Being a little overweight or slightly obese was linked to about a 6 percent lower risk of dying, compared to people considered "normal weight," according to a review of almost 100 past studies covering nearly three million people. Being severely obese, however, was still tied to an almost 30 percent higher risk of death.
The study comes from a federal researcher who drew controversy in 2005 with a report that found thin and normal-weight people had a slightly higher risk of death than those who were overweight. Many experts criticized that work, saying the researcher — Katherine Flegal of the Centers for Disease Control and Prevention — painted a misleading picture by including smokers and people with health problems ranging from cancer to heart disease. Those people tend to weigh less and therefore make pudgy people look healthy by comparison.
Flegal's new analysis bolsters her original one, by assessing nearly 100 other studies covering almost 2.9 million people around the world. She again concludes that very obese people had the highest risk of death but that overweight people had a 6 percent lower mortality rate than thinner people. She also concludes that mildly obese people had a death risk similar to that of normal-weight people.
However, independent experts say the methods are too flawed to make those claims. Critics again have focused on her methods. This time, she included people too thin to fit what some consider to be normal weight, which could have taken in people emaciated by cancer or other diseases, as well as smokers with elevated risks of heart disease and cancer.
"Some portion of those thin people are actually sick, and sick people tend to die sooner," said Donald Berry, a biostatistician at the University of Texas MD Anderson Cancer Center in Houston.
The problems created by the study's inclusion of smokers and people with pre-existing illness "cannot be ignored," said Susan Gapstur, vice president of epidemiology for the American Cancer Society.
A third critic, Dr. Walter Willett of the Harvard School of Public Health, was blunter: "This is an even greater pile of rubbish" than the 2005 study, he said. Willett and others have done research since the 2005 study that found higher death risks from being overweight or obese.
Flegal defended her work. She noted that she used standard categories for weight classes. She said statistical adjustments were made for smokers, who were included to give a more real-world sample. She also said study participants were not in hospitals or hospices, making it unlikely that large numbers of sick people skewed the results.
The idea that being somewhat overweight could be linked to better health has been dubbed the "obesity paradox," even though actual obesity is generally not associated with the apparent "benefit."
"This is actually the common finding," said Flegal, a senior scientist from the CDC.
The paradox, as scientists have called it, is based on past findings that suggest overweight and obese people - even those with additional health problems - live longer than their thinner counterparts
Some have argued that the pattern is a statistical one only because being thin, especially in old age, is often a sign or a result of serious illness - so the thinner people seem to have higher mortality.
The study results certainly do not give people permission to pack on extra pounds, according to Dr. Steven Heymsfield, the executive director of the Pennington Biomedical Research Center in Baton Rouge, Louisiana.
Heymsfield, who co-authored an editorial accompanying the new report, said the difference in mortality between overweight and normal weight people is probably very small.
"That's actually a very small number. It's probably only statistically significant because of the large number she had in her study," he added.
Also, there are concerns that body mass index (BMI) - a measurement of weight in relation to height - is not an accurate measure of someone's health risks.
For example, Heymsfield said a soldier may be considered overweight but still be healthy, because he or she has more muscle mass.
"It's not a good marker for body fat or health risk," he said.
There is also confusion around what BMI should be considered "normal," which is why Flegal and her colleagues conducted their analysis.
Past studies looking at the link between BMI and death used varying ranges to describe normal weight, overweight and obesity.
"There seems to be a lot of confusion about this whole area, and part of the confusion is that people are using a bunch of different categories," said Flegal.
For the new analysis, the researchers, who published their results in the Journal of the American Medical Association, used data from past studies, and classified the risks according to BMI categories accepted by the World Health Organization and the U.S. National Heart, Lung, and Blood Institute.
Those organizations consider a BMI between 18.5 and 24.9 normal weight, between 25 and 29.9 overweight, and 30 or above obese. They further subdivide the obese category, though - with a BMI between 30 and 34.9 designated grade-1 obesity, and anything above 35 grades 2 or 3.
The researchers reviewed databases of medical research and found 97 studies that looked at weight and mortality risk. Combined, Flegal and her colleagues had information on about 2.9 million people from around the world and 270,000 deaths.
Being obese - in general - was linked to an 18 percent greater risk of death, compared to being normal weight. Being severely obese - grades 2 and 3 - was linked to a 29 percent greater risk of death.
However, being merely overweight was linked to a 6 percent decreased risk of death compared to a normal weight person, while being slightly (grade 1) obese was linked to a 5 percent lower risk.
The study cannot say why there seems to be a link between being overweight or slightly obese and a lower risk of death.
"We don't have the data to look at the physiological mechanisms, and that wasn't our goal," said Flegal.
"Our contribution - I hope - is just to summarize it to show what other articles are showing," she said.
For Heymsfield, the findings reinforce the common belief that increased weight is tied to an increased risk of death, but highlights the fact that someone classified as "overweight" is not necessarily "very unhealthy."
On the other hand, he warns, important markers of health, such as blood pressure and cholesterol, do respond to minor changes in weight.
"So gaining that extra 10 or 20 pounds can put you into a dangerous category, and it's important to find out if you're one of those people," he said.
Flegal's boss, CDC Director Dr. Thomas Frieden, said in a written statement, "we still have to learn about obesity, including how best to measure it," "However, 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."
The Associated Press and Reuters contributed to this report
More from TODAY Health: