Milk is classified by the amount of different types of protein present in the beverage. A1 and A2 milk actually vary due to genetic differences between the animals which produce the two types of milk. The amount of each type of milk available on the market also vary by country, with a majority of cows in the U.S. being A1 variety, while countries like New Zealand have a general mix of A1 and A2.
Proteins themselves are made up of long chains of amino acids. Among the proteins in milk is one called beta casein. Beta casein is further classified as either A1 or A2 depending on a single amino acid out of a strand of 209. A2 beta casein, considered the original version of the protein, contains the amino acid Proline in position 67 of the strand. A1 beta casein, a deviation from the original, includes the amino acid Histidine in that position. A1 is wholly unique among all mammal milks for having Histidine included in its chain. The presence of this acid is inherited from the genes of the cow's mother and father, meaning that the production of the protein is based upon the cow's genetic make-up, hence certain breeds of cow are more likely to have a higher A1 or A2 content in their milk. Depending on the genes present in their ancestors, and how that breed has developed over generations, the cow may have a greater A1 content in their milk. The highest percentage found in diary cows is roughly a 50-50 split between the two.
A1 beta casein is actually a mutation of A2 beta casein, believed to have been caused by the genetic changes in breeds of cow that have undergone significant cultivation. Guernsey cows in particular are known for having the highest A2 protein content with very little A1 protein present. Holstein cows, the most abundant breed in the United States, produce milk that is roughly equal between the two proteins, giving it the highest level of A1 protein from any breed of milking cow. This is due to the significant genetic changes the Holstein breed has undergone during its cultivation as a milking breed.
While Guernsey cows have the highest A2 content, and therefore any milk produced is considered A2 milk, other breeds also include A2 proteins at significantly higher levels than A1. The second highest A2 proteins are from the Brown Swiss breed, while Jersey, Ayrshire and Milking Shorthorn also produce higher levels of A2 milk. Both Friesian and Holstein cows produce milk that is roughly equal between A1 and A2.
Interest in A1 and A2 milk has increased over the last several years as studies have been conducted which demonstrate a possible link between A1 beta casein and a rise in risk of such illnesses as coronary heart disease, autism, type 1 diabetes and schizophrenia. While certain breeds of cow have a higher likelihood of producing A1 or A2 proteins, concern has also risen due to the tendency of milk from different cows being mixed together during the packaging process.