Rising obesity rates may undo all the health benefits accrued from reduction is smoking rates, a new study by University of Michigan and Harvard researchers has revealed.
"Obesity plays a large role in life expectancy," said co-author Allison B. Rosen, assistant professor in the Department of Internal Medicine at the University of Michigan.
"Despite the fact that we are smoking less, body-mass indexes (BMI) are going up. These increases in obesity are overtaking these changes in smoking behaviours," Rosen added.
Using a technical analysis that includes forecasting future trends based on historical data, the researchers found that despite declines in smoking, the remaining life expectancy of a typical 18-year-old would be held back by 0.71 years by the year 2020 because of the increased body-mass index of the general population.
The researchers also looked at quality of life. That same 18-year-old could expect to give up 0.91 years of increased quality-adjusted life expectancy.
If all U.S. adults became non-smokers of normal weight by 2020, their life expectancy would be forecast to increase by 3.76 years or 5.16 quality-adjusted years.
However, the researchers say the study's results don't imply that life expectancy will fall - more likely, life expectancy will continue to rise due to other factors, but less rapidly than it otherwise would.
Lead author Susan T. Stewart, a Harvard University research associate for the joint project of the National Bureau of Economic Research and Harvard's Program for Health System Improvement, said: "In the past 15 years, smoking rates have declined by 20 percent, but obesity rates have increased by 48 percent. If past trends continue, nearly half of the population - 45 percent - is projected to be obese by 2020."
The study has been published in the New England Journal of Medicine.