Casein protein, like whey, comes from cow's milk. It accounts for roughly 80 percent of milk's total protein content, with whey constituting the other 20 percent. Casein is insoluble, which means it's the solid protein in milk.
Casein's crazy ability to gel and glob makes it a unique and diverse protein supplement. During digestion, casein gels as it hits stomach acid. This reduces the rate of digestion and allows for a slower, steadier, more efficient release and utilization of casein's amino acids. (Amino acids are the building blocks of protein.)
This basically means that your muscles receive a "trickle" of food over a longer period of time.
A slow digestion rate is also beneficial because it might reduce protein breakdown and amino acid oxidation-the burning of amino acids for energy. Casein can even increase your feeling of fullness, so you'll feel like you've got a full gut without having to carry one.
Casein And Whey:
Casein can be a slow-digesting, double-edged sword, especially when compared to whey. On the plus side, casein's ability to slow down digestion extends the release of amino acids to muscle tissue and provides a more sustained boost to nitrogen balance. (A positive nitrogen balance is crucial to building muscle.)
However, casein's slow release of aminos also tends to reduce the body's peak anabolic response. This means that casein might not stimulate muscle protein synthesis as powerfully or quickly as whey. Because it digests slowly, casein is less anabolic (muscle-building) than whey when compared gram-to-gram.
Additionally, casein protein has a relatively low leucine content (8%) compared to whey (11%). Leucine is the amino acid responsible for the peak anabolic response to protein ingestion. Basically, leucine signals the body to stimulate protein synthesis and build muscle.
Research from my PhD thesis demonstrated that the anabolic response to a meal was closely associated with that meal's leucine content. Because casein contains less leucine than whey, it's not as directly anabolic.