(TIME.com) -- The longest lived among us aren't necessarily those who are of normal weight, says a new study.
According to new research this week in the Journal of the American Medical Association (JAMA), researchers say that being overweight may lead to a longer life.
The somewhat surprising conclusion comes from an enormous, detailed review of over 100 previously published research papers connecting body weight and mortality risk among 2.88 million study participants living around the world.
The new research confirms that obese people, and particularly those who are extremely obese, tend to die earlier than those of normal weight. But the findings also suggest that people who are overweight (but not obese) may live longer than people with clinically normal body weight.
The new report is the largest and most comprehensive review of how weight, measured as body mass index (BMI), a measure comparing the ratio of height to weight, can influence longevity. Previous studies that have exposed the link in the past, however, have raised questions about whether the overweight advantage is real.
"We published an article in 2005 that showed, among other things, that (being) overweight was associated with lower mortality -- and we got an awful lot of negative feedback from that," says the current study's lead author, Katherine Flegal, a senior research scientist at the Centers for Disease Control and Prevention (CDC). Since that study, however, dozens of others have reached the same conclusion -- even if it was hard for researchers and the public to accept.
"I think there's a lot of under reporting of this finding ... and so people are sort of repeatedly surprised by it," Flegal says. Because many researchers don't expect to find a benefit associated with being overweight, she suggests, they may not believe their results are valid if they find such a connection, which may make them more hesitant to publish them and invite review and discussion about what may be driving the trend.
For the new study, Flegal and her colleagues analyzed every study they could find that broke down death risk broken by the standard BMI categories set by the World Health Organization (WHO) in the late 1990s: with underweight defined as BMI less than 18.5, normal weight being BMI between 18.5 and 25, overweight being BMI between 25 and 30, and obese as BMI of over 30.
Men or women who are 5 feet 4 inches tall would have "normal" BMI if they weighed between 108 and 145 pounds, for example, and overweight if they weighed 146 to 174 pounds, and obese if weighed more than that.
Overall, people who were overweight but not obese were 6% less likely to die during the average study period than normal-weight people. That advantage held among both men and women, and did not appear to vary by age, smoking status, or region of the world. The study looked only at how long people lived, however, and not how healthy they were whey the died, or how they rated their quality of life.
Why would overweight people live the longest?
Flegal and her co-authors suggest that it's possible that overweight and obese people get better medical care, either because they show symptoms of disease earlier or because they're screened more regularly for chronic diseases stemming from their weight, such as diabetes or heart problems.
There is also some evidence that heavier people may have better survival during a medical emergencies such as infections or surgery; if you get pneumonia and lose 15 pounds, it helps to have 15 pounds to spare, for example.
Another possible explanation may involve "reverse causation": maybe being thin doesn't make you sick, as some experts argue, but instead being sick can make you thin. Being overweight may be associated with longer lives if people who lose weight because of diseases such as cancer, for example, contribute to earlier death among individuals who weigh less.
Flegal also says her findings may necessarily be contrary to previous studies about the relationship between BMI and mortality because those analyses used a variety of different BMI categories with different cut-points for the various weight groups. In the new JAMA study, Flegal and colleagues only looked at research using the WHO categories.
Even so, she acknowledges that interpreting the results may be confusing, since the names of the WHO "normal" and "overweight" categories don't necessarily correspond to commonly held perceptions.
Today, roughly 33% of U.S. adults are clinically overweight, according to WHO standards, and an additional 36% are obese. By those standards, the average American is not clinically normal weight at all, but considered overweight. So in fact, the overweight people in the study who tend to live longest may not be fatter than most people at all. Among Americans, at least, they may actually be of average weight.
In addition, say doctors, weight alone may not be enough to understand an individual's risk of developing disease and dying early.
The latest research shows, for example, that it's not just the fat that comes with weight gain, but the type of fat, particularly fat that accumulates around the belly, that might be more life-threatening. All of which suggests that the connection between weight and health is a complicated one that may not be measured simply in years lived.
This story was originally published on TIME.com.