Americans have swapped one vice for another, and it could be taking months, if not years, off our lives. New research suggests that although the number of smokers in the United States has decreased by 20% in the past 15 years, the number of obese Americans has increased by 48%. If the trends continue, the dangers of obesity will overshadow the public health gains from reduced smoking over the next decade.
The study draws upon data from national health surveys conducted over the past few decades, some stretching back to 1971, which covered a few hundred thousand people. The authors calculated how smoking and obesity affected life span between 1990 and 2005 (the most recent year for which data were available) and extrapolated those rates forward until 2020. According to this analysis, obesity will rob a typical 18-year-old in 2020 of 0.71 years (260 days) of life and 0.91 years (332 days) of quality of life.
More specifically, the average life span gain from not smoking, 0.31 years, was one-third of the average harm done by adding weight, 1.02 years. The paper also reports that, in the highly unlikely event that all Americans quit smoking and reach a normal weight by 2020, the average life span would increase by 3.76 years and 5.16 quality-adjusted years.
The study, which appears  today in The New England Journal of Medicine, comes with a number of caveats, says lead author Susan Stewart, an expert on aging at the National Bureau of Economic Research in Cambridge, Massachusetts. "We don't want to be alarmist," she says. "For an individual, it's probably still worse to smoke than be obese," because smoking causes many acute health problems whereas obesity takes longer to corrode someone's health. It's just that more people are gaining weight than are cutting out smoking, so things look worse from a national perspective.
What's more, the paper does not argue that life span will decrease in absolute terms. Obese people will live longer in the future than they do today because of advances in medical care. But they would live even longer if they curbed their weight. As co-author David Cutler, an economist at Harvard University, puts it, "If you get a $10,000 raise, are you happy if someone steals $2000 from you?"
Stewart says the paper should help doctors and health officials decide where to invest funds to help the greatest number of people. However, other public health experts question the findings. David Williamson, an epidemiologist at Emory University in Atlanta, says that the researchers don't furnish the usual indicators of statistical significance for their projections, such as error bars. "I don't know how seriously to take it."
Williamson also criticizes the somewhat arbitrary decision to use a 15-year trend line to make projections. The national health surveys incorporated into the study yield different information about how detrimental obesity really is. He says that more-recent data, covering the years 2000 to 2005, suggest that being overweight is not as bad as doctors once feared--slightly overweight people even seem to live longer than people of normal weight. Indeed, when Cutler and Stewart calculate in their paper how much the average life span would decrease if their trend line reached back only 5 years, they found less-dramatic results: a reduction of 0.10 years (37 days) and 0.17 quality-adjusted years (62 days) by 2020 for an average 18-year-old.
Regardless of the specific numbers, Cutler and Stewart note that the overall trend is the same: The rise in obesity will overwhelm the benefits from a smoking reduction in the future. That's true, they note, even if obesity rates increase as little as 0.15% per year over the next 15 years.