Incidence of insulin-dependent diabetes mellitus among Sardinianheritage children born in Lazio region, Italy

Background The relative importance of genetic and environmental factors in causing insulin-dependent diabetes mellitus (IDDM) is unknown. We studied this question by assessing the incidence of the disease in children, born in a region with a low incidence of IDDM (Lazio), but whose parents came from a region with high incidence (Sardinia). Methods We identified all IDDM cases that occurred between 1989 and 1994. We used as the denominator the number of children aged 0–14 born in Lazio of Sardinian parents to calculate incidence. We compared this rate with the incidences of IDDM in the populations of Lazio and Sardinia. Findings The age-adjusted incidence of IDDM in Sardinian-heritage children born and living in Lazio was 33•8 per 100 000 per year (95% CI 7•0–99•0) for those with two Sardinian parents, and 15•9 (8•7–26•6) for those with only one parent from Sardinia. The former incidence was not different from that recorded in Sardinia (34•4, 31•3–37•9), but was fourfold that of Lazio-heritage children (7•9, 7•1–8•8). Interpretation Our results show that two different ethnic groups living in the same region have a fourfold difference in incidence of IDDM. Children of Sardinian-heritage born in Lazio have the same incidence as the population of origin, which is genetically prone to the disease. Moreover, children with one Sardinian parent had a rate half that of Sardinians and double that of the indigenous population. We conclude that in a given population genetic susceptibility determines the frequency of IDDM in response to the environmental challenge.