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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 experts and government officials maintain, America's excess pounds would slowly but surely melt away.

Scientists are less optimistic. 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. (1)Bypassing these physical systems is not just a matter of willpower.

More than 66 percent of Americans are overweight of-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” “Nostalgia is going to get us nowhere,” he added.

(2)Neither will wishful misconceptions about the efficacy of exercise. First, the federal government told Americans to exercise for half an hour a day. Then, dietary guidelines issued in 2005 changed the advice, recommending 60 to 90 minutes of moderate exercise a day. There was an uproar; many said the goal was unrealistic for Americans. But for many scientists, the more relevant question was whether such an exercise program would really help people lose weight.

The leisurely after-dinner walk may be pleasant, and it may be better than another night parked in front of the television. But modest exercise of this sort may not do much to reduce weight, evidence suggests.

“People don't know that a 20-minute walk burns about 100 calories,” said Dr. Madelyn Fernstrom, director of the weight-management center at the University of Pittsburgh Medical Center. “People always overestimate the calories consumed in exercise, and underestimate the calories in food they are eating” Controlling the balance is far more difficult than most people imagine, said Dr. Jeffrey Friedman, an obesity researcher at Rockefeller University. The math ought to work this way: There are 3,500 calories in a pound. If you subtract 100 calories per day by walking for 20 minutes, you ought to lose a pound every 35 days. Right?

Wrong. First, it's difficult for an individual to hold calorie intake to a precise amount from day to day. Meals at home and in restaurants vary in size and composition; the nutrition labels on purchased foods － the best guide to calorie content － are at best rough estimates. Calorie counting is therefore an imprecise art.

Second, scientists recently have come to understand that the brain exerts astonishing control over body composition and how much individuals eat. “There are physiological mechanisms that keep us from losing weight,” said Dr. Matthew W. Gilman, the director of the obesity prevention program at Harvard Medical School Pilgrim Health Care.

Scientists now believe that each individual has a genetically determined weight range spanning perhaps 30 pounds. (3)Those who force their weight below nature's preassigned levels become hungrier and eat more; several studies also show that their metabolisms slow in a variety of ways as the body tries to conserve energy and regain weight. People trying to exceed their weight range face the opposite situation: eating becomes unappealing, and their metabolisms shift into high gear.

The body's determination to maintain its composition is why a person can skip a meal, or even fast for short periods, without losing weight. It's also why burning an extra 100 calories a day will not alter the judgment on the bathroom scales. Struggling against the brain's innate calorie counters, even strong-willed dieters make up for calories lost on one day with a few extra bites on the next. And they never realize lt. “The system operates with 99.6 percent precision,” Dr. Friedman said.

The temptations of our environment － the sedentary living, the ready supply of rich food － may not be entirely to blame for rising obesity rates. In fact, new research suggests that the environment that most strongly influences body composition may be the very first one anybody experiences: the womb.

According to several animal studies, conditions during pregnancy, including the mother's diet, may determine how fat the offspring are as adults. Human studies have shown that women who eat little in pregnancy, surprisingly, more often have children who grow into fat adults. More than a dozen studies have found that children are more likely to be fat if their mothers smoke during pregnancy.

The research is just beginning, true, but already it has upended some old myths about dieting. The body establishes its optimal weight early on, perhaps even before birth, and defends it vigorously through adulthood. As a result, weight control is difficult for most of us. And obesity, the terrible new epidemic of the developed world, is almost impossible to cure.

【Adapted from “For the Overweight, Bad Advice by the Spoonful”, The new York Times, August 30,2007】

• obesity 肥満
• scapegoat 罪を負わされるもの
• uproar 大騒ぎ
• physiological 生理学上の
• metabolism 新陳代謝
• innate 生まれつきの
• upend ～をひっくり返す
• optimal 最適の
• epidemic 流行病

• 1 本文の内容に合っている英文には○を、合っていない英文には×をつけなさい。
  • ア An individual's weight is determined only by environmental influences such as diet and exercise.
  • イ In the United States, the number of obese women has been increasing recently.
  • ウ The restaurants in New York now have to put warning labels on high calorie food on their menus.
  • エ Today, people spend a longer time sitting than those in the past, which may have much to do with the recent rise in obesity.
  • オ Evidence suggests that the leisurely after-dinner walk may not help us reduce weight.
  • カ If you walk for 20 minutes every day, you will lose a pound in 35 days.
  • キ We can count on the nutrition labels on purchased foods because they tell us the precise calories contained in them.
  • ク It is the mechanisms in our brain that prevent us from losing weight.
  • ケ The body's determination to maintain its composition is so strong that burning an extra 100 calories a day will not change the figure on the scales.
  • コ If women eat little in pregnancy, their children are likely to grow into lean adults.
• 2 下線部(1)を、theseの内容を明らかにして日本語に直しなさい。
• 3 下線部(2)と(3)を日本語に直しなさい。