Soy protein can come in concentrates or isolates, but is usually 70-90% protein by weight. It is technically an incomplete protein, meaning it lacks one essential amino acid needed for growth (however this is debatable). Soy is popular in vegan athletes, since it is a 100% vegetable based protein source. Soy beans in general have a high allergen content, being second only to peanuts in the number of people allergic to it, which makes it undesirable for many. Soy also has isoflavones that can be considered estrogenic/anti-estrogenic. This isn’t a big deal to me, but many people fear the estrogenic effects of soy along with it possibly messing with T3 levels so it is typically avoided by many bodybuilders. Soy is a good source of protein for supplemental intake, since you will need to make up that missing amino acid from other protein sources, so it is best mixed with other protein sources in a blend or complex. Soy isn’t very popular with bodybuilders these days because of its flavones and amino acid profile, but some formulas still contain it.