The two most common whole proteins used in dietary supplements are casein and whey. These proteins from bovine milk have different digestive properties. Casein, accounting for 80% of the protein in milk, forms a gel or clot in the stomach following ingestion that makes it slow to digest. As a result, casein provides a sustained but slow release of amino acids into the bloodstream, sometimes lasting for several hours (Boirie et al. 1997). Whey protein accounts for the other 20% of the protein in bovine milk and contains high amounts of the essential and branched chain amino acids (Hoffman and Falvo 2004). Whey protein is the translucent liquid part of milk that remains following the cheese manufacturing process (coagulation and curd removal); as a result, it is absorbed into the body much more quickly than casein.
Boirie and colleagues (1997) examined differences between casein and whey protein supplementation. They demonstrated that a 30 g feeding of casein had significantly different effects on postprandial protein gain than whey ingestion. Whey protein ingestion resulted in a rapid appearance of amino acids in the plasma; casein ingestion resulted in a slower rate of absorption, producing a slow but sustained elevation in plasma amino acid concentrations. Whey protein ingestion resulted in a peak increase in protein synthesis of 68%, while casein ingestion stimulated a peak increase in protein synthesis of 31%. Analysis of postprandial leucine balance at 7 h postingestion showed that casein ingestion resulted in a significantly higher leucine balance, and no change from baseline was seen at that time point following whey consumption.
Providing further support, Tipton and colleagues (2004) also suggested that the differences in digestive properties between whey and casein result in a fast and slow increase in muscle protein synthesis, respectively. However, ingestion of 20 g of these proteins at 1 h following resistance exercise resulted in a similar net muscle protein synthesis over a 5 h examination period. The results suggested that although whey protein can stimulate a rapid increase in protein synthesis, a large part of this protein is oxidized (used as fuel), while casein may result in a greater protein accretion over a longer duration. In a comparison of single versus multiple ingestions of whey protein (total protein consumed was equivalent) over 4 h, the repeated pattern of ingestion resulted in a greater net leucine oxidation than a single feeding of either casein or whey (Dangin et al. 2002). Even though casein and whey are both complete proteins, their amino acid composition is different. Whey protein contains a much greater leucine content than casein. Given that leucine has an important role in muscle protein metabolism, the benefits of whey protein consumption may be seen in its enhanced absorption capability and the resultant increase in protein synthesis. This may be especially relevant for enhancing muscle remodeling and recovery with regard to a potential window of adaptation (immediately before, after, or both before and after a workout) in which the heightened sensitivity of the muscle causes it to respond differently than at other times (Esmark et al. 2001; Cribb and Hayes 2006; Hoffman, Ratamess, Tranchina, et al. 2010).
Both casein and whey are effective in stimulating muscle protein synthesis. However, differences in the digestive properties of these proteins appear to stimulate a different pattern of protein synthesis. Whey protein ingestion results in a greater acute response compared to a more gradual rise in protein synthesis following a feeding of casein. Although the total net muscle protein synthesis appears to be similar between these proteins, it is possible that the acute elevation seen following whey protein ingestion postexercise provides a greater window of opportunity for enhancing the recovery and remodeling of skeletal muscle.