Whey and casein are different types of proteins that are both present in milk. These proteins differ in their rate of digestion as well as their amino acid profiles. Whey is digested more quickly by the body and may be most beneficial when consumed immediately after workouts. Casein is a slower-digesting protein that is used less efficiently by the body.
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Milk Protein Composition
Thirty-eight percent of the solid matter in milk is protein. Of that total protein, 80 percent is casein and 20 percent is whey. According to the National Strength and Conditioning Association, casein is present in milk curds, while whey protein is located in the liquid portion of milk. The organization also notes that the protein composition of milk is ideal for replenishing nutrients after workouts and keeping energy levels up.
The biological value of a protein is a measure of how efficiently that protein can be utilized by the body. Proteins with high biological values supply more essential amino acids -- amino acids that must be consumed through dietary sources because is body is unable to produce them. According to a 2004 paper published in the "Journal of Sports Science and Medicine," whey protein has a very high biological value of 104, with egg protein as a reference point of 100. Casein protein has a biological value of 77, which means it is used less efficiently by the body when compared to whey.
Rate of Digestion
The National Strength and Conditioning Association notes that fast-acting whey protein is broken down and absorbed quickly by the body. In addition, some supplement manufacturers break down whey proteins even further for enhanced digestion. Casein protein, on the other hand, takes longer for the body to digest, which means protein will remain in the system for a longer period of time. This could theoretically provide its own benefits for athletes attempting to build or preserve muscle tissue.
Effectiveness in Muscle Building
Regardless of the superior digestibility and amino acid profile of whey protein, casein protein may be equally beneficial in promoting muscle growth. A study published in 2004 in "Medicine and Science in Sports and Exercise" found that consuming whey and casein protein after exercise resulted in similar increases in muscle protein synthesis -- or muscle growth. Another study published in 2013 in the "Journal of Sports Science and Medicine" had similar findings. This study, which tested the effects of both whey and casein protein on female collegiate athletes after exercise, found no difference in performance markers between casein and whey protein.