WASHINGTON - Being overweight boosts the risk of dying from diabetes and kidney disease but not cancer or heart disease, and carrying some extra pounds appears actually to protect against a host of other causes of death, federal researchers reported yesterday.
The counterintuitive findings, based on a detailed analysis of decades of government data about more than 39,000 Americans, suggest that being overweight does carry risks, but the dangers may be less dire than specialists thought.
"The take-home message is that the relationship between fat and mortality is more complicated than we tend to think," said Katherine Flegal, a senior research scientist at the Centers for Disease Control and Prevention in Atlanta, who led the study. "It's not a cookie-cutter, one-size-fits-all situation where excess weight just increases your mortality risk for any and all causes of death."
The study, published in the Journal of the American Medical Association, was greeted with sharply mixed reactions.
Some praised it for providing persuasive evidence that the dangers of fat have been overblown.
"What this tells us is the hazards have been very much exaggerated," said Steven Blair, a professor of exercise science, epidemiology, and biostatistics at the University of South Carolina. "It's just not as big a problem as people have said."
But others dismissed the findings as fundamentally flawed, saying an overwhelming body of evidence has documented the risks of being either overweight or obese.
"It's just rubbish," said Walter Willett, professor of epidemiology and nutrition at the Harvard School of Public Health. "It's just ludicrous to say there is no increased risk of mortality from being overweight. . . . From a health standpoint, it's definitely undesirable to be overweight."
The proportion of flabby Americans has been rising steadily, and two-thirds are now classified as overweight, including about one-third who have put on so many pounds they qualify as obese. The trend has triggered widespread warnings of an impending epidemic of diabetes, heart disease, cancer, and other ailments.
Flegal and her colleagues raised the possibility two years ago that being overweight was less risky than feared. Their analysis of data from decades of federal surveys concluded that people who were overweight - but not obese - had lower mortality rates than those of normal weight. But their study came under heavy criticism.
In the new research, the team sought to confirm and expand on the original findings, examining additional data from later surveys and parsing individual causes of death across a range of weights. The analysis is based on the best health statistics federal scientists collected between 1971 and 2004, including cause-of-death data from 2.3 million adults from 2004.
The researchers used federal definitions of "overweight" and "obesity" based on body mass index, or BMI. A BMI between 25 and 30 classifies someone as overweight, and above 30 as obese. For example, a 5-foot-4 woman is considered overweight at 145 pounds and obese at 175.
The researchers calculated that in 2004, obesity was associated with as many as 112,000 excess deaths from heart disease and more than 45,000 deaths from diabetes and kidney disease. Obesity was not, however, associated with an overall excess in cancer deaths, though it was linked to as many as 19,000 excess deaths from malignancies commonly blamed on fat, including breast, uterine, ovarian, kidney, colon, esophageal, and pancreatic cancer.
The most surprising finding was that being overweight but not obese was associated only with excess mortality from diabetes and kidney disease, not cancer or heart disease. Moreover, the researchers found an apparent protective effect against all other causes of death, such as tuberculosis, emphysema, pneumonia, Alzheimer's, and injuries. An association between being overweight and nearly 16,000 deaths from diabetes and kidney disease was overshadowed by a reduction of up to 133,000 deaths from all other noncancer, nonheart disease causes. Even moderately obese people appeared less likely to die of those causes.
Although the study did not examine why being overweight might guard against dying from some diseases, Flegal said other research has suggested that extra heft might supply the body with vital reserves to draw upon to fight illness and aid recovery.
"You may not just have more fat. You may also have more lean mass - more bone and muscle," Flegal said. "If you are in an adverse situation, that could be good for you."
Willett and others, however, said the study was too small to account fully for factors such as smoking and preexisting illnesses that could skew the findings. They added that being overweight not only increases the risk of dying from many major diseases, but also has many other consequences.
"I think it would be very unfortunate if these findings made us complacent about becoming overweight," said JoAnn Manson, chief of preventive medicine at Brigham and Women's Hospital in Boston. "We know being overweight is linked to increased incidence of major chronic disease, including diabetes, hypertension, and cardiovascular disease, impairs physical function, and decreases quality of life."