Obesity is a national health crisis--that  much we know. If current trends continue, it will soon surpass smoking in the U.S. as the biggest single factor in early death, reduced quality of life and added health care costs. A third of adults in the U.S. are obese, according to the Centers for Disease Control and Prevention, and another third are overweight, with Americans getting fatter every year. Obesity is responsible for more than 160,000 “excess” deaths a year, according to a study in the Journal of the American Medical Association . The average obese person costs society more than $7,000 a year in lost productiv­ity and added medical treatment, say researchers at George Washington University. Lifetime added medical costs alone for a person 70 pounds or more overweight amount to as much as $30,000, depending on race and gender.

All this lends urgency to the question: Why are extra pounds so difficult to shed and keep off? It doesn’t seem as though it should be so hard. The basic formula for weight loss is simple and widely known: consume fewer calories than you expend. And yet if it really were easy, obesity would not be the nation’s number-one lifestyle-related health concern. For a species that evolved to consume energy-dense foods in an environment where famine was a constant threat, losing weight and staying trimmer in a modern world of plenty fueled by marketing messages and cheap empty calories is, in fact, terrifically difficult. Almost everybody who tries to diet seems to fail in the long run--a review in 2007 by the American Psychological Association of 31 diet studies found that as many as two thirds of dieters end up two years later weighing more than they did before their diet.

[More]