Extremely obese people — those who are 80 or more pounds over a normal weight — live three to 12 fewer years than their normal-weight peers, a new study shows.

Just being overweight or moderately obese, however, has little or no effect on life span, the research found. The finding adds to the growing body of evidence that being slightly overweight may have no influence on life expectancy, but being severely overweight trims years off people's lives.

Overall, about 66% of adults in the USA are either overweight or obese. About one-third of people are in the obese category, meaning they have a body mass index of 30 or greater.

Body mass index, or BMI, is a measure based on height and weight. About 6% of people are extremely obese — that is, they have a BMI of 40 or greater.

Economists with RTI International, a non-profit research organization in Research Triangle Park, N.C., analyzed national data on 366,000 people. Among the findings being published online in the journal Obesity:

•Overall, excess weight was responsible for the loss of roughly 95 million years of life in the USA in 2008.

•Non-smokers who are obese — those who are about 30 or more pounds over a healthy weight — have a shorter life span by a year or less.

•Non-smokers who are overweight — about 29 pounds over a healthy weight — do not have shortened lives.

•Smoking takes a toll, too, and very heavy smokers are affected most. An 18-year-old white male who is normal weight and does not smoke can expect to live to age 81. If he's extremely obese and a smoker, his life expectancy is 60, a difference of 21 years.

The effect of extreme obesity appears to be greater for men than women and for whites than blacks, says Derek Brown, a health economist with RTI International and co-author of the study.

Lead author Eric Finkelstein says being moderately overweight may not affect people's life span because there are so many effective treatments to manage the health problems that often come with extra pounds, such as high cholesterol, high blood pressure and diabetes.

For instance, of the top 25 most prescribed medications, 10 target high blood pressure, high cholesterol or diabetes, he says.

Finkelstein and obesity experts with the Centers for Disease Control and Prevention have conducted additional research that shows the high medical costs of extra pounds. They recently published a study that showed obese Americans cost the country an estimated $147 billion in weight-related medical bills in 2008, double the amount a decade ago.

Obesity now accounts for 9.1% of all medical spending, up from 6.5% in 1998, they found. Overall, an obese patient has $4,871 in medical bills a year compared with $3,442 for a patient at a healthy weight.