America's Obesity Crisis:Eating Behavior: Why We Eat

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The hormones leptin and insulin are longer-term signals. Produced by fat cells, leptin helps manage just how much fat you store around your organs and under your skin through a complex feedback loop. If your fat deposits start to shrink--for example, when you lose weight--the amount of leptin in your body falls, a situation that the brain interprets as a result of starvation. The whole system of chemicals and neurological impulses shifts in an attempt to get the body to burn fewer calories so that it can regain the weight. The greater the weight loss, the stronger the signals to eat more and replenish fat stores.

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?