By Allison Aubrey, NPR

Being a little overweight may tip the odds in favor of living a long life, according to a new analysis. Researchers say there may be some benefit to having a little extra body fat.

This isn’t the first time researchers have raised questions about the link between body weight and how long someone will live. While there’s no debate that being severely obese will raise the risk of all kinds of illnesses and even cut some lives short, it’s less clear what happens to people who are less overweight.

When Katherine Flegal of the Centers for Disease Control and Prevention set out to answer this question, she wanted to include as many people as possible in her analysis — from as many places around the world as she could find.

“We searched all the literature, thousands of articles, found almost 100 articles with 3 million people, that really addressed this question head on,” Flegal says.

And she concludes that being overweight is actually associated with a lower risk of death. It’s certainly not dramatic, but about a 6 percent decreased risk.

“It’s statistically significant,” Flegal says. The findings appear in the latest Journal of the American Medical Association.

So who are we talking about here? A lot more people than you might think. About one-third of all Americans fall into this category of overweight. And Flegal found that even among those who are technically, slightly obese, there was no increased risk of death.

At a time when we’re bombarded with weight-loss messages, Flegal says it isn’t popular to suggest that heavier people may live longer. In fact a few years back, when she published a paper that came to a similar conclusion, her findings were attacked.

“Our article got called rubbish and ludicrous,” Flegal says. “So it really opens you to lots of criticism. I discovered much to my sorrow that this is kind of a flashpoint for people.”

One of the experts who takes issue with Flegal’s conclusions is epidemiologist Walter Willett of the Harvard School of Public Health. He’s read her new paper and says he’s not buying it.

“This study is really a pile of rubbish and no one should waste their time reading it,” he says.

Willett says it’s not helpful to look simply at how peoples body mass indexes, or BMIs, influence their risk of death — as this paper did without knowing something about people’s health or fitness. Some people are thin because they’re ill, so of course they’re at higher risk of dying. The study doesn’t tease this apart.

Also, he says the analysis doesn’t address the bigger, more important issues of quality of life. If an overweight person does live longer — is he or she living with chronic diseases?

“We have a huge amount of other literature showing that people who gain weight or are overweight, have increased risk of diabetes, heart disease, stroke, many cancers and many other conditions,” Willett says.

For those of you who want to know whether your body weight is a problem, Willett says rather than comparing your BMI to those around you, think about what you weighed when you were 20 years old.

“For most people, our ideal weight, if we weren’t seriously overweight at age 20, is about what we weighed then,” Willett says. That’s why weight change is a good number to keep an eye on. It can be an early warning sign that you’re on the path to more weight gain.

Not everyone’s convinced the new paper is rubbish. Dr. Steven Hymsfield, of the Pennington Biomedical Research Center in Baton Rouge, La., says there are a couple of scenarios in which extra body weight might help people live longer.

“If you fall and you fall on vulnerable bone, like the hip, having a little extra fat there might protect you from hip fracture,” Hymsfield says.

Or he says, if an illness leaves you unable to eat, extra body fat could be useful.

Hymsfield acknowledges this is just speculation. He says while this paper won’t end the debate over whether a little extra body fat could be a good thing, it does show that the relationship between weight and health may be more complicated than just a simple calculation.

In an editorial accompanying Flegal’s paper, Hymsfeld and a colleague wrote that determining a person’s BMI is only one step in figuring out future health risks.