Amy R. Weinstein, M.D., M.P.H., formerly of Brigham and Women's Hospital, Boston, and colleagues investigated the combined relationship of BMI and physical activity with diabetes to understand whether increasing physical activity levels reduces the elevated risk of diabetes from obesity. The study included 37,878 participants from the Women's Health Study, an ongoing clinical trial. The participants were free of cardiovascular disease, cancer, and diabetes at the beginning of the study, and follow-up averaged about 7 years. Weight, height, and recreational activities were reported at study entry. Normal weight was defined as a BMI of less than 25; overweight, 25 to less than 30; and obese, 30 or higher. A 5'4" woman would have a BMI of 25 if she weighed 145 lbs.; a BMI of 30 if she weighed 174 lbs. Active was defined as expending more than 1,000 kilocalories on recreational activities per week.

The researchers found that individually, BMI and physical activity were significant predictors of incident diabetes. Compared with normal-weight individuals, overweight individuals had a 3.2 times increased risk for diabetes; obese individuals, a 9.1 times increased risk. For overall activity (kilocalories expended per week), compared with the least active first quartile, a decreased risk of diabetes in the other quartiles ranged from 9 percent to 18 percent.

"We observed a modest reduction in the risk of diabetes with increasing physical activity level compared with a large increase in the risk with increasing BMI," the authors write. "These findings underscore the critical importance of adiposity [level of fat content] as a determinant of type 2 diabetes. Because physical activity is a significant individual predictor and has a beneficial effect on BMI, it remains an important intervention for diabetes prevention. Our study suggests that to further reduce the risk of diabetes with physical activity, it should be performed in conjunction with achieving weight loss. By furthering our understanding of the relative influence of BMI and activity on diabetes, we may improve our ability to risk stratify patients and in turn may reduce the incidence of diabetes," they write.

(JAMA. 2004; 292:1188-1194. Available post-embargo at JAMA.com)

Editor's note: This study was supported by research grants from the National Institutes of Health, Bethesda, Md.. Dr. Weinstein is now at Beth Israel Deaconess Medical Center, Boston.

Editorial: The Fitness, Obesity, and Health Equation – Is Physical Activity the Common Denominator?

In an accompanying editorial, Steven N. Blair, P.E.D., and Tim S. Church, M.D., M.P.H., Ph.D., of the Cooper Institute, Dallas, write that the findings of Wessel et al and Weinstein et al provide a timely opportunity to examine an ongoing debate and offer a resolution.

"The results presented by Weinstein et al suggest that increased BMI is substantially more important for incident diabetes, and Wessel et al suggest that inactivity or low fitness is a greater threat to health in terms of CVD outcomes. In recent years, the 'fitness vs. fatness' issue has led to controversy and heated debate. Although the debate may never be fully resolved the relative contribution of fitness and obesity to overall health and risk actually may be a trivial matter because a common treatment is already available for both low fitness and excess body weight. Increasing regular physical activity results in predictable increases in fitness, and it is widely accepted that regular physical activity is a core component of successful weight loss programs and, more importantly, of long-term weight loss maintenance.

"In essence, physical activity is the common denominator for the clinical treatment of low fitness and excess weight, making the 'fitness vs. fatness' debate largely academic. Thus, physicians, researchers, and policymakers should spend less energy debating the relative health importance of fitness and obesity and more time focusing on how to get sedentary individuals to become active," they write.