There are 2 main types of protein in milk: casein and whey. Approximately 80% of milk protein is casein, and whey makes up the remaining 20%. Casein forms a gel in the gut, and is digested and absorbed more slowly. Whey on the other hand, is quickly absorbed because it is dissolved in the water component of milk. The combination of whey and casein in milk allows for rapid yet sustained delivery of protein to the body.