It's a common medical refrain: Carrying extra pounds raises the risk of ills such as heart disease and diabetes and therefore the risk of a premature death.

But does that heightened risk of early death apply across the board to those who are merely overweight? A new analysis of nearly 3 million people suggests maybe not.

The finding, published online Tuesday in the Journal of the American Medical Association, pooled data from 97 studies encompassing adult men and women in the United States, Canada, Europe, Australia, China, Taiwan, Japan, Brazil, India and Mexico.

A total of 270,000 people died of any cause during the studies. When the scientists crunched the numbers, they found, as expected, that people who were significantly obese -- with a body mass index (BMI) of 35 or more -- had shorter life spans on average than those who were of normal weight, defined as having a BMI of 18.5 to 24.9.

But the scientists also found that people classed as overweight (with a BMI of 25 to 29.9) died at slightly lower rates -- not higher -- than those of so-called normal weight. And they found that those who were mildly obese (with a BMI of 30 to 34.9) died in no greater numbers than did their normal-weight peers.

Study lead author Katherine Flegal, an epidemiologist with the Centers for Disease Control and Prevention, said she and her colleagues could not say what lay behind the apparent survival edge for overweight people. But she noted that it had been observed before in other studies.

Flegal added that smoking -- which raises the risk of early death but also tends to keep people thinner -- doesn't appear to be the explanation, since that factor was carefully controlled for in the analysis.

The paper didn't make any recommendations for doctors or members of the public, Flegal added. "Our goal is really to summarize existing information and not conclude what people should do, other than follow good health practices."