Get a taste of what awaits you in print from this compelling excerpt.

In one sense, the obesity crisis is the result of simple math. It's a calories in, calories out calculation. The First Law of Fat says that anything you eat beyond your immediate need for energy, from avocados to ziti, converts to fat. "A calorie is a calorie is a calorie," says Lawrence Cheskin, director of the Johns Hopkins Weight Management Center, whether it comes from fat, protein, or carbohydrate. Cheskin, who is six foot one (185 centimeters) and weighs 160 pounds (70 kilograms), has never had a weight problem himself. "Who said life is fair?" he observes.

The Second Law of Fat: The line between being in and out of energy balance is slight. Suppose you consume a mere 5 percent over a 2,000-calorie-a-day average. "That's just one hundred calories; it's a glass of apple juice," says Rudolph Leibel, head of molecular genetics at Columbia University College of Physicians and Surgeons. "But those few extra calories can mean a huge weight gain." Since one pound of body weight is roughly equivalent to 3,500 calories, that glass of juice adds up to an extra 10 pounds (five kilograms) over a year. Alternatively, you'd gain 10 pounds if, due to a more sedentary lifestyle—driving instead of walking, taking the escalator instead of the stairs—you started burning 100 fewer calories a day.

"We know people get fat by overeating slightly more than they burn, but we don't know why they do it," Leibel says. "I'm convinced our overeating is not willful or the result of a deranged upbringing. It's the genes talking, but it's a very complicated language. Genetics are everything."

In the 1960s James V. Neel, a geneticist at the University of Michigan, listened in on one genetic conversation. In his "thrifty gene" hypothesis, Neel suggested that some of us inherited genes that make us exceptionally efficient in our intake and use of calories. Our bodies are good at converting food into fat and then hanging on to it. This trait may have helped our ancestors survive when calories were few and far between, Neel speculated.

But fast-forward to the 21st century, when calorie supply isn't a problem, and genes that favor gaining weight have outlived their usefulness. Evolution betrays us. We store fat for the famine that never comes. "If we understood the genetics well enough," says Anna Mae Diehl, a professor of medicine at Johns Hopkins School of Medicine, "we could fingerprint people when they are born and say: Ah, good genes. Lucky you. You can eat whatever you want. Or: Uh-oh. Poor kid. Better never have a doughnut."

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