Aims: The present study sought to investigate whether the effect of obesity on the incidence of type 2 diabetes varies with age among Japanese.
Methods: Employing two independent cohorts, one in Chiba City from 1994 to 2005 and the other in Kashiwa City from 2002 to 2006, the combined effect of body mass index (BMI) and age on the incidence of type 2 diabetes was evaluated by Cox regression analysis. A total of 37,564 and 26,959 subjects were enrolled in the cohorts and the follow-up rate was 94.7% and 93.3%, respectively.
Results: In the Chiba cohort, the hazard ratio for incidence of type 2 diabetes was significantly higher in obese subjects (25.0 kg/m(2) < or = BMI) than in normal weight subjects (18.5 kg/m(2) < or = BMI < 25.0 kg/m(2)) across all age groups, with the highest hazard ratio observed in the youngest group aged 40-59 years. In the Kashiwa cohort, the hazard ratio was also significantly higher in obese subjects than in normal weight subjects in men aged 40-59 and 70-79 years and in women aged 40-59 years. Analysis for the interaction between age groups and obese subjects versus normal weight subjects revealed significant weakening of the effect of obesity in women in both cohorts in subjects aged 60-69 and 70-79 years compared to younger subjects aged 40-59 years. In men, however, a significant weakening of the effect was observed only in subjects aged 60-69 years in the Chiba cohort. The interaction between four BMI categories including an extremely obese group (30.0 kg/m(2) < or = BMI) and age category was significant in women (P < 0.001) but not in men (P = 0.113) in the Chiba cohort.
Conclusion: Based on data from the two independent cohorts, the effect of obesity on the incidence of type 2 diabetes was found to decline with age in Japanese women but not in men.