WEDNESDAY, Dec. 1, 2010 (HealthDay News) -- Being too fat can shorten your life, but being too skinny may cut longevity as well, a new study suggests.
Using data on almost 1.5 million white adults culled from 19 separate analyses, researchers from the U.S. National Institutes of Health (NIH) found that 5 percent of the U.S. population can be classified as morbidly obese -- a number five times higher than previously thought.
With a body mass index (BMI) of 40 or higher, the morbidly obese had a death rate more than double that of those of normal weight, according to study author Amy Berrington de Gonzalez.
BMI is a measurement of body fat based on height and weight. Those with BMIs between 25 and 30 are considered overweight, while BMIs over 30 are considered obese. The study, which sought to establish an optimal BMI range, showed it to be between 20 and 25 in those who never smoked, and 22.5 to 25 in those who did.
Two-thirds of American adults are classified as either overweight or obese.
"We were focusing mostly on high BMI -- over 25 -- and the aim was to clarify the relationships [between weight and longevity] rather than expect to find anything completely new," said Berrington de Gonzalez, an investigator with the National Cancer Institute's division of cancer epidemiology and genetics in Bethesda, Md.
Although her team did not calculate the number of life years potentially lost due to obesity, they determined the highest death rates for this group were from cardiovascular disease. About 58 percent of study participants were female, and the median baseline age was 58.
More than 160,000 participants died during the time they were followed, which ranged between five and 28 years, and 35,369 of those deaths were among people who had never smoked and had no history of cancer or heart disease. Results proved similar for men and women, whose median baseline BMI was 26.2.
The large sample included in the study, reported in the Dec. 2 issue of the New England Journal of Medicine, enabled researchers to evaluate differences according to age, gender, follow-up time and physical activity level. Researchers decided to focus only on non-Hispanic whites because the relationship between BMI and mortality may differ across racial and ethnic groups.
"This confirms that the population is getting fatter -- that's been known," said Dr. Michael J. Joyner, a professor of anesthesiology at the Mayo Clinic with experience in exercise physiology, human physiology and body composition issues. "I see this data as confirmatory."
Joyner and Berrington de Gonzalez noted that the study results also associated being underweight with higher mortality rates, though the reasons why aren't completely clear.
Study participants with very low BMIs -- between 15 and 18 -- died at higher rates than those with BMIs between 22.5 and 24.9, according to the research, which attributed this at least partially to pre-existing diseases in the underweight group.
The association between low BMI and death rates was somewhat weaker among those who exercised than those who were inactive. Smokers accounted for one-quarter of the study participants in the lowest BMI category, but only 8 percent of those in the highest BMI category of 40 to 49.9. Pre-existing cancer and emphysema were slightly more common in the low-BMI categories, while pre-existing heart disease was more common as BMIs increased.
"One interpretation is that people had low BMIs because they lost weight because they were already ill," Berrington de Gonzalez said. "Or that being underweight puts you at a higher risk of death. We can't say for certain which explanation is the right one."
For more on BMI, go to the U.S. Centers for Disease Control and Prevention.