Epidemiological and geographic factors in diabetes.

Both non-insulin-dependent (type 2) diabetes mellitus (NIDDM) and insulin-dependent (type 1) diabetes mellitus (IDDM) show a wide variation in incidence and prevalence in different populations. The prevalence of NIDDM varies from close to zero in some populations to 40-50% in the adults of Nauru (Pacific) and in the Pima Indians (North America). The incidence of IDDM in children under 16 years ranges from around 30/100,000 children per year in Finland and other Northern European countries to less than 1/100,000 per year in Japanese children. Many genetic and environmental factors combine to produce this variation. Among the suggestions made for NIDDM are the 'thrifty genotype' and, more recently, the 'thrifty phenotype' hypotheses of Neel and of Hales and Barker respectively. Genetic and environmental factors in IDDM in children have combined to create an apparent gradient of decreasing incidence from northern to southern European countries but with at least one local 'hot spot': the island of Sardinia. The factors responsible for this pattern, and the increasing incidence over time, require further clarification.