In the mid-1960s, a team of physiologists in Dallas decided to compare the effects of sedentariness with those of exercise on health by paying five healthy twenty-year-olds first to spend three weeks in bed and then to undergo an intensive eight-week exercise program. The bed rest was ruinous. When they were finally allowed to arise from their beds, the volunteers’ bodies resembled forty-year-olds’ by many metrics: they were fatter, had higher blood pressure, higher cholesterol levels, less muscle mass, and lower fitness.40 The eight ensuing weeks of exercise, however, not only reversed the
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