Effects of Obesity and Smoking on U.S. Life Expectancy

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Abstract
Mortality from adult obesity and from persistent smoking have already been reliably assessed in studies of tens of thousands of deaths.1-4 A common measure of obesity is the body-mass index (BMI, the weight in kilograms divided by the square of the height in meters).2 An increase of 2 in the BMI in overweight populations and an increase of 10% in the prevalence of smoking reduce the life span in men comparably, each by about 1 year ( Figure 1 ). Public debate by economists should use such assessments. Instead, Stewart et al. (Dec. 3 issue)5 overestimate the hazards of obesity and underestimate the hazards of tobacco use. They mainly use data from a study involving only 3000 deaths that were analyzed, without epidemiologically appropriate precautions,1-4 in 32 separate subgroups, yielding unreliable relative risks (see Table A3 in the Supplementary Appendix, available with the full text of their article at NEJM.org). Also, they present their underestimated hazards of tobacco use as the relatively small gain in nationwide life expectancy they forecast if, from 2005 through 2020, the prevalence of smoking in the United States decreases from 24% to 19%. This is their predicted decrease; however, the prevalence may decrease faster, since the federal tax on cigarettes just increased by 150%. Even if the hazards were corrected, such calculations could obscure tobacco's substantial importance for individual smokers.4 Stopping smoking can lead to a gain in life expectancy of about 10 years, far more than a smoker could expect to gain from weight control.1