Does smoking or obesity affect life expectancy more?

Originally published in MedPage Today

by Crystal Phend, MedPage Today Senior Staff Writer

Gains in life expectancy from lower smoking rates over the next decade will be offset, to some degree, by reductions in life expectancy based on the rise in obesity, researchers estimated.

Does smoking or obesity affect life expectancy more? If obesity and smoking rates had held steady, the average 18-year-old would have seen a 2.98-year increase in life expectancy over a 15-year period, according to Susan T. Stewart, PhD, of Harvard and the private nonprofit National Bureau of Economic Research in Cambridge, Mass., and colleagues.

But a 48% rise in obesity overrode the expected gain from a 20% reduction in smoking rates seen over the past 15 years, the researchers reported in the Dec. 3 New England Journal of Medicine.

Bottom line: a predicted net impact of 0.71 fewer life-years through 2020 — one quarter of the anticipated increase.

Overall, life expectancy isn’t expected to fall over the next decade, the researchers cautioned. Instead, their estimates suggest that “life expectancy will continue to rise but less rapidly than it otherwise would.”

Still, the findings should be a wake-up call for policymakers and physicians, Stewart said in an interview.

“We know that the effects of obesity are not quite as intense as the effects of smoking, but obesity is more widespread,” she said. “It was a little discouraging to see that obesity was winning.”

“But if we were to put the same kind of effort into addressing obesity as we have fairly successfully put into addressing smoking, then perhaps we could have the same kind of positive effects for the future,” she added.

The researchers forecast life expectancy and quality-adjusted life expectancy for a representative 18-year-old for each year from 2005 through 2020.

Since both obesity and smoking impact quality of life, the researchers also estimated quality-adjusted life expectancy using 2003 Medical Expenditure Panel Survey data.

Four iterations of the National Health Interview Survey from 1978 through 2006 revealed an average 1.4% decrease in smoking rates per year in the 15 years prior to 2005.

But Body Mass Index (BMI) trends based on National Health and Nutrition Examination Surveys (NHANES) from 1971 through 2006 showed an average 0.5% increase per year over the 15 years before 2005.

Assuming a continuation of past trends for the next 15 years, 21% of current smokers would quit by 2020, the researchers estimated.

Based on this factor alone, life expectancy for the typical 18-year-old would increase 0.31 years, with an extra 0.41 years of quality-adjusted life expectancy.

But over the same time frame, the normal weight population would drop by 35% in the U.S. with an estimated 45% of Americans expected to be obese by 2020.

The impact of this change alone would reduce life expectancy by 1.02 years and quality-adjusted life expectancy by 1.32 years.

Thus the net effect of the two risk factors together would be a 0.71-year reduction in life expectancy and 0.91-year drop in quality-adjusted life expectancy relative to the trend.

This same pattern was forecast for every year from 2005 to 2020, with the disproportionate effects of obesity becoming even more pronounced over time.

Even in sensitivity analyses based on more rapid declines in smoking and slower rises in obesity, the effects of obesity exceeded those of smoking on life expectancy.

The trends could be expected to have less absolute impact for older adults, who have fewer years of remaining life expectancy, Stewart said.

However, the results might underestimate the impact of obesity on youth, considering that earlier onset leaves more time for risks such as diabetes to arise, she said.

“Though perhaps not achievable,” completely eliminating both smoking and obesity would increase life expectancy by 3.76 years while quality-adjusted life expectancy would rise by 5.16, the researchers wrote.

They cautioned that these population-level forecasts do not apply at the individual level to smokers who quit or people who lose weight.

Nevertheless, “even modest weight loss and reductions in smoking at the individual level can have substantial effects on population health,” they concluded.

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