Gina Kolta

Americans have been getting fatter for years, and with the increase in waistlines has come a surplus of conventional wisdom. If we could just return to traditional diets, if we just walk for 20 minutes a day, exercise gurus and government officials maintain, America’s excess pounds would slowly but surely melt away.

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

Over 66% of Americans are obese, says the federal Centers for Disease Control and Prevention, in Atlanta. Although the number of obese women in the US appears to be holding steady at 33%, for most Americans the risk is growing. There have been proposals to put warning labelson sodas like those on cigarettes. 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, asociology professor at the University of Southern California.