About 15 years ago, I broke the news to the physique world that a few studies had shown that soy results in testicle shrinkage and lowered testosterone levels. Still, some people steadfastly clung to the notion that soy protein is at least a good muscle builder.
They had somewhat of a leg to stand on, albeit a short stubby one. Their reasoning was that soy protein was different from soy in that the isoflavones implicated in lowering testosterone and the shrinking of testicles were removed in processing and none of them were present in the protein itself. That may or may not be true, as some reports indicate that the isoflavones are indeed present in some brands of soy protein. Regardless of which position you support, recent research gives us an altogether different reason to avoid soy.
Soy Protein Is No Better Than Water
A study at McMaster University found that when it comes to muscle protein synthesis – building muscle basically – soy is no better than water at least when it comes to elderly men. The researchers gave 30 old guys 0 grams of protein, 20 grams of soy, or 40 grams of soy at rest and after resistance exercise. They then compared the results to a group of men who had used 20 or 40 grams of whey protein instead.
While 40 grams of soy increased muscle protein synthesis a little, 20 grams of soy worked as well as 0 grams of soy. Both 20 and 40 grams of whey, however, increased muscle protein synthesis significantly. The researchers theorized that whey worked well (and soy didn't) because whey has a much higher percentage of leucine, the "master amino acid" for muscle building, than soy. Likewise, a higher percentage of the amino acids in soy, including leucine, are diverted towards oxidation, which makes them unavailable for protein synthesis.
Clearly, if you want to grow muscle, it's best to avoid soy protein.