It may not be true after all that being overweight protects people against certain types of mortality -- other than cancer and cardiovascular disease -- as some recent research has suggested.
According to a new study published in the New England Journal of Medicine, being overweight or obese is associated with a higher rate of death from all causes.
Researchers led by Amy Berrington de Gonzalez, an investigator at the National Cancer Institute in Bethesda, Md. analyzed data from more than 1.5 million white adults who participated in 19 past studies that evaluated risk for developing cardiovascular disease.
"We conducted the study to try and clarify the relationship between BMI [body mass index] and all-cause mortality, in part to answer two questions: what the optimal BMI range is and the risk associated with being overweight, or having a BMI of 25 to 30," said de Gonzalez. "There's particular uncertainty with regard to this second question."
de Gonzalez said she and her team selected only subjects who were healthy non-smokers.
"Both poor health and smoking can bias the relationship between BMI and mortality, she said. "It's one of the possible explanations of the previous study findings that found the protective effect of being overweight."
De Gonzalez and her fellow researchers found that overweight participants had a 13 percent higher mortality risk than people in the lower BMI ranges, contrary to those previous findings. The beneficial effect of being overweight that other researchers found could also be due to other factors in addition to smoking and poor health, said obesity experts.
"We call it the 'obesity paradox,'" said Dr. Robert Kushner, clinical director of Northwestern Memorial Hospital's Comprehensive Center on Obesity in Chicago. "In certain subgroups, such as people with certain types of cancers, there may be a protective effect of being overweight."
"Those studies in general deal with older people," said Dr. Richard Atkinson, clinical professor of pathology at Virginia Commonwealth University in Richmond. "As you get older, having a little extra fat allows you to survive a heart attack, survive trauma and other things. It's nice to have a little extra reserve." "Some of these studies that found that being slightly overweight was protective didn't necessarily assess for the type of body fat," said Dr. Philip Schauer, director of The Cleveland Clinic Bariatric and Metabolic Institute. "For example, fat on the belly is worse than fat on the buttocks or thighs."
But one biostatistician said that the association between overweight participants and mortality is small, and is likely due to confounding variables.
"What [the authors] don't point out is that the study model they used might also incompletely adjust for unmeasured confounding factors," said Bruce Levin, professor and chair of the Department of Biostatistics at Columbia University's Mailman School of Public Health in New York.
De Gonzalez acknowledged that some variables may have interfered with their data.
"The increased risk for overweight people was small," said de Gonzalez. "You can never rule out confounding, but we ruled out everything we had information on."
Another of the study's limitations was the analysis of only white adults.
"Different ethnic groups, especially Native Americans, African Americans and people from South Asia seem to tolerate obesity less and are more prone to obesity-related comorbidities," said Schauer. "The association might be stronger in those groups."
Others believe the association would be similar to the one seen in whites, and the authors wrote, "Similar analyses are underway in other populations."
The study also found an association between being underweight and a higher risk for mortality.
"With a BMI below normal, we see a rise in mortality. People who have cancer could lose body mass," said Schauer. "It could be an indicator they have another chronic illness."
Despite the study's weak points, obesity experts say the fact that it's very large makes it statistically powerful. They also say the study drives home a long-held belief.
"This study is an updated version of what we've had for many years," said Atkinson. "Back in the 60s and 70s, there were [results] that looked just like this."
"The association between BMI and premature mortality has been known for quite a while, but it strengthens the whole argument," said Schauer.
"It shows the lowest mortality in the BMI range between 20 and 25. This coincides with the guidelines we currently have when we assess someone's body weight," said Kushner. "It's the healthy weight range."