Diet and exercise are the cornerstones for preventing type 2 diabetes. These measures are especially important for people with risk factors for diabetes, such as prediabetes, obesity, a family history of diabetes, diabetes during pregnancy (gestational diabetes), or an ethnic background (black, Hispanic, Asian, or Native American) that puts them at higher risk.

While being overweight and not getting enough exercise can both raise your risk for type 2 diabetes, a new study reported in Diabetes Care (Volume 30, page 53) suggests that shedding pounds may be more important for preventing diabetes.

Researchers followed close to 69,000 participants in the ongoing Harvard Nurses' Health Study. None of the women had a history of diabetes, cardiovascular disease, or cancer when the study began. Sixteen years later, 4,030 women had been diagnosed with diabetes. After adjusting for age, smoking, and other diabetes risk factors, increased waist size and body mass index (BMI) had a substantial impact on developing the disease.

Women who were considered very obese -- with a body mass index (BMI) of 40, for example -- had a 28 times higher risk of diabetes than normal or underweight women whose BMI was below 21. A lack of physical activity was not as big a factor. Even in those who exercised at least 20 hours a week, the risk of diabetes was 11 times greater in obese participants than in lean ones.

Bottom Line on Diabetes: These findings don't mean you should scale back on physical activity, which often goes hand and hand with losing weight. But since exercise may not always be enough to help you drop the pounds, watching what you eat should be an equally important goal.