Around 80% of milk protein is casein, and around 30% of that is beta casein. There are several types of beta casein, with A1 and A2 being the most common. Beta casein, like all proteins, consists of a long chain of amino acids. A1 beta casein was noted as a natural genetic change in European cattle when a histidine molecule was replaced with a proline molecule in this amino acid chain. The ingestion of A1 beta casein (but not A2) leads to the formation of beta-casomorphin-7 (BCM7), a peptide that has been associated with several health concerns. In vitro and animal studies have shown an association between the BCM7 peptide and type 1 diabetes, allergies, and cardiovascular disease. However, there is no evidence showing a causative link between the ingestion of A1 beta casein and any chronic health condition in humans.
Some human studies have shown greater gastrointestinal distress with A1-containing milk than with A2-containing milk, possibly due to an inflammatory response to the BCM7 peptide. This could be helpful to people with symptoms of lactose intolerance, because it is possible that A2-containing milk could cause fewer symptoms than mixed A1/A2 milk. However, all milk products still contain whey protein, so any symptoms caused by whey protein will not be improved by A2 milk.[1][2][3]