Casein makes up 80% of the protein in milk and is said to be the slowest-digesting protein you can consume. This results in a slow, steady release of amino acids into the bloodstream – a process that lasts for several hours. For this reason, casein is marketed as a way to fuel muscle recovery and growth during sleep. There are two types of casein: Casein hydrolysate and micellar casein. The hydrolyzed form is said to be ‘pre-digested’ and the quicker-acting of the two, while micellar casein is taken directly milk. Each casein protein powder on the market will have a different ratio of fat, carbohydrates and protein and may be formulated to help users achieve different goals.