Soy and Whey manufacturers did a superb job of confusing the public. For five years now the health community has been drinking up soy.
Then studies showed that due to its fast gastric emptying properties most of the protein taken in via whey protein is metabolized and burned up in the liver. The casein manufacturers first responded by increasing the number of grams per serving. Why not? More protein would have to be consumed, thus more protein is sold.
Soy got popular with vegetarians. The industry watched when whey sales spiked, thus, to get their own position, they targeted on the "health" marketplace. Many of the studies are valid, however, they do not represent the entire picture.
In one particular study published in France, two groups of apparently healthy men were fed isocaloric diets with one group using soy protein, the other using animal products as a protein source. The goal was to see if there was a reduced propensity for gallstones if animal proteins are replaced by soy. Since gallstone accumulation can be linked with crystallization of cholesterol, at its conclusion the study examined cholesterol levels.
Findings from that study demonstrated that cholesterol crystallization was decreased in the soy group. This finding led to the idea that soy can aid in preventing gallstones. The study failed to show some important things. There was no control over vegetable and fruit consumption. Soy is part of the group of vegetables that are high in isoflavones.
Isoflavones have been closely linked to health benefits including a lowering of cholesterol. We've known for years that a predominance of animal meats can have adverse affects on cholesterol, but that doesn't mean that someone consuming soy would have better cholesterol levels than someone eating combinations of chicken breast, turkey breast, fish, and egg whites and a variety of natural vegetables. You can see how the conclusions drawn from studies can be misconstrued and over-emphasized in significance.
Scientific studies indicate that there are cancer defeating benefits of a meal plan rich in vegetables like soy. Most people would not assume that people who ingest soy instead of milk, egg-whites, and lean protein will demonstrate improved health, if a physical activity ingredient is included, the study would be invalidated.
Much of the information promoted specific to soy is published by companies with a vested interest in Soy product sales, like The USB (United Soybean Board). This doesn't mean that soy isn't an excellent source of protein, rather it shows that the health benefits have been taken out of context to sell Soy products.
Metabolically, recent data suggest that you should consume a mixture of casein and whey protein for optimal results. Unless you are a vegetarian, you'll see the best results from consuming a wide variety of foods as part of your nutrition program.