California, San Diego. "Drugs hijack the food-reward pathways."
So it's not surprising that food can become as much of an obsession as controlled substances. The question is how. Like human addicts, Hoebel's sugar junkies develop hypersensitive dopamine receptors that overreact to a variety of drugs of abuse, and the changes are long-term; even after a month of abstinence, the taste of sugar incites the rats to addictive behavior. Boggiano's Oreo-bingeing rats have long-term changes in their brain opioids that somehow make them unusually responsive to highly palatable foods. Even a single morsel of cookie will trigger a binge, much as a single drink can send an alcoholic on a bender. The physiology behind dopamine and opioid sensitization isn't yet clear. What's alarming for ordinary human eaters are the circumstances that create such obsession.
While extreme diets may prime the brain's reward system for bingeing behavior, researchers are finding, to their dismay, that any kind of weight-loss diet sets you up for biochemical warfare with the stomach-hunger network. The body, it seems, doesn't "know" when it's overweight; it only "knows" when it's in jeopardy of losing weight.
Weight-loss dieting, by definition, requires lowering food intake below the amount the body needs to maintain its present form. So, in a valiant attempt to regain homeostasis, the dieter's stomach-hunger system lowers levels of the satiety signals leptin and insulin and pumps the hunger hormone ghrelin into the bloodstream. Scientists still don't know how the brain- and stomach-hunger systems interact to support or override each other. What's certain is that, when you diet, you're up against your appetite's wants as well as its needs; you get clobbered with both kinds of hunger.
"A man can do what he wants, but he can't want what he wants," Schopenhauer declared. The German philosopher's grim perspective certainly applies to appetite. Between the twin imperatives of brain and stomach hunger, there doesn't seem to be much room for free will. Yet if the potency of appetite seems excessive, consider this: How many meals would you skip if you never had any cravings? The answer is probably too many. If eating were an experience as neutral as breathing, would we bother to spend hours each day shopping, cooking, and blowing our paychecks in restaurants? Or would we have to remind ourselves, every so often, to take time out for a bite—the way we often catch ourselves holding our breath, and remind ourselves to breathe?—Karen Wright
Stress Eating: Dieting Makes You Do It
Lab rats aren't bingers by nature; they have to be trained to binge by repeated cycles of food restriction and junk-food feasts. The feast-and-famine cycles look a lot like yo-yo dieting, says UAB's Mary Boggiano. She suspects that extreme dieting, combined with occasional sugar-and-fat-filled sprees, might prime a person's neurochemistry for bingeing behavior, creating the same long-term sensitization in the dopamine and opioid systems that occurs in addiction. The changes could make a person especially vulnerable to bingeing in response to stress, because stress hormones can also stimulate cravings for high-calorie comfort foods.
"Half of us overeat when we're stressed, and half undereat," Boggiano says. The people who overeat tend to be those with a history of dieting. "That's a real message. You can't prevent stress, but you can control whether or how you diet."
Diets to avoid include crash diets that severely limit calories, she says, or those that eliminate particular food groups, such as the Atkins diet. Diets that emphasize moderation and allow all food groups, such as the Weight Watchers approach, are less likely to oversensitize the brain's dopamine and opioid systems.
Help Yourself
Whether you weigh 130 or 330 pounds, your hunger networks are engineered to defend your body's existing weight. Still, that's no cause for diet defeatism. We can still attempt to regulate weight.
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