Casein is the most abundant type of protein in cow’s milk, representing approximately 80% of the protein content. Caseinates exist as small spheres of 100-300µm in size, known as micelles. All types of caseinates contain a rich combination of essential and non-essential amino acids. However, it is important to note that caseinates contain a lower percentage of branched-chain amino acids, and a lower protein bioavailability to humans than whey protein.

Casein is a slow digesting source of protein and the amino acids in casein have been shown to be absorbed well following consumption. Elevated levels of amino acids have been shown in the bloodstream and muscle tissue up to seven hours following the consumption of casein, and because it does not cause an acute spike in amino acids like that seen in whey protein, less protein is oxidised and wasted. Casein reduces muscle breakdown far more powerfully than whey protein, and it has a better overall protein balance. Casein is an ideal choice as a night time protein. However due to its slow digestion, it is not an ideal choice as a post-workout source of protein if consumed on its own, and casein typically increases muscle protein synthesis to a lesser extent than whey protein.

Various forms of casein are available on the commercial market. The most commonly found forms are micellar casein, an isolated form of undenatured casein, in which the micelles are preserved through filtration. It has the highest percentage of protein fractions of caseinates (approximately 85-92%), and contains extremely low levels of lactose and fats; and calcium caseinate, another isolated form of casein, in which the manufacturing process results in the micelles being broken down to form a calcium salt, wherein the nutritional content of the casein is preserved (approximately 90% protein and low levels of fat).