The largest study ever conducted of the health risks of obesity has found that it increases the likelihood of premature death but not as much as many medical experts had suspected.
The research, being published Thursday in The New England Journal of Medicine, analyzed the fates of 324,135 white adults who were followed for 12 years. The study found that the excess risk of dying associated with being fat was relatively modest and declined as people grew older. By age 65, the increased risk was slight, and by age 74 it had disappeared.
People who were moderately overweight but not obese had no increased risk of premature death, the study found.
Medical experts who discussed the results disagreed about how to advise Americans. Some, including the lead author of the study, said that any increased risk of premature death was reason enough to view obesity as a serious public health hazard.
Others said that the dangers of obesity had been exaggerated, and Americans' faith in the redemptive powers of weight loss may be misplaced.
The two top editors of the journal wrote in an editorial that losing weight was "an ill-fated New Year's resolution."
"The cure for obesity may be worse than the condition," the editors, Dr. Marcia Angell and Dr. Jerome P. Kassirir, wrote, explaining that the cures tend to be dire and ultimately ineffective.
Although there is a widespread belief that obesity places people at high risk of premature death and that when obese people lose weight they reduce their risk of dying young, previous studies were "limited, fragmentary and often ambiguous," the two doctors wrote. And, they added, even though it is often said that obesity causes 300,000 deaths a year in the United States, "that figure is by no means well established."
It was not clear why obesity should have smaller and smaller effects on death rates as people age. One hypothesis is that fat people who are susceptible to the calamitous consequences of excess weight die early, so those who remain are comparatively resistant to the effects of being fat, said Dr. Charles Hennekens, the chief of preventive medicine at Brigham and Women's Hospital in Boston.
The study did not look at the incidence of diseases associated with obesity, like diabetes, high blood pressure and high levels of blood cholesterol. About 6 percent to 8 percent of obese people develop diabetes, said Dr. Jules Hirsch, an obesity researcher at Rockefeller University, adding that "diabetes heads the list" of diseases associated with being fat.
The new study avoided many of the problems of earlier ones simply because it was huge, as large as all previous such studies combined, Hennekens said.
The lead author of the study, Dr. June Stevens, an epidemiologist and nutritionist at the University of North Carolina in Chapel Hill, said that she and her colleagues analyzed data collected by the American Cancer Society for a study of cancer prevention, reasoning that it constituted a pool of a million adults who had been followed for years and whose ages, weights and causes of death were known.
The investigators excluded people who had ever smoked because smokers tend to be thinner and also tend to have a higher death rate. They also excluded people who were ill or who had had a recent unexplained weight loss. But that still left more than 300,000 people who had been followed from 1960 to 1972.
In analyzing the data, Dr. Stevens and her colleagues considered body mass index as a measure of fatness. Body mass is defined as 4.89 times the weight in pounds divided by the height in feet squared.
The ideal body mass is usually said to be 21. For a woman who is 5 feet 5 inches tall, that corresponds to a weight of about 126 pounds. Every 6 pounds of weight gained would increase her body mass index by 1 point.
A man who is 5 feet 10 inches tall and had a body mass index of 21 would weigh 146 pounds. Every 7 pounds he gained would increase his body mass index by 1 point. (To calculate how many pounds equal one point in your body mass index, take your height in feet, square it, and multiply it by 0.204.)
But, the data show, even among the youngest people in the study, those ages 30 to 44, among whom the effects of obesity were most profound, there were no excess deaths associated with obesity until the body mass index was above 27 and, even then, the effects were relatively modest -- less than a 25 percent to 50 percent excess death rate -- until the body mass index was about 32, when the risk of death approximately doubles for those ages 30 to 54.
For those ages 55 to 64, a body mass index of 32 or greater was associated with a 50 percent increased risk of death. These are people who are well over 100 pounds overweight, Dr. Stevens noted.
Dr. Stevens said that she found the increase in death rates caused by obesity at younger ages to be alarming. And even though there is no good treatment, or even a proven prevention, that does not mean that people should be complacent, she said.
"Obesity is an extremely important public health problem," Dr. Stevens said. And, she added, with one third of Americans overweight, defined as body mass indexes above 27, "Americans are getting so fat that it's incredible."
Hennekens agreed. "The United States is not just the heaviest society in the world but probably the heaviest society in the history of the world," he said. And the new study "only reinforces the hazards of body weight," he said.
But Dr. Reubin Andres, the chief of metabolism at the National Institute on Aging in Baltimore, said it was not clear whether even if fat people lost weight, their risk of dying would correspondingly decline. He said some studies showed increased death rates in fat people who lost weight.
The mortality risk of obesity "is slight," Andres said, and nonexistent for the elderly.
"In effect, it says that when you are old, you do not have to pay much attention to body weight," Andres said.
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