Casein and whey are both proteins found in milk. They both contain all the essential amino acids, although they have slightly different amino acid profiles. Notably, whey protein has a higher leucine content (about 11-12% versus casein’s 9%).[1]
Because of its chemical structure, casein does not combine well with acids. When it comes into contact with stomach acid, it becomes gel-like in consistency, and this coagulated substance is the reason that casein is digested so slowly. Whey protein is absorbed more quickly, releasing amino acids into the system within an hour.[2][3] These differences can affect the way we use these protein supplements. For example, taking a whey protein supplement after training makes sense, as the body can immediately use the amino acids for muscle growth and recovery. However, casein is a better choice prior to sleeping, since it gives the body a continuous supply of amino acids while fasting for (hopefully) 8 hours while sleeping.[4]
Some people prefer the texture of whey protein as a drink, since it mixes more easily with water. Both proteins are good for muscle growth and recovery, and both can increase satiety.[5][6][7]