WEDNESDAY, Dec. 2, 2009 (HealthDay News) -- The overall health of the U.S. population has improved over the past three decades, largely because people have quit smoking in droves, but a new study suggests those gains might soon be wiped out if the rising obesity rates among Americans don't level off or drop.
If current trends in both smoking and obesity continue unchanged, the average life expectancy in America will be reduced by almost nine months, according to the study, which is published in the Dec. 3 issue of the New England Journal of Medicine.
On the other hand, the researchers calculated what would happen if everyone in America maintained a normal weight and no one smoked. If these two behavior changes were to occur, Americans would gain nearly four years of life.
"Although overall life expectancy is likely to increase, when we look at these two unhealthy behaviors we see the potential that it could have risen this much higher without obesity and smoking," said study author Susan Stewart, a research specialist in aging at the National Bureau of Economic Research in Cambridge, Mass.
"Even small improvements in these risk factors can make a difference," she added.
It's estimated that obesity is responsible for between 5 percent and 15 percent of deaths each year in the United States, according to background information in the study. Smoking still accounts for about 18 percent of deaths each year.
Along with their effect on mortality, obesity and smoking can both have a large impact on quality of life as well.
For the current study, the researchers used data from three nationally representative surveys that included data from as far back as 1971 through 2006.
The researchers projected that past trends in obesity and smoking would continue, which meant that obesity rates would continue to increase, while smoking rates would continue to drop. The researchers estimated that if current trends continue, nearly half of the U.S. population will be obese by 2020.
"If smoking continues to decline at past rates and obesity continues to increase at past rates, obesity will win this horse race, and over time, the increasing effects of obesity will outweigh the declines in smoking," she said.
"Clearly, these are two trends that are important in influencing public health," said Dr. David Meltzer, an associate professor of medicine at the University of Chicago Medical Center. "We've known that obesity has big health consequences and smoking definitely matters, and it's important, though perhaps not shocking, to learn that the increase in obesity is more than the decrease in smoking."
But, he pointed out, these findings only hold true if behaviors don't change. "One does need to remember that behaviors can change. There was a time when smoking looked like it would just go up and up, but then it declined," Meltzer noted.
Stewart and her colleagues acknowledged that their analysis is based on past trends continuing unchanged, but noted that the information is useful for showing where medical interventions can have the best value.
"We don't want to be just another alarmist paper," Stewart said. "We don't feel that there's no hope. Even modest weight loss and cutting down on smoking can result in improved health."
Learn more about the health effects of obesity from the National Heart, Lung, and Blood Institute.