Protein is an excellent energy source in milk with 8 grams (g) per 8-ounce (oz) glass. Milk contains two primary proteins, casein and whey. Casein is responsible for 80% of the protein in milk, while other casein types include beta-casein. A1 and A2 are both varieties of beta-casein found in cow's milk.
Back in the old days, local cows solely produced the A2 form of beta-casein. But, the milk sold in stores today majorly comprises the A1 protein.