Since the late 1990’s medical literature has identified two genetic sub-types of beta casein in cow’s milk. The older form of A2 beta casein varies from the more recent appearance of A1 beta casein by a single amino acid subsitution of histidine for proline at position 67 on the casein molecule.
This single mutation results in a casein molecule that is metabolized by gut bacteria to yield a bioactive peptide, beta casomorphin-7, which binds with opioid receptors in the gut as well as the nervous system. Beta casomrphin-7 has been characterised not only as an opiate receptor agonist, but also as a peptide with the ability to catalyse the oxidation of LDL in a noncation-dependant fashion. The uptake of oxidized LDL by endothelium-bound macrophages has been widely implicated in the pathogenesis of atherosclerosis.
Since A2 beta casein is not broken down into beta casomorphin-7, the risk of ischemic heart disease related to milk that does not contain A1 beta casein is suspected to be reduced. Since other factors play a role in development of type 1 diabetes mellitus, the presence of A1 beta casein in milk products is potentially a contributing factor to developing this disease, along with viral illness and vitamin D deficiency.
Breastfeeding of infants and delaying exposure to cow milk protein correlates with decreased incidence of type 1 diabetes mellitus in infants.