The hypothesis that early exposure of the infant to cow's milk (or lack of breast-feeding) predisposes the child to type 1 diabetes dates from the 1980s. It has important implications, but remains controversial because the evidence on which it is based has been indirect and is open to criticism. Two meta-analyses of multiple studies in which diabetes prevalence was associated retrospectively with infant feeding revealed only a marginal increase in relative risk. Two recent prospective studies found no apparent association between development of antibodies to islet antigens and feeding patterns in high-risk infants with a first-degree type 1 diabetic relative. Studies reporting increased humoral and cellular immunity to cow's milk proteins in children with type 1 diabetes often lack appropriate controls and standardization and do not, in themselves, establish a causal connection to disease pathogenesis. A review of published data leads to the conclusion that increased immunity to cow's milk proteins is not disease-specific, but reflects genetic predisposition to increased immunity to dietary proteins in general, associated with the HLA haplotype A1-B8-DR3-DQ2 (A1*0501, B1*0201), which also predisposes to celiac disease and selective IgA deficiency. We suggest that the cow's milk hypothesis could be productively reframed around mucosal immune function in type 1 diabetes. Breast milk contains growth factors, cytokines, and other immunomodulatory agents that promote functional maturation of intestinal mucosal tissues. In the NOD mouse model, environmental cleanliness may influence diabetes incidence through mucosal mechanisms, and exposure of the mucosa to insulin (present in breast milk) induces regulatory T-cells and decreases diabetes incidence. The mucosa is a major immunoregulatory barrier, and cow's milk happens to be the first dietary protein it encounters. The basic question is whether impaired mucosal immune function predisposes to type 1 diabetes.

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