I-Min Lee and colleagues (July 21, p 219), on the basis of a meta-analysis of published studies, derive an estimate of 5.3 million deaths worldwide attributable to physical inactivity, and contrast this estimate with the 4.8 million deaths attributable to smoking in 2000. However, whereas the uncertainty range for the tobacco-related estimate is roughly +-20%, the estimate for physical inactivity is discussed as "conservative" but no measure of uncertainty is provided. People who are obese or who have a chronic illness, as well as smokers, are more likely to be inactive than are those who are lean, healthy, or who do not smoke. This confounding leads to an overestimate of inactivity-related risk, which is unlikely to be totally accounted for by the use of multivariate risk estimates. Additional caution is indicated by the observation that the relative risks for the four diseases regarded as being strongly related to inactivity are not higherthan that for all-cause mortality, and the proportion of all cancer deaths attributed to lack of exercise in high-income countries is only l-2%.
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