But she says the difference in death rates appears to be small between normal-weight people and those who are overweight or mildly obese.

The study has raised new questions about “body mass index,” or BMI. This is a measurement of body fat as a ratio of height to weight. BMI guidelines were used as a basis for the study. In recent years, many public health experts have promoted body mass index as a way to predict the risk of health problems. But a person's BMI can be misleading in some cases.

Steven Heymsfield is the executive director of the Pennington Biomedical Research Center in Baton Rouge, Louisiana. He says people can be physically fit and in good health, but might weigh more because they are more muscular.

“It’s very common in the military, for example, where you have young men and women who are very physically fit -- their BMI can be a little higher. And so the military knows that, and they check people with body fat measurement if they exceed the BMI guideline.”

Still, Dr. Heymsfield says people should not think gaining extra weight is OK just because of the new findings. He says being at a healthy weight lowers the risk for heart disease and diabetes. He and a colleague wrote an editorial that appeared along with the study in the Journal of the American Medical Association.