Familial Risks for Type 2 Diabetes in Sweden

OBJECTIVE Our aim was to characterize familial risks for type 2 diabetes by the type and number of affected family members, including half-siblings, adoptees, and spouses, to quantify risks and estimate the contribution of environmental effect. RESEARCH DESIGN AND METHODS Families were identified from the Multigeneration Register, and type 2 diabetic patients were obtained from the Hospital Discharge Register. Standardized incidence ratios were calculated for offspring with type 2 diabetes whose family members were hospitalized for type 2 diabetes at ages >39 years compared with those lacking affected family members. RESULTS The number of hospitalized type 2 diabetic patients was 157,549. Among 27,895 offspring, 27.9% had a parent or sibling also hospitalized for type 2 diabetes. The familial relative risk (RR) ranged from 2.0 to >30, depending on the number and type of probands. The highest RRs of type 2 diabetes were found in individuals who had at least two siblings affected by type 2 diabetes, irrespective of the parental disease. Adoptees showed no risk from adopted parents. CONCLUSIONS The study, the largest yet published, showed that familial RRs varied by the number and type of affected family member. However, much of the familial clustering remains yet to be genetically explained. The high risk should be recognized in clinical genetic counseling. The data from adoptees confirmed the genetic basis of the familial associations, but those from half siblings and spouses suggested that a smaller part of familial clustering may be accounted for by environmental factors.