The evidence for the association between obesity and the risk of type 2 diabetes has been derived mainly from the analysis of the degree of obesity. The role of the duration of obesity as an independent risk has not been fully explored. The objective of the present study was to investigate the association between the duration of obesity and the risk of type 2 diabetes.

Prospective cohort study.

The Framingham Heart Study (FHS), follow-up from 1948 to 1998.

A total of 1256 FHS participants who were free from type 2 diabetes at baseline, but were obese on at least two consecutive of the study’s twenty-four biennial examinations, were included. Type 2 diabetes status was collected throughout the 48 years of follow-up of the study. The relationship between duration of obesity and type 2 diabetes was analysed using time-dependent Cox models, adjusting for a number of covariates.

The unadjusted hazard ratio (HR) for the risk of type 2 diabetes for men was 1·13 (95 % CI 1·09, 1·17) and for women was 1·12 (95 % CI 1·08, 1·16) per additional 2-year increase in the duration of obesity. Adjustment for sociodemographic variables, family history of diabetes, health behaviour and physical activity made little difference to these HR. For women the evidence of a dose–response relationship was less clear than for men, particularly for women with an older age at obesity onset.

The duration of obesity is a relevant risk factor for type 2 diabetes, independent of the degree of BMI.