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  3. How exercise affects body weight is one of the more intriguing and vexing issues in physiology. Exercise burns calories.no one d

How exercise affects body weight is one of the more intriguing and vexing issues in physiology. Exercise burns calories.no one d

admin2012-12-30  10
问题     How exercise affects body weight is one of the more intriguing and vexing issues in physiology. Exercise burns calories.no one doubts that, and so it should, in theory, produce weight loss, a fact that has prompted countless people to undertake exercise programs to shed pounds. Without significantly changing their diets, few succeed. "Anecdotally, all of us have been cornered by people claiming to have spent hours each week walking, running, stair-stepping, etc. , and are displeased with the results on the scale or in the mirror, "wrote Barry Braun.an associate professor of kinesiology at the University of Massachusetts at Amherst.
    But a growing body of science suggests that exercise does have an important role in weight loss. That role, however, is different from what many people expect and probably wish. The newest science suggests that exercise alone will not make you thin, but it may determine whether you stay thin, if you can achieve that state. Until recently, the bodily mechanisms involved were mysterious. But scientists are slowly teasing out exercise’s impact on metabolism, appetite and body composition, though the consequences of exercise can vary. Women’s bodies, for instance, seem to react differently than men’s bodies to the metabolic effects of exercise. None of which is a reason to abandon exercise as a weight-loss tool. You just have to understand what exercise can and cannot do.
    "In general, exercise by itself is pretty useless for weight loss, "says Eric Ravussin.an expert on weight loss. It’s especially useless because people often end up consuming more calories when they exercise.The mathematics of weight loss is, in fact, quite simple, involving only subtraction. "Take in fewer calories than you burn.put yourself in negative energy balance, lose weight, "says Braun.who has been studying exercise and weight loss for years. The deficit in calorics can result from cutting back your food intake or from increasing your energy output the amount of exercise you complete or both. When researchers affiliated with the Pennington center had volunteers reduce their energy balance for a study by either cutting their caloric intakes by 25 percent or increasing their daily exercise by 12. 5 percent and cutting their calories by 12.5 percent, everyone involved lost weight.They all lost about the same amount of weight too about a pound a week. But in the exercising group, the dose of exercise required was nearly an hour a day of moderate-intensity activity. What the federal government currently recommends for weight loss is "a lot more than what many people would be able or willing to do, " Ravussin says.
    At the same time, as many people have found after starting a new exercise regimen, working out can have a significant effect on appetite. The mechanisms that control appetite and energy balance in the human body are elegantly calibrated. "The body aims for homeostasis, "Braun says. It likes to remain at whatever weight it’s used to. So even small changes in energy balance can produce rapid changes in certain hormones associated with appetite, particularly acylated ghrelin, which is known to increase the desire for food, as well as insulin and leptin, hormones that affect how the body burns fuel.
    The effects of exercise on the appetite and energy systems, however, are by no means consistent. In one study presented last year at the annual conference of the American College of Sports Medicine, when healthy young men ran for an hour and a half on a treadmill at a fairly high intensity, their blood concentrations of acylated ghrelin fell, and food held little appeal for the rest of that day. Exercise blunted their appetites. A study that Braun oversaw had a slightly different outcome. In it, 18 overweight men and women walked on treadmills in multiple sessions while either eating enough that day to replace the calories burned during exercise or not. Afterward, the men displayed little or no changes in their energy-regulating hormones or their appetites, much as in the other study. But the women uniformly had increased blood concentrations of acylated ghrelin and decreased concentrations of insulin after the sessions in which they had eaten less than they had burned. Their bodies were directing them to replace the lost calories. In physiological terms, the rcsults"arc consistent with the paradigm that mechanisms to maintain body fat are more effective in women, "Braun and his colleagues wrote. In practical terms, the results are scientific proof that life is unfair. Female bodies, inspired almost certainly"by a biological need to maintain energy stores for reproduction, "Braun says, fight hard to hold on to every ounce of fat. Exercise for many women increases the desire to eat.  
We can infer from the text that
选项 A、going on diet will definitely lead to weight loss.
B、the theory of exercise burning calories is challenged.
C、it is often disappointing to lose weight by doing exercise.
D、most people tend to spend hours walking, running, etc.
答案C
解析 推断题。首段首句提出“how exercise affects body weight”这一主题之后，第三句指出结果“Without significantly changing their diets，few succeed”, 之后引用Barry Braun的话做进一步说明“Anecdotally，all of us have been cornered by people claiming to have spent hours each weekwalking，running，stair-stepping，etc．，and are displeased with the results on the scale or in the mirror”，可见，多数人都无法通过锻炼达到减肥的目的，让人失望是必然的结果，故[C]为答案。
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