NEW YORK - Although obesity and lack of physical activity both raise the risk of type 2 diabetes in women, obesity appears to be the more important factor, researchers report in the journal Diabetes Care.

Dr. Frank Hu of the Harvard School of Public Health, Boston, and colleagues note that the relative contribution of obesity and inactivity to the risk of developing type 2 diabetes remains controversial.

To investigate further, the researchers monitored 68,907 women taking part in the Nurses' Health Study, a large ongoing study that is evaluating women's health over time. The women in the current trial had no history of diabetes, cardiovascular disease or cancer at study entry. During 16 years of follow-up, there were 4,030 incident cases of type 2 diabetes.

After allowing for age, smoking, and other diabetes-associated factors, the risk of type 2 diabetes increased progressively with increasing body mass index (BMI - the ratio of height to weight often used to determine if someone is overweight or too thin). The risk also increased with waist circumference, and decreased with physical activity levels.

Using women who had a healthy weight (BMI of less than 25) and were physically active as the reference group, the relative risks of type 2 diabetes were 16.75 in women with a BMI of 30 or more and were inactive. The corresponding risk in obese women who were active was 10.74. In women who were lean but inactive, the relative risk was 2.08.

Although both variables were significant predictors of type 2 diabetes, the researchers found that the association for waist circumference was substantially stronger than that for physical inactivity.

They researchers conclude that "the magnitude of risk contributed by obesity is much greater than that imparted by lack of physical activity," and therefore "weight loss and maintenance of healthy weight should be emphasized as an eventual goal to prevent the onset of type 2 diabetes."

SOURCE: Diabetes Care, January 2007.