In contrast, having a high body-mass index -- a measure of weight in relation to height, often used to gauge obesity -- was not associated with early death in the study.
The investigators said the findings support the idea that BMI is a fairly crude measure that may not reflect a person's body composition, or be a good indicator of health.
Someone with a lot of muscle mass, for example, may have a high BMI and, technically, fall into the "overweight" category, explained researcher Dr. William Leslie.
So the relationship between body size and health "is more nuanced than the number on your bathroom scale," said Leslie, a professor of medicine and radiology at the University of Manitoba, in Winnipeg, Canada.
"It's important to be attuned to what you're made of, rather than just how much you weigh," Leslie said.
The findings, published online March 8 in the Annals of Internal Medicine, may offer one explanation for the so-called "obesity paradox."
That refers to a counterintuitive pattern that's been seen in a number of studies: Overweight and moderately obese people with heart disease or other chronic ills tend to outlive thinner people with those same conditions.
But those studies have often relied on BMI, Leslie explained. And it's possible that higher BMI reflects greater muscle mass and fitness, or less weight loss from a chronic disease -- as opposed to some protective effect of body fat, he added.
For their study, Leslie's team combed through data on more than 54,000 adults, mostly in their 60s, who'd undergone DXA scans to measure their bone density. Those bone scans have the bonus of allowing an estimate of a person's body fat percentage.
It turned out that men and women with the greatest amounts of body fat were more likely to die over the next four to seven years, the study showed.
Men in the top 20 percent had at least 36 percent body fat. And those with highest body fat were up to 59 percent more likely to die during the study period, versus men whose body fat was in the 28 percent to 32 percent range -- which was about average for the group, according to the study.
The difference was smaller among women. Still, those with the highest percentage of body fat -- about 39 percent fat or higher -- were 19 percent more likely to die during the study period, compared with women in the 30 percent to 34 percent range (about average for the group), the study found.
In contrast, people with a BMI high enough to land them in the "obese" category didn't show an increased death risk. And they were actually less likely to die than men and women with the lowest BMIs -- lower than 24 or 25, which includes people in the "normal" weight range, Leslie pointed out.
In these older adults, he explained, a lower BMI may reflect waning muscle mass or frailty.
A researcher not involved in the study agreed.
"I think these findings help clarify some of the confusion around the obesity paradox," said Rebecca Shenkman, director of the MacDonald Center for Obesity Prevention and Education at Villanova University College of Nursing, in Pennsylvania.
More importantly, she said, the findings highlight the limits of BMI as a health indicator. "We really need to take a step back and look at everything's that going on the body," Shenkman said.
And it's not only about body fat, she noted. Fitness levels matter, too: Studies have found that people who stay fit through exercise typically enjoy a longer life than couch potatoes -- even if they're overweight.
And, Shenkman said, it's possible to be thin and out-of-shape.
"Healthy eating and regular exercise are more important than being skinny," she said.
Leslie made the same point. "In our society," he said, "there's been this mantra that thin is 'in,' and being heavy is 'bad.' But health is about more than the number on your scale."
He did not, however, suggest that people run out to have their body fat measured. Taking a tape measure to your waistline, for example, is a simpler way to estimate how much fat you're carrying, Leslie noted.
Women who have a waist circumference of more than 35 inches have a higher risk for heart disease and type 2 diabetes, according to the U.S. National Heart, Lung, and Blood Institute. For men, a waist circumference greater than 40 inches indicates an increased risk of health problems, the NHLBI says.More information
The U.S. Centers for Disease Control and Prevention has more on maintaining a healthy weight.
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