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 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.