A recent Harvard study shows that getting more exercise may not be enough to counteract the deadly effects of obesity.

The findings, part of a study led by Frank Hu, Associate Professor of Nutrition and Epidemiology at the School of Public Health, correlated body-mass index and physical activity to the death rate of 116,564 female nurses over a period of 24 years.

“We did this research to disprove the very misleading hypothesis that if you’re fit, you don’t need to worry about extra weight,” Hu says. “We found that even a modest weight gain during adulthood, independent of physical activity, is associated with a higher risk of death.”

The results, published last month in the New England Journal of Medicine, indicate that while active women were less likely to die than inactive ones, active, obese women still had a significantly higher risk of death than their lean counterparts. Hu defined active obese women as those who spent 3.5 or more hours exercising per week with a body-mass index—weight in kilograms over the square of the person’s height in meters—of 30 or higher.

Over two-thirds of Americans are overweight, and one-third are obese. According to the Centers for Disease Control and Prevention, obesity now rivals smoking as the leading preventable cause of death.  

Scientists have long debated whether exercise can counteract the effects of excess weight, which would make physical activity a more important risk inhibitor than body-mass index. Proponents of the “fat but fit” hypothesis say that body-mass index is too crude a measure because it ignores body composition and levels of fitness. 

“Existing data shows it is absolutely possible to be overweight, active and healthy,” said Timothy Church, Medical Director at the Cooper Institute, a large non-profit research and teaching institute focusing on diet and health.  “We are overly fixated as a society on obesity, which misses the point of the problem, which is poor diet coupled with sedentary lifestyles.”

A similar 1999 study led by Chong-Do Lee, then at West Texas A&M University, had tracked 22,000 men over an average of 8 years and concluded that a high body-mass index conveyed no extra risk in the most fit people.

Critics like Church also say that stigmatizing obese people makes the public health problem worse. “Most obese individuals who try and lose weight are going to fail, and when that happens they tend to give up even trying to be fit,” he said. His research suggests that fitness is more important than fatness in minimizing risk, and that up to 40 percent of obese individuals are “fit.”

But Hu’s study—the largest of its kind—agrees with the position of scientists like those at the American Heart Association (AHA), who hold that obesity is an independent predictor of cardiovascular disease. 

While existing data only covers women, Hu said the findings hold for men as well.

“The self-reported physical activity data was successfully tested against multiple independent fitness markers, and we cited several other studies that found similar trends in male populations,” he said.

According to Barbara Moore, president and CEO of Shape Up America!, a nonprofit obesity-awareness organization, exercise is the only long-term way to safely lose weight and stay fit.

“The bottom line is that we should be aiming for both healthy weight and fitness. I think they are both equally important for long-term good health,” she said.