WASHINGTON -- Physical activity can offset some of the harmful health consequences of being overweight but cannot fully erase them, according to the biggest study to examine the relative benefits of being fit versus fat.
The study of more than 115,000 female nurses found weight and activity levels are both powerful predictors of longevity, and that being either overweight or sedentary independently increases the risk of death.
But the study concluded that the healthiest people are those who are both thin and physically active, a blow to those who have argued that fitness is more important than fatness and can offset the risks of being overweight.
Women who were obese and inactive had the highest mortality: 2 times that of women who were thin and active. Those who were physically active despite being obese were still nearly twice as likely to die as those who were lean and active.
''Physical activity reduces the effects of being overweight, but it's far from removing all the increased risk associated with obesity," said Frank Hu, an associate professor of nutrition and epidemiology at the Harvard School of Public Health, who led the study being published in today's New England Journal of Medicine.
Proponents of fitness welcomed the findings that physical activity is important but questioned the conclusion that being overweight and healthy were mutually exclusive.
''We live in a society where it's, 'obesity, obesity, obesity,' and we're missing the point. Obesity is a symptom of poor behavior, which is physical inactivity and poor diet," said Timothy Church, medical director of the Cooper Institute, a Dallas research center that focuses on fitness. ''I'm still convinced it is possible to be overweight, active, and healthy."
But other specialists said the findings provide the most compelling evidence to date that while physical activity helps, any excess weight carries risks.
More than two-thirds of Americans are overweight, and one-third are obese, raising alarm that the nation is facing a health crisis because being overweight increases the risk for a host of health problems, including heart disease, cancer, and diabetes.
But a small but vocal cadre of researchers have argued that while being overweight can increase the risk for health problems, there has been too much emphasis on losing weight, which is difficult for most people, and not enough on increasing physical activity, which is more attainable and could be more important.
The new study involved data collected between 1976 and 2000 on 116,564 women participating in the Nurses' Health Study, an ongoing project in which thousands of nurses are being tracked for years to study various factors affecting health.
Those who were physically active for at least 3 hours each week were considered active, and therefore fit. Lean was defined as having a body mass index -- a standard measurement of height and weight -- below 25.
The results show that ''physical activity attenuates the effects of obesity, but it's far from removing all the increased mortality associated with obesity," Hu said.
That held true for women who were not obese but were still overweight. Women who were overweight and inactive had a 64 percent elevated risk of mortality, compared to 28 percent higher risk for those who were overweight but active.