Cow’s milk contains several proteins, the most abundant of which is casein (which forms the structure of curds in cheese and supplies essential amino acids). One important type of casein is beta-casein – a protein that influences digestion and how milk interacts with the gut. Beta-casein exists in two major variants, A1 and A2, which differ by just one amino acid. This small difference affects how the body breaks them down.
A1 milk contains the A1 beta-casein variant, commonly found in cows of European origin such as Holstein, Friesian, Ayrshire, and British Shorthorn. During digestion, A1 beta-casein can release a peptide called beta-casomorphin-7 (BCM-7), which some studies suggest may be harder to digest and has been linked to discomfort in sensitive individuals. However, large-scale evidence for broader health risks of A1 milk remains inconclusive.
A2 milk comes from cows that produce only the A2 beta-casein variant. Breeds such as Gir, Sahiwal, Ongole, Red Sindhi, Tharparkar (Indian native cows), Jersey, and Guernsey cows naturally produce A2 milk. Research suggests that A2 milk releases significantly less BCM-7, making it easier to digest for many people who experience bloating or mild intolerance with regular milk. It is worth noting that not all A2-labeled milk is entirely free of A1 beta-casein due to breeding and production factors, though the levels are typically minimal.
The naming can be misleading because A2 is the original milk from older cattle breeds, while A1 is the result of a later genetic mutation in European cows. The first identified common variant was called A1 beta-casein and the second variant discovered was called A2 beta-casein. Scientists named them in the order of discovery, not as a ranking of quality.
