Why We Eat

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Families can change all that. Picking better foods and preparing them healthfully certainly helps. But so does a return to the time when eating was seen not just as a way to fill up but as an opportunity to transact the business of being human. A set table and a balanced meal take a lot more work than a carry-out pizza. But the rewards are infinitely richer.

FOR BIOLOGICAL REASONS Our desire for food--and lots of it--is hardwired into our cells. Do our bodies want to be fat?

There's no question that some pretty strong social, emotional and behavioral forces play a part in determining what, when and how much we eat. But if you really want to know why some people are fat and others aren't, you have to take a good look at biology as well. Mother Nature simply can't afford to leave anything so important to human survival as eating to the whims of cultural fashion. Ten years after the discovery of the first obesity gene, scientists are only beginning to understand just how hardwired our desire for food--and lots of it--truly is.

What they are finding is an exquisitely fine-tuned system of chemical and neurological checks and balances that regulates both what we eat and how much our bodies store as fat. The average American consumes about 1 million calories a year--and, under normal circumstances, burns almost exactly that amount. The body achieves that balance by automatically increasing or decreasing the efficiency with which it performs various tasks, thus consuming fewer or more calories. (Most of the calories we expend are used to breathe, maintain body temperature, keep the brain chugging along, etc. Depending on how much you move, physical activity typically accounts for 15% to 30% of the total.) If you pack on a couple of pounds over the course of the year, your body's error rate is still less than 1%.

Accomplishing that feat requires a lot of communication and coordination among the fat cells, the liver, the muscles, the brain, the stomach and the gastrointestinal tract. Sometimes the signal is a molecule. Other signals are actually conducted along nerve paths. There are even mechanical signals, like the stretching of the stomach, which is one way the body says, "I'm full."

As if all that weren't complicated enough, the body must also regulate its food intake and manage its weight over time. "There are short-term signals and long-term signals," says Judith Korner, an endocrinologist at Columbia University in New York City. "Some signals are both short term and long term, and then there are medium-term signals."

As you might expect, the short-term signals are involved mostly with the initiation and completion of meals. Ghrelin, a hormone produced by the stomach, tells the brain, "It's time to eat!" When enough food leaves the stomach and reaches the small intestine, another hormone, called cholecystokinin, signals that the meal is over--and triggers the release of enzymes in the gallbladder and the pancreas.