The rats in Bart Hoebel's laboratory at Princeton University are sugar junkies. They binge on sugar syrup every day, pressing a feeding bar frantically for hours at a time. Deprived of sugar for just 24 hours, they show signs of withdrawal any human addict would recognize: chattering teeth, trembling paws, wobbly heads.
Hoebel's sugar addicts aren't the only rodents jonesing for junk food. Rats at Pennsylvania State University in State College are fat fiends; they gorge themselves on Crisco. At the National Center for Scientific Research in Bordeaux, France, rats overindulge in a cocoa-flavored breakfast cereal called Choc and Crisp. At the University of Alabama at Birmingham (UAB), the rat junkies binge on Double Stuf Oreo cookies, consuming twice as many calories per day as normal rats when they're stressed out, a mood state induced by mild electrical shocks delivered through the metal floor grid.
All the animals are unwilling recruits in the scientific quest to understand appetite, a fundamental human drive whose complexities have long frustrated researchers and dieters alike. Craving and bingeing are anomalies in rodents, but they're common in people. Studies of eating behavior show that most men and women go on occasional eating binges and experience food cravings that feel overwhelming.
Such findings should come as no surprise; eating is more important to an individual's survival than even sex, and it's in everybody's interest to make sure the urge to consume stays strong. If you're one of the millions trying to lose weight or lower your cholesterol, you know firsthand just how powerful a force appetite can be. In this land of plenty, where there's little chance of starving, our appetites have become downright dangerous.
"We're overeating, eating way beyond our caloric need," says Mary Boggiano, a psychologist at UAB. "Sixty-four percent of the population is overweight." Half that number—fully one-third of the U.S. population—is not just overweight but obese, facing increased risks of heart disease, high blood pressure, cancer, diabetes, and arthritis. The steep rise in obesity over the past two decades has been called an epidemic and deemed a factor in hundreds of thousands of deaths a year. Despite policies aimed at reversing the trend, "current data indicate that the situation is worsening," warns the U.S. Centers for Disease Control and Prevention.
So researchers are studying rat junkies—and human subjects, too—in the hopes that a deeper understanding of appetite will yield strategies to obstruct it. The studies explore psychology as well as physiology because, as every dieter knows, appetite afflicts the mind as much as the body. Hunger and craving may originate in the flesh, but they manifest in that zone of consciousness where want and need are easily confused. To clarify the difference, it helps to distinguish between "brain hunger"—the mind's desire to eat for pleasure—and "stomach hunger"—the body's demand to eat for energy.
"It doesn't take that much food to quell stomach-hunger pains," says Boggiano, who struggled with bulimia for years. Rather, it's brain hunger, she says, that triggers bingeing behavior and cravings for specific foods. A double whammy of brain and stomach hunger may make it feel impossible to trim off extra pounds. It's clear no theory of appetite will succeed unless it can account for both kinds of hunger. No diet will, either.
The Orchestra Within
Stomach hunger is appetite at its most basic. It's created by a network of interrelated metabolic pathways that monitor and maintain the body's energy status in a balance known as homeostasis. If you skip dinner to work late and get stuck in traffic on the way home, your growling stomach will remind you that regular meals are a necessity, not a luxury. An empty stomach churns out a hormone called ghrelin, which stimulates hunger; and as blood-sugar levels drop between meals, the level of insulin in the bloodstream declines, too. If you pull over and get a bite to eat, your blood sugar and insulin levels will spike, and a hormone called leptin, produced by fatty tissue, will be released to initiate feelings of satiety. These visceral messengers—ghrelin, leptin, blood sugar, insulin—and others communicate with the central nervous system, whose job it is to stir up or discourage appetite.
The relay center that translates into hunger or satiety all the metabolic signals from the outlying districts is the hypothalamus, a cherry-size structure deep in the brain. Its central role in appetite regulation was clearly demonstrated in the 1940s, when scientists discovered that rats with surgical damage to a region of the hypothalamus called the ventromedial nucleus ate voraciously. The postoperative animals, barely recovered from anesthesia, chowed down with such abandon that some literally inhaled food particles until they had trouble breathing. They gained more than 10 percent of their body weight in the first 18 hours post-op.
People with comparable brain lesions behave the same way. In the 1960s, doctors admitted to New York Hospital a young woman with a hypothalamic tumor who threw fits of violent rage whenever food was withheld from her. In two months at the hospital, the woman gained nearly a pound a day before dying of a heart attack. Such observations amply confirmed that the ventromedial hypothalamus (VMH) is the satiety center, in charge of suppressing appetite. In the 1950s, a nearby region—the lateral hypothalamus (LH)—was implicated in triggering appetite. Rats with LH lesions ate less and got skinny.
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