I recall reading that the only non-animal food that is a "complete" protein is the soybean. Now I realize that many vegan types with try to convince you that this is propaganda. I go to a gym 3 times a week and I don't think that you'll find many serious weight lifters/personal trainers who would endorse this type of diet for very long. I think you probably risk losing muscle tissue which is undesirable. If the body is starved of protein it will start to live off of itself.