Throughout the world, on the other hand, the incidence of obesity and diabetes is increasing at an alarm in grate. Obesity levels in the United States remained relatively constant from the early 1960s through 1980, between 12 and 14 percent of the population; over the next twenty-five years, coincident with the official recommendations to eat less fat and so more carbohydrates, it surged to over 30 percent. By 2004, one in three Americans was considered clinically obese. Diabetes rates have increased apace. Both conditions are associated with an increased risk of heart disease, which could explain why the incidence of heart disease is not decreasing. It is also possible that obesity, diabetes, and heart disease all share a single, underlying cause. The surge in obesity and diabetes occurred as the population was being bombarded with the message that dietary fat is dangerous and that carbohydrates are good for the heart and for weight control. This suggests the possibility, however heretical, that this official embrace of carbohydrates might have had unintended consequences.

I first heard this notion in 1998, when I interviewed William Harlan, then associate director of the Office of Disease Prevention at the National Institutes of Health. Harlan told me that public-health experts like himself assumed that if they advised all Americans to eat less fat, with its densely packed calories, weights would go down. “What we see instead,” he said, “is actually weights have gone up, the portion sizes have gone up, the amount we eat has gone up…. Foods lower in fat became higher in carbohydrates and people ate more.”

The result has been a polarization on the subject of nutrition. Most people still believe that saturated fat, if not any and all fat, is the primary dietary evil—that butter, fat, cheese, and eggs will clog our arteries and put on weight—and have reduced their intakes. Public-health experts and many in the media insist that the obesity epidemic means the population doesn’t take their advice and continues to shun physical activity while eating fatty foods to excess. But a large number of people have turned to the message of Banting and one remarkably best-selling diet book after another: Eat Fat and Grow Slim (1958), Calories Don’t Count (1961), The Doctor’s Quick Weight Loss Diet (1968), Dr. Atkins’ Diet Revolution (1972), The Complete Scarsdale Medical Diet (1978), The Zone (1995), Protein Power (1996), Sugar Busters! (1998), and The South Beach Diet (2003). All advocate an alternative hypothesis: that carbohydrates are the problem, not fat, and if we eat less of them, we will weigh less and live longer. All have been summarily dismissed by the American Heart Association, the American Medical Association, and nutritional authorities as part of a misguided fad.