The latest research involving about 1.5 million people concluded that healthy white adults who were overweight were 13 percent more likely to die during the time they were followed in the study than those whose weight is in an ideal range.
"Having a little extra meat on your bones -- if that meat happens to be fat -- is harmful, not beneficial," said Dr. Michael Thun of the American Cancer Society, senior author of the study.
The study's conclusions, published in Thursday's New England Journal of Medicine, are similar to three other large studies, said the lead author, Amy Berrington of the National Cancer Institute.
"Now there's really a very large body of evidence which supports the finding that being overweight is associated with a small increased risk of death," Berrington said.
For their government-funded analysis, the researchers pooled 19 long-term studies of mostly white adults. They used each person's body mass index -- a measure of height and weight -- and checked to see who died during the follow-up periods, which ranged from five to 28 years.
They focused on people who were healthy at the beginning of the studies, excluding smokers and those with heart disease or cancer because those affect death rates and researchers wanted to see the impact of weight alone.
The lowest death rate for healthy women who had never smoked was in the high end of the ideal body mass index range -- between 22.5 and 24.9. Compared with that group, those who were overweight had an increased risk of death of 13 percent. The increased risk ranged from 44 to 88 percent for those who were obese. The morbidly obese were 2 1/2 times more likely to die prematurely. The results for men were similar.
Most of the participants in the studies were white so the research focused on them. Results may be different in other ethnic and racial groups, Berrington said. She said evidence suggests that for the same BMI level, African-Americans might have a lower risk of death and Asians a higher risk.
Two-thirds of U.S. adults are either overweight or obese. Overweight begins at a BMI measurement of 25, obese at 30 and morbidly obese at 40. A 5-foot-6 person is considered overweight at 155 pounds, obese at 186 pounds and morbidly obese at 248 pounds.
Because of its size and the diversity of studies included, the research "provides strong evidence against the position that it's a good thing for health to be overweight," Thun said.
The notion that a "bit of reserves" would help keep you from getting sick probably stems from the days when food was scarce, he said.
The latest research was launched after a controversial 2005 study by the Centers for Disease Control and Prevention that concluded being overweight didn't raise the risk of death; that report included smokers and those with pre-existing illnesses.
University of South Carolina obesity researcher Steven Blair said the results were consistent with other studies and the "massive effort" was commendable. But he said there wasn't enough information available about fitness level or physical activity. A proponent of the "fit and fat" theory, Blair said his research has shown that obese people who are tested and deemed fit did not face increased risks of dying.
"If we want to get to the bottom of the health hazards of overweight and obesity, we have to have better data on physical activity," Blair said. "Until we do that, there's uncertainty of how important BMI is as an important predictor of mortality."