June 7, 2004
There are many other factors that affect this delicate balance. For example, laboratory evidence suggests that a diet that boosts your triglycerides—typically, one high in fatty, fried or highly refined foods—may interfere with both leptin's and insulin's actions on the brain, leading to an erroneous signal that the body is in danger of starving. The same receptors in the brain that are responsible for a marijuana high also boost appetite, which is why pot smokers get the munchies.

The more scientists learn about these biochemical, neurological and dietary factors, the more they marvel that anyone in our culture manages to stay thin, given the abundance and easy availability of food. If there's some kind of biological mechanism that protects certain people against weight gain, researchers haven't discovered it. By contrast, the evidence in favor of one that protects against weight loss is increasingly strong. Genetic variations clearly push some people toward bigger appetites, slower metabolisms and greater weight gain than others. "There are genes in the population that predispose to obesity," says Dr. Jeffrey Friedman, a molecular geneticist at the Rockefeller University in New York. "Obviously, there's an environmental contribution, but no one questions that genes are involved."

So the next time you stare in judgment at a fat person on the bus or bemoan your physique in the mirror, remember that nature has stacked the deck against weight loss. Trimming 25 lbs. from your figure may not be that difficult. But try shedding 100 lbs., and your body is going to scream. Whether willpower, exercise, drugs or even surgery is enough to quiet the body's basic need for fat is still an open question.

FOR PSYCHOLOGICAL REASONS
What deep inner urges drive some people to overeat and others to starve themselves?

Sometimes in order to fully understand a problem you have to study its opposite. That's why researchers trying to figure out what makes some of us so prone to obesity are taking a close look at patients at the other end of the bathroom scale: anorexics who starve themselves and bulimics who binge and purge. Could over-and undereating, scientists wonder, be two sides of the same coin, different forms of the same biological circuitry gone awry?

Although anorexia and obesity look nothing alike in clinical terms, there are similarities. People with both disorders tend to organize their days around eating and allow food to loom too large in their lives. "People who are anorexic and people who are overweight often begin to get phobic about food," says Dr. William Davis, of the Renfrew Center in Philadelphia, which treats patients with eating disorders. Food for them is much more than a source of nourishment; it can become a substitute for self-esteem and a vehicle for exercising—or losing—control over the body.

But scientists who have studied both groups say that underneath these similarities are two very different syndromes. "Obesity is at its base a metabolic disorder," says Dr. Lee Kaplan, director of the Obesity Research Center at Massachusetts General Hospital. Anorexia and bulimia, he maintains, "are primary psychiatric disorders."

The human body is designed to eat, and eating stops under normal circumstances only when the body senses it has enough energy for its immediate needs and enough stored away for future tasks. "It's hard to lose weight because the body wants to gain it back," says Kaplan. "In a competition between willpower and the body, the body always wins."

Except when an eating disorder has hijacked the process. Anorexics and bulimics are more successful at losing weight than people on diets because they have managed to throw this basic drive to eat into reverse. Through sheer force of will, anorexics convince their body that it doesn't need food. "Anorexics are able to do things that are clearly beyond what a normal person is capable of doing," says Kaplan. "Theirs is extreme behavior often driven by an inappropriate and distorted body image."

By contrast, people who gain weight are following the body's natural urge to hoard calories. Slight changes in the way you burn and store calories can lead, over time, to piling on the pounds. And while some of the factors responsible for these changes are within your control—how much you exercise or whether you take a second helping of ice cream—most are either inherited or the inbred responses of an organism that is designed to protect itself from starvation. Stress, sleep deprivation and long days packed with constant activity have a tendency to accumulate weight. "If we took away cars and television and computers, and stopped eating fast food, but were still exposed to the other stresses of modern life, I don't think we would eliminate obesity," says Kaplan. "We might dampen it, but we wouldn't eliminate it altogether."

And that's why treatments for obesity differ so much from those for anorexics. People who starve themselves tend to respond better to therapies and behavior-modification approaches that address their distorted body image and underlying emotional issues. People with overeating problems are often successfully treated with a more physiological approach, usually through diet and exercise and, sometimes, medications that curb appetite or burn calories more efficiently.

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