BAKERSFIELD, Calif. -- Over the past 15 years, smoking rates have decreased by 20 percent -- which means we have less heart disease, fewer cancer cases, and a longer life expectancy.

But at the same time, obesity has increased by 48 percent, and Harvard researchers say that our expanding waistlines have the potential to wipe out all the gains we've seen from less smoking.

Their scientific modeling estimates that between 2005 and 2020, a typical 18-year old would gain about four months of life expectancy from reduced smoking rates -- but this would be counteracted by a 1-year loss from obesity.

Researchers still predicted that improvements in medical treatments will keep overall life expectancy going up, but the improvements will be slower due to Americans' collective weight gain.

Other experts in the report said obesity's impact on the quality of life may be its greatest toll. People battling excess weight have higher rates of disability, diabetes and heart disease -- disorders that make them sicker at younger ages and dependent on medications for years longer.