Scientists are less sanguine. Many of the so-called facts about obesity, they say, amount to speculation or oversimplification of the medical evidence. Diet and exercise do matter, they now know, but these environmental influences alone do not determine an individual's weight. Body composition also is dictated by DNA and monitored by the brain. Bypassing these physical systems is not just a matter of willpower.

More than 66 percent of Americans are overweight or obese, according to the federal Centers for Disease Control and Prevention, in Atlanta. Although the number of obese women in the United States appears to be holding steady at 33 percent, for most Americans the risk is growing. The nation's poor diet has long been the scapegoat. There have been proposals to put warning labels on sodas like those on cigarettes. There are calls to ban junk foods from schools. New York and other cities now require restaurants to disclose calorie information on their menus.

But the notion that Americans ever ate well is suspect. In 1966, when Americans were still comparatively thin, more than two billion hamburgers already had been sold in McDonald's restaurants, noted Dr. Barry Glassner, a sociology professor at the University of Southern California. The recent rise in obesity may have more to do with our increasingly sedentary lifestyles than with the quality of our diets.

"The meals we romanticize in the past somehow leave out the reality of what people were eating,"he said. "The average meal had whole milk and ended with pie.... The typical meal had plenty of fat and calories."