Likewise, the components of our diet have undergone a radical change. The flesh of the wild game that made up our ancestors' diet had just 3% to 4% fat, whereas prime beef has 30% or more. And prior to the domestication of crops such as wheat and corn, humans consumed a variety of wild grains filled with fiber, which slows digestion. The process of highly refining foods, which allows carbohydrates to be quickly absorbed by the digestive system, wasn't widespread. As Rutgers University anthropologist Lionel Tiger puts it, human metabolism "did not evolve for prime beef, but, one would surmise, neither did it evolve to eat heaping plates of white rice and pasta." Nor heaping plates of doughnuts and chocolate-covered croissants, for that matter.

Why, then, do we like these foods so much? For answers, researchers are once again turning to laboratory animals, which exhibit many of the same dietary proclivities we do. Rats, for example, will labor mightily to obtain a sugar pellet even after they have dined on rat chow and aren't particularly hungry. The reason, thinks Allen Levine, director of the University of Minnesota's obesity center, has a lot to do with sugar's impact on mood-enhancing circuits in the brain. Sugar gives rats—and by extension humans—a buzz.

There is also reason to think that our penchant for making unhealthy choices might be enhanced by the abundance of particular foods. Consider the results of an experiment recently conducted at Philadelphia's Monell Chemical Senses Center. Rats given one cup each of fat, protein and carbohydrate were found to make balanced diet choices, eating a tad more protein than carbohydrates and a tad more carbohydrates than fats. But rats presented with more cups of fat and carbohydrates than protein dramatically increased their consumption of the former while sharply curtailing their intake of the latter—to the point, in some cases, that the rats became protein deficient. Why would rats do this? Perhaps because their brains are hard-wired to take advantage of sudden windfalls of food, and in the wild, of course, such windfalls do not occur every day.

They do, however, in restaurants. Soft drinks are now delivered in one-liter cups, observes Judith Stern, vice president of the American Obesity Association. Even the venerable Joy of Cooking has capitulated to the trend; recipes that used to provide meals for six now feed only four. So cheap are carbohydrates and fats that supersizing costs the food industry next to nothing.

Adults are free agents. They can overeat and gain weight if they want to. But perhaps what is most disturbing about their overeating is that they are unwittingly, and in myriad ways, passing on that tendency to children. It's not just that children pick up their parents' bad habits. There is growing evidence that what you eat early in life can permanently boost your body's desire for food.

If we were to start from scratch, how would we design a diet to keep our weight under control? For starters, we could concentrate on diets geared for life rather than quick and easy weight loss. "The people I see are great dieters, beautiful dieters," says Dr. Cheryle Hart, founder and medical director of the Wellness Workshop in Spokane, Wash. "They can deny themselves, but only for so long. Then they snap. We all would." Second, we could stop paying such close attention to every jot and tittle in the diet debate. It will take decades for researchers to unravel all the reasons we eat what we do, and why we like to eat so much of it. But a few insights are emerging that should point us in the right direction, as long as we don't turn them into inviolable dietary laws.

Not all carbohydrates seem to have been created equal. So-called simple carbohydrates, those found in white bread and cake, are so quickly digested by the body that they trigger a very rapid rise in the levels of glucose in the blood. The pancreas releases a massive amount of insulin to mop up the excess. Soon enough, however, blood glucose levels plummet to the point where our brains may feel woozy, we become excessively hungry and are driven to eat again. Complex carbohydrates, on the other hand, particularly those rich in fiber, do not elicit the same kind of spike-and-crash response.

Researchers refer to the amount of glucose a single serving of a particular food releases in the bloodstream as its glycemic load. And there is growing evidence that we can manipulate it somewhat to control our hunger. Broccoli and peanuts, for example, have a low glycemic load, while instant rice and baked potatoes have an extremely high one. Avoiding sugar-laden processed foods and increasing the consumption of fruits and vegetables is a first step toward sensible eating, says Dr. David Ludwig, director of the Optimal Weight for Life Program at Children's Hospital Boston. Choosing small portions of pasta and bread made from coarsely ground wheat is a good second step.

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