Higher incidence of diabetic nephropathy in type 2 than in type 1 diabetesin early-onset diabetes in Japan

Background. Whether the type of diabetes, race, and year and age of diagnos is affect the incidence of diabetic vascular complications is unknown. That both type 1 and type 2 diabetes occur in the young Japanese population pro mpted us to investigate whether the type of diabetes and the year of diagno sis are related to the incidence of nephropathy. Methods. Of the 17,256 diabetic patients who visited the outpatient clinic at our diabetes center between 1965 and 1990, 1578 (9.1%) had early-onset d iabetes (diagnosed before the age of 30); of these, 620 (39%) had type 1, a nd 958 (61%) had type 2 diabetes. The incidence of nephropathy was analyzed in the patients according to postpubertal duration and year of diagnosis. Results. The cumulative incidence of nephropathy after 30 years of postpube rtal diabetes was significantly higher (P < 0.0001) in type 2 diabetic pati ents (44.4%, 95% CI, 37.0 to 51.8%) than in type 1 diabetic patients (20.2% , 95% CI, 14.9 to 25.8%). The incidence of nephropathy among type 1 diabeti c patients has declined during the past two decades, whereas it has not amo ng type 2 diabetic patients. The rate ratio for type 2 diabetic patients di agnosed between 1980 and 1984 relative to type 1 diabetic patients diagnose d in the same period was 2.74 (95% CI, 1.17 to 6.41). Conclusions. The incidence of nephropathy has declined in Japanese patients with type 1 but not in those with type 2 diabetes. In young Japanese patie nts, because of the higher incidence of nephropathy in type 2 diabetes and the higher prevalence of type 2 than type 1 diabetes, type 2 diabetes is li kely the major cause of diabetic nephropathy.