(Health.com) -- Although fewer people are smoking -- and therefore less likely to die from cigarette-related causes -- the obesity epidemic may negate any gains in life span, according to a new study.
By 2020, the typical 18-year-old will gain 0.31 years due to the drop in smoking rates (above and beyond life span increases caused by other factors). But the increase in obesity rates during the same period will reduce life expectancy by 1.02 years, the researchers say.
During the next 10 years, in other words, we'll lose 0.71 years of our life span, time that we would have gained if so many people weren't overweight, according to the estimates published this week in the New England Journal of Medicine (NEJM).
In addition, the increase in quality-adjusted life expectancy -- a measure that takes into account levels of disability and other quality-of-life factors -- will be reduced by 1.32 years. If all U.S. adults were nonsmokers of normal weight, life expectancy would increase by 3.76 years, or 5.16 quality-adjusted years, according to the study.
"Life expectancy is not going to decline," says the study's lead author, Susan T. Stewart, Ph.D., a researcher at the National Bureau of Economic Research, in Cambridge, Massachusetts. "But it could have risen by that much more if it weren't for the increases in obesity."
Stewart and her colleagues forecast life expectancy through the year 2020 using national survey data. Smoking, a major risk factor for lung disease, heart disease, and cancer, has decreased by 20 percent in the United States in the past 15 years, according to the study.
Over the same period, obesity has increased by 48 percent. Obesity contributes to a host of serious health problems, including heart disease, diabetes, joint problems, stroke, and some sleep disorders.
By 2020, the report predicts, smoking will decrease by 21 percent, but 45 percent of the population will be obese.
Prior research has examined the effects of obesity on longevity, but this study is the first to examine the combined effects of obesity and smoking.
"No one ever has really done quite this linkage between smoking and obesity," says S. Jay Olshansky, Ph.D., a professor of epidemiology and biostatistics at the School of Public Health at the University of Illinois at Chicago. "Some people have suggested we're on the verge of dramatic increases in life expectancy because of reductions in smoking, but these authors are saying, 'Hold on a minute; the negative effect of obesity is much greater.'"
The extent of obesity's impact on life span "might be a real eye-opener," says Stewart.
Many people will question how a sedentary lifestyle can be as unhealthy as a deadly habit such as smoking, she says, adding that this is exactly why she and her colleagues believe this research is important. "We wanted to bring attention to the health of a population [that] is already not as healthy as it could be, and will continue and worsen," she says.
The study does have limitations. The authors based their projections on a steady rate of change in obesity, for instance.
However, "childhood obesity has been rising dramatically, so the trends in the future are going to change by how long people have been obese," says Olshansky, who did not participate in the current research, but projected similar obesity trends in a 2005 paper in the NEJM. "Younger generations are going to carry the obesity with them much longer," leading to additional or more serious weight-related health risks, he says.
"If we don't intervene, we are in trouble," Olshansky adds.
Reversing the obesity trends reported in the study will likely require a concerted public health campaign similar to the one that has reduced smoking rates.
"There are larger social issues to be addressed in combating the roots of obesity," Stewart says. "These roots include sedentary lifestyles, widespread availability of high-calorie food in large portions, and reduced time for at-home food preparation."
"Fixing obesity is going to require a change in our modern relationship with food," Olshansky says. "I'm hopeful that we [will] begin to see a turnaround in this childhood obesity epidemic."
The smoking trends used in the study were based on data from the National Health Interview Survey, and the body-mass index (BMI) trends were derived from the National Health and Nutrition Examination Survey. BMI levels were classified according to the World Health Organization's guidelines for obesity.
Copyright Health Magazine 2011