To the Editor: The valuable contribution of Paffenbarger et al. (March 6 issue)* to the understanding of exercise and longevity overlooks a central question in most studies comparing life-style factors. It is a basic chicken-and-egg question that must be acknowledged even if it is unanswerable. Does exercise account for longer life, or does whatever factor predisposes to physical activity — e.g., constitutional vigor, life styles providing for regular recreation, or both — account for longevity and exercise?.
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