It's a common medical refrain: carrying extra kilos raises the risk of ills such as heart disease and diabetes and therefore the risk of premature death. But does that heightened risk of early death apply across the board to those who are merely overweight?
A comprehensive analysis of nearly 3 million people worldwide suggests maybe not.
The finding, published online this week in the Journal of the American Medical Association (AMA), pooled data from 97 studies encompassing 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 merely 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 even 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 Centres 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 - does not appear to be the explanation, since that factor was carefully controlled in the analysis.
The paper did not make any recommendations for doctors or members of the public, Flegal added.
"Our goal is really to summarise existing information and not conclude what people should do, other than follow good health practices, no matter what their weight."
There are a range of reasons why people who are overweight might fare better in studies than those who are of normal weight, said obesity researcher Dr Steven Heymsfield, executive director of the Pennington Biomedical Research Centre in Baton Rouge, Louisiana.
But only some suggest that carrying a few extra kilos truly makes someone healthier.
For example, some studies suggest that people who are overweight or mildly obese are treated more aggressively by their doctors for blood pressure or cholesterol problems than those who are of normal weight, said Heymsfield, who was not involved in the study but co-wrote a commentary accompanying it.
Preferential treatment could more than compensate for a higher rate of health problems among those who are overweight or mildly obese.
Health professionals also know that the BMI - a score that is calculated by dividing a person's mass in kilograms by their height in metres, squared - is not a perfect indicator of how much extra fat someone carries because people who have more muscle mass may score as overweight when they are not, for example, Heymsfield said.
But there also could be real reasons why carrying extra weight could confer a survival advantage. Fatter people are not as prone to osteoporosis and have more padding to protect the bones should a patient take a tumble, lowering the risk of a life-endangering hip fracture.
"I think we should be open-minded and ask, 'OK, what could be helpful about fat?'" Heymsfield said.
The survival edges reported in the analysis were not large. Those who were overweight were on average 6 per cent less likely to die during the studies than those whose BMI was normal.