The predictive effect of body mass index on type 2 diabetes in the Norwegian women and cancer study

Background: Several studies have analyzed the association of body mass index (BMI) with either the prevalence or incidence of type 2 diabetes (T2D), but no study from Europe or North America has yet analyzed and compared the association of BMI with both incident and prevalent T2D cases. Methods: Stratified logistic regression was used to calculate odds ratios (OR), and stratified Cox proportional hazards regression was used to calculate hazard ratios (HR) of the effect of BMI on the prevalence, and incidence of T2D. Wald chi-square statistics were applied when comparing the risk estimates. Results: Among prevalent T2D cases, overweight women (BMI 25-29.9 kg/m(2)) had an OR of 2.83 (95% confidence interval [CI], 1.92-4.18) and obese women (BMI >= 30 kg/m(2)) had an OR of 12.12 (95% CI, 8.32-17.68) when compared with normal weight women (BMI <25 kg/m(2)). Among incident T2D cases, overweight women had a HR of 5.01 (95% CI, 3.59-6.98) and obese women had a HR of 15.99 (95% CI, 11.39-22.46) when compared with normal weight women. After stratification by level of physical activity, and adjustment for age, smoking status, and education level, the Wald chi-square statistic for BMI was 180.90 for prevalent T2D cases, and 262.03 for incident T2D cases. Conclusion: The predictive effect of BMI was found to be stronger for T2D incidence than T2D prevalence.