( This is an updated version dated 17th April'2016 with answers to some questions from esteemed readers. )

What are the controversy around A1 and A2 ß-casein in milk?

ß-Casein is one of the major proteins in the milk of cows, buffaloes, and other mammals.  Research studies have shown that there are different variants of ß-casein in milk that evolved over hundreds of years due to mutation and selective breeding for high milk and protein yields.  There are 15 known variants of ß-casein out of which A1 and A2 are major. ß-casein-A1 (alleged bad casein) differs from A2-casein only by one amino acid at position 67 (proline in A2 and histidine in A1).  This substitution increased the susceptibility of A1 ß-casein to hydrolysis by digestive enzymes to produce physiologically active ß-casomorphin (BCM), a decapeptide.