WEDNESDAY, Dec. 22, 2004 (HealthDayNews) -- Increased physical activity can't overcome the potentially deadly effects of obesity, a new study concludes.
The 24-year study of more than 116,000 women found that obesity "predicted a higher risk of death regardless of the level of physical activity," according to a report in the Dec. 23 issue of the New England Journal of Medicine.
The study was done by researchers at the Harvard School of Public Health.
It's a potentially controversial finding, said study author Dr. Frank B. Hu, an associate professor of nutrition and epidemiology. The reason: A theory has been floated that "if you are fat but physically fit you don't have to worry about your weight because physical activity can cancel out the risk associated with obesity."
"This is not supported by our data," Hu said. "Our data suggest that obesity is a risk factor independent of physical activity. People who are obese and inactive have the highest risk."
But an accompanying editorial in the journal suggests the new findings may not settle the debate because the definition of physical activity used in the study was not as strict as in some other studies that found exercise did have a protective effect.
"One large study that looked at men found that physical activity was a much truer predictor of mortality rate than body mass index [BMI]," said Mark A. Pereira, an assistant professor of epidemiology at the University of Minnesota, and co-author of the editorial.
BMI, a ratio of weight to height, is a standard measure of fatness. Anyone with a BMI of 25 to 29.9 is regarded as overweight; a BMI of 30 or greater is defined as obese.
The new study found that obesity and lack of physical activity were independent predictors of the risk of death. A high level of physical activity did not eliminate the risk of premature death associated with obesity.
For example, excess weight plus limited physical activity -- less than three-and-a-half hours of exercise a week -- accounted for 31 percent of all premature deaths; 59 percent of deaths from cardiovascular disease; and 21 percent of deaths from cancer among nonsmoking women.
Inevitably, women who were physically inactive and obese fared the worst. Their risk of death was two-and-a-half times higher than for physically active, lean women.
Pereira said a weakness of the study was that it relied on self-reporting by the participants to measure physical activity. Any woman who reported less than 3.5 hours of physical activity per week was listed as inactive.
"The difference is that the relevant study of men used a treadmill test to measure physical fitness," he said. "A related limitation [of the new study] is that they did not assess how much time women spent in sedentary activities, such as watching television."
And so, Pereira said, "Where the study leaves us is that BMI is a good marker of disease risk and mortality, but a far from perfect marker. In and of itself, BMI says nothing about body composition and lifestyle. Lifestyle is going to predict longevity."
Hu was in basic agreement with that assessment.
"Some people cannot maintain a healthy weight even with exercise, if they don't pay attention to diet," he said. "It is an issue of both diet and exercise, not of one or another. Really, both are important."
The American Heart Association offers guidelines for a healthy lifestyle.