Cow’s milk contains a protein structure made up of mostly casein (about 80%) and whey (about 20%). Whey is a simpler protein, whereas casein is a more complex protein, and takes longer to digest. There are several types of casein proteins, with the most common ones being A1 and A2 beta-casein protein. A2 milk contains almost exclusively A2 milk protein and no A1 protein. Regular cow’s milk contains both A1 and A2, although the proportions vary with different breeds. Most babies (without lactose intolerance) will do well on cow milk formula, as it is a similar protein ratio to breastmilk. Some babies will need an A2 formula, as breastmilk mostly contains the A2 variant of beta casein. For premature infants who have an underdeveloped digestive system, along with full-term babies who are having digestive issues, A2 formula may be a good option since it will likely be gentler on their tummies.