Lack of activity destroys the good condition of every human being, while movement and methodical physical exercise save it and preserve it.—Plato

One credible hypothesis about the source of the epidemic of overweight and obesity in the United States is the progressive decrease in physical activity expended in daily life—for work, transportation, and household chores. Some researchers contend that the average caloric intake of Americans has not substantially increased; instead by reducing daily physical activity, the caloric imbalance between calories consumed and expended has shifted to favor weight gain. While no data conclusively prove this hypothesis, evidence does support it.

Among the recent studies that support the premise that Americans' sedentary lifestyles have precipitated the obesity epidemic is research that examined the diets of an Amish...