Dairy (whey, casein) and egg proteins are better for athletes, but not because soy is estrogenic.
In support of soy proteins…
1) While it’s true that the Protein Efficiency Ratio (PER) and Biological Value (BV) of other protein sources are higher than soy, the Protein Digestibility Corrected Amino Acid Score (PDCAAS) is a considered to be a superior method of measuring protein quality because it’s based on human, not animal, amino acid and nitrogen needs. Soy proteins have the same (PDCAAS), as whole egg and milk proteins.
2) Comparatively, soy contains considerably more arginine and nearly double the glutamine found in whey protein. Soy protein isn’t as rich in BCAAs as whey, casein, or egg, it does contain a respectable percentage.
3) Soy proteins may provide non-performance benefits including cardiovascular protection, increased bone mineral density, protect against certain types of cancer, and help blunt the appetite.
4) Soy is vegan, so it’s suitable for vegetarians who don’t drink milk or eat eggs in addition to meat, poultry, and seafood.
5) Soy is relatively cheap – especially given what’s happened to diary prices this past year.
In support of whey, casein, milk, and egg proteins…
1) Whole milk proteins have been shown to be superior to soy for muscle building and retention – at least according to one recent study. As with other research, the overall picture is likely to be much more complicated as subsequent trials are done.
2) Higher total Essential Amino Acid (EAA) content. Like all legumes, soy is lower in methionine than milk, eggs, and animal foods. However, by consuming them with whole grains or other plant foods that are higher in methionine, you can overcome this inherent weakness.
3) Taste better. Soy protein isolates are certainly not the best tasting proteins you can get. Dairy and eggs win hands-down here.
4) Mix easier. Unlike whey and egg proteins, most soy proteins are non-instantized. You almost always need to use a blender of shaker to get them to mix up well.
I personally believe that every protein has its own strengths and weaknesses. That's why I use all of the above. Most of my daily soy protein consumption is through foods (e.g., veggie burgers, textured vegetable proteins), my whey protein mainly through supplementation, and the casein and egg protein that I eat is obtained through a variety of whole foods and protein powders
