Heres something I wrote on it and used bodybuilding.com for some articles on it:
Soy protein is made up of 30% carbohydrates (15% from fiber), 38% protein, 18% oil (85% which is from unsaturated fats) and 14% moisture. It has potassium, zinc, iron, vitamin-E, phosphorous and the full B-Complex. Soy protein has high amounts of arginine and glutamine.
Soy protein is seen as a feminine product because phytoestrogens. Bodybuilders and fitness enthusiasts avoid taking soy protein because they believe that it raises estrogen levels and decreases testosterone levels. By believing this claim, it leads them to hypothesize that soy protein will cause a person to have thyroid problems. Also low energy, infertility, and digestive troubles are believed to be caused by soy protein.
The American College of Sports and Medicine with the sponsor of US National Dairy Council, showed that what the common bodybuilder views on soy protein were wrong. Taking the soy protein as a post-workout shake showed that taken soy with milk helped with muscle hypertrophy. There was no change in testosterone levels as believed.
Soy protein is gradually digested while whey protein is digested more quickly. This reminds me of the combined effects of casein and whey protein that many bodybuilders observe.