A multi-centre, blinded international trial of the effect of A1 and A2 β-casein variants on diabetes incidence in two rodent models of spontaneous Type I diabetes
Aims/hypothesis. The diabetes-inducing potential of cows' milk is still debated and there is no consensus on the diabetogenicity of individual milk proteins. A1-β-casein has been associated with increased diabetes frequency in ecological studies and in NOD mice. Our aim was to ascertain whether A1-β-casein was more diabetogenic than A2 and to test the diabetogenicity of a milk-free diet in animals representing different forms of spontaneous Type I (insulin-dependent) diabetes mellitus. Methods. Defined diets were coded and shipped to laboratories in New Zealand (NOD/NZ), Canada (BB) and the UK (NOD/Ba). Base diets were Pregestimil (PG) and ProSobee (PS). Purified fractions of whole casein (WC), A1 or A2-β-casein were added at 10%. A milk-free, wheat-predominant, NTP-2000 diet was the control. Animals were fed from weaning up to 150 or 250 days, and insulitis, diabetes frequency and expression of pancreatic cytokines were assessed. Results. Diabetes incidence was highest in three locations in animals fed NTP-2000. PG and PS diets were protective except for NOD/Ba mice fed PG+WC where incidence was similar to NTP-2000. A1 and A2 diets were protective in both models, but A1 β-casein was slightly more diabetogenic in PS-fed BB rats. The New Zealand study was confounded by an infection. Conclusion/interpretation. A milk-free, wheat-predominant diet was highly diabetogenic in three widely separate locations in both animal models. A previous result that A1 β-casein was more diabetogenic than A2 β-casein in NOD mice was not confirmed; both β-casein variants were protective in BB rats and NOD mice. Whole Casein promoted diabetes in NOD/Ba but protected BB showing that unique diabetes haplotypes react differently to dietary proteins. A1- was more diabetogenic than A2-β-casein only in PS-fed BB rats. Neither the analysis of insulitis nor of pancreatic cytokine gene expression showed a difference between A1 or A2 β-casein fed animals. Milk caseins are unlikely to be exclusive promoters of Type I diabetes, but could enhance the outcome of diabetes in some cases. Other diet components such as wheat could be more important promoters of Type I diabetes.