Soy protein is one of the few plant-based proteins that contains all the essential amino acids. Some research studies have found soy protein to be equally as effective in building muscle as whey protein. However, soy protein contains around 15% less leucine, the primary driver of protein synthesis, than whey protein. Common sources of soy protein include tofu, edamame beans, and soy-protein supplements. Soy isoflavones, a class of plant-based compounds that can mimic estrogen, are removed from soy protein isolate supplements during the purification process. This makes them the soy protein source of choice for people looking to limit exposure to these phytoestrogens.