Migrant populations and the incidence of type 1 diabetes mellitus: an overview of the literature with a focus on the Spanish-heritage countries in Latin America.

Type 1 diabetes mellitus (DM) is a 'chronic' autoimmune disorder leading to the destruction of the pancreatic beta cell. The natural history of diabetes includes a long subclinical (prediabetes) period. The pathogenesis is multifactorial and characterized by the interaction of environmental factors, with predisposing genes, most of which are associated with the HLA DR DQ loci. The relatively recent development of worldwide incidence registries for Type 1 DM has allowed us to compare the epidemiological results obtained in most parts of the world. This approach is particularly valuable in analysing the effects of migration of populations from one area of the world where the incidence of Type 1 DM is different (usually lower) to a new geographic setting. Properly designed migrant studies may be valuable in uncovering whether the genetic background remains more important than the new 'exposure' as illustrated by the Sardinian migration to Lazio and Lombardy. The presence of some putative 'protective' environmental exposures or the absence of those prevalent in the country of origin may explain the usually lower Type 1 DM incidence observed in most countries (Chile, Peru, Mexico) sharing a 'Spanish caucasoid genetic pool', and even in relatively genetically homogeneous groups such as Japanese populations migrating to Hawaii. In fact, the disease is caused by both genetic and environmental factors and to convince the scientific community of this fact is a primary responsibility for epidemiologists.