Milk is a mixture of two major proteins, whey and casein. Whey protein is frequently sold as a stand-alone supplement or used as the sole protein source in meal replacements. Not surprisingly, however, research is beginning to suggest that leaving casein and whey together (as in a multi-protein blend) may have multiple benefits for your body.
Whey is commonly described as a fast-release protein, meaning it is quickly digested and absorbed. Casein is considered slow-release by comparison. Thus, by supplementing your diet with a blend of casein and whey, presumably you can receive the benefits of both "speeds" of protein (i.e., rapid yet sustained delivery of protein building blocks to your growing muscles).
There may be other benefits to combining your whey and casein. Whey protein is a source of a number of biologically active peptides (i.e., short chains of amino acids). Some of these peptides may provide healthful properties (i.e., lower blood pressure, improve mood). Casein helps to preserve the functions of such peptides. This effect appears to be unique to casein.
Casein seems to block the breakdown of whey peptides in your intestine. It can increase the lifetime of some peptides by over 15 times. That gives the peptides a greater opportunity to do your body good. In the same way, casein may also render your whey protein more useful to your muscles by slowing down its digestion and absorption.
To sum it up, a protein blend supplement containing casein and whey may give you not only the bodybuilding benefits of slow- and fast-release proteins, but also a host of other healthful properties that might be absent when consuming either protein on its own.