The health of the U.S. population has improved over the past three decades, mostly because people have quit smoking in droves, but a study suggests that those gains may soon be removed from the rising obesity rates among Americans.

If the present trends in both smoking and obesity continue unchanged, the average life expectancy in America will be reduced by almost nine months as per the study which was published in an issue of the New England Journal of Medicine.

The researchers calculated what would happen if everyone in America maintained a normal weight and no one smoked. The life expectancy of 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.

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.