The soy protein vs whey protein for enhanced muscle building argument has been going on for many years. One type of protein's supporters will sponsor a study that provides "irrefutable evidence" that their favored protein source is better. This study will be answered by another study from the opposing camp, with results favorable to their protein source. The argument has been going on so long and loudly that it's hard to distinguish fact from fiction anymore and decide which one medical and biological science says is the best source of protein for building stronger and healthier muscles.
What Protein Is
Most of you know what protein is. "It's that nutrient your body needs to make long lasting energy efficiently." That's partly right. Protein is a series of amino acids that your body uses for various biological functions, such as energy and building and repairing damaged muscle tissue. For people trying to build lean muscle mass, one gram of protein per kilogram of weight is required daily (a kilogram is 2.2 pounds). There are six main sources of protein available: soy, whey, casein, meats, eggs and milk. No single protein source should be favored over the others because each has amino acid chains of different lengths, each of which your body needs for different purposes. A few years ago, a scale called the Biological Value was introduced to rank the various types of proteins, according to how "available" they immediately are for your body. Eggs were given a Biological Value of 100 because they're the most biologically available naturally occurring protein source.
A few years later, science was able to isolate whey protein from milk and they discovered that, according to the process used to isolate and test the BV, whey protein has a BV of between 104 and 154. This means that whey protein is more readily available to your body to use for biological processes. However, this isn't necessarily what someone trying to build lean muscle mass wants. The more readily available the protein is, the easier and faster it can be broken down into its constituent amino acid chains and the faster it passes through the digestive tract as waste matter. Whey protein's major benefit for the body comes if you use it mixed with a little milk to slow its absorption after your workout, to help your body recover and rebuild itself.
According to most bodybuilding information, soy protein by itself is pretty worthless. However, the other nutrients in soybeans are quite good for your body. Soy protein has a low BV, so if used sparingly in a diet high on eggs, milk and whey protein, it will help slow down the absorption of these more biologically available proteins so your body can garner more benefit from them. Also, soy has been shown to help lower the bad kind of cholesterol, low density lipids, which have been shown to be major factors in heart disease and heart attacks.
There really is no single protein source that is more important to your body than the others. You should be sure your diet contains a quantity of each of the six sources, since they each have their benefits. However, whey protein is more easily absorbed by your body, which is both a negative as well as a positive.