The growing incidence of overweight and obesity has caused an increase in the number of people with type 2 diabetes. Other contributors are lack of physical activity, poor diet, and smoking. Because these habits often go hand-in-hand, Harvard University researchers examined the effects of combinations of various risk factors and published their findings in The New England Journal of Medicine.

The 85,000 female volunteers were part of the on-going Nurses' Health Study. All were healthy and free of diagnosed cardiovascular disease, diabetes, and cancer at baseline in 1980. Approximately every two years, the volunteers completed detailed questionnaires about their weight, diet, physical activity, health status, smoking habits, and alcohol consumption.

Defining the risk factors

The investigators classified each volunteer from low to high risk for diabetes, based on criteria similar to that used to analyze coronary disease. Low risk was described as a body mass index (BMI) of less than 25, at least 30 minutes of moderate to vigorous exercise each day, no current smoking, about half an alcoholic drink per day, and a diet high in fiber and polyunsaturated fat and low in trans fat and glycemic load. New cases of diabetes, as defined by National Diabetes Data Group standards, were also recorded during the 16 years of follow-up.

Body mass index was the strongest predictor of type 2 diabetes; as it increased so did risk. The relative risk was 7.59 (95% CI: 6.27, 9.19) for women with a BMI of 25 to 30, and 20.1 (95% CI: 16.6, 24.4) for those with a BMI of 30 to 35. Even those with a BMI at the high end of normal, 23 to 25, had a substantially higher risk (RR: 2.67, 95% CI: 2.13, 3.34) than those who had a BMI less than 23.

However, lack of exercise, a poor diet, current smoking, and abstinence from alcohol also significantly increased diabetes risk, regardless of BMI. The researchers looked at diabetes risk after combining these factors and found that women who were classified as low risk in three categories (BMI, diet, and exercise) had a relative risk of diabetes of only 0.12.

Type 2 diabetes is largely preventable

Most striking was the population attributable risk. Researchers estimate that 87% (95% CI: 83, 91) of the new diabetes cases could have been prevented by three lifestyle modifications: maintaining a BMI less than 23, consuming a healthful diet, and following a regular exercise program. Yet, less than 10% of the women studied fit these criteria.

Appealing to patients

Because BMI is such as important determinant of type 2 diabetes, weight control is probably the most effective way to reduce risk. But even among overweight and obese people, a healthful diet, moderate exercise, and smoking cessation could substantially lower risk. In addition, diet and exercise are the primary factors in determining weight loss. And that, more so than diabetes risk, may be the real motivator for patients.