Casein is a dairy protein and accounts for almost 80% of all proteins found in cow's milk. It contains a very rich pool of amino acids that are almost identical to whey protein, which is a by-product of cheese. This so-called pool of amino acids is high in essential aminos, which are responsible for muscle protein synthesis.
When ingested, casein forms a clot in the stomach. This clot slows down the rate of gastric emptying, which affects the rate at which amino acids get absorbed into the bloodstream. In so many words, because of this clot, the casein takes longer to be digested in the stomach, which slows down the amount of amino acids being released into the small intestine, where they are absorbed.
This slow, steady flow of amino acids creates a prolonged yet low rate of muscle protein synthesis. This synthesis can last for up to seven hours after casein ingestion! This makes casein the ideal protein supplement to take when going to bed, or when you know you will be going prolonged periods without the opportunity to ingest food. The most popular casein supplements are milk protein and calcium caseinate. The most effective is micellar casein-which is a natural, undenatured form of casein found in milk. Micellar casein is separated from milk by means of ultrafiltration, without the use of chemicals, which increases the amount of bioactive milk peptides that not only support immune function but also increase muscle growth.
Whey protein is a rich source of branched chain amino acids (BCAAs), containing the highest known levels of any natural food source. Whey protein is one of 2 types of protein that comes from milk (the other being casein protein, as mentioned above). It's made during the process of cow's milk being turned into cheese. When the milk is being turned into cheese, the whey protein is a by-product.
Whey protein can be found in protein powders, protein bars and some protein drinks, however there is no actual food that contains whey. There are 2 major types of whey protein, whey protein concentrate and whey protein isolate. Whey protein concentrate is low in fat, has a highest biological value and is typically about 75% pure protein by weight. Whey protein isolate is whey in its purest form. Isolates are processed to remove fat and lactose. Unfortunately, many of the health/immune system benefits are also lost in this process. Typically whey protein isolate is about 90%+ pure protein by weight and has zero carbohydrates, lactose or fat.
Unlike casein protein, once ingested, whey protein gets into the system fast. Whey protein supplements play a huge roll in post-workout nutrition, when your body is in a catabolic state and needs a fast injection of protein. Whey protein has been said to help increase lean muscle mass and improve muscle repair time. It is also said to reduce post-workout muscle breakdown and help increase metabolic rate.