WASHINGTON—Turns out a few extra pounds may not be such a bad thing, according to a new analysis of nearly three million adults that showed people who are overweight or slightly obese may live longer.

But experts were quick to caution that the possible benefits dropped off when the “few” extra pounds turned into many.

The researchers used data from nearly 100 studies from around the world, with health information from more than 2.8 million adults.

Among the sampled population, there were around 270,000 deaths within the study period.

Even after controlling for other factors, such as age, sex, smoking, those whose weight and height put them in the “overweight” category were six percent less at risk of dying than those in the “normal” category.

And those who were “slightly obese,” with heights and weights that gave them BMIs of 30 to 35, were five percent less at risk of dying in a given period.

But for those who were more significantly obese, with BMIs of 35 and higher, the mortality rate soared by 29 percent compared to “normal” weight subjects, according to the authors of the meta-analysis, published Tuesday in the Journal of the American Medical Association.

BMI, which stands for body-mass index, is calculated by dividing weight in kilograms by height in meters, squared.

The authors suggested several possible reasons to explain why some extra weight may be good, but too much is bad, including that those with a few extra pounds may be more likely to receive “optimal medical treatment.”

They said it was also possible that increased body fat provided metabolic benefits that protect the heart, or that having extra reserves of fat could be helpful for those whose sicknesses make it hard to eat.

Lead researcher Katherine Flegel, of the US Centers for Disease Control and Prevention, or CDC, published a controversial study in 2005 that indicated there was a link between excess weight and living longer.

This time, her analysis was based on a much larger number sample pool, across different countries in North America, Europe, Asia and South America.

These studies and others show that small amounts of excess fat “may provide needed energy reserves” during illness, or help in other ways that need to be investigated, wrote biomedical researchers Steven Heymsfield and William Cefalu in an editorial also published Tuesday in the JAMA.

“Not all patients classified as being overweight or having grade 1 obesity, particularly those with chronic diseases, can be assumed to require weight loss treatment,” they emphasized.

CDC director Thomas Friedan said in a statement that “we still have to learn about obesity, including how best to measure it.”

However, he insisted that “it’s clear that being obese is not healthy, it increases the risk of diabetes, heart disease, cancer, and many other health problems.”

“Small, sustainable increases in physical activity and improvements in nutrition can lead to significant health improvements.”

According to CDC statistics, a third of US adults are considered obese.