Training techniques and genetics are the critical determinants of muscle size. Protein can't be stored for later use, if the athletes body gets more protein than it needs, extra is broken down and stored as fat or used as energy. High protein diets have never been shown to be uniquely beneficial to athletes. In addition, these processes result in residual nitrogen, which must be discarded through the urine as urea and ketones. This step requires the loss of water, which increases the athlete's chance of dehydration.
Protein is the toughest nutrient to digest.
Your body expends a lot of energy just breaking down high protein foods. Therefore high protein diets should be avoided before training. Contrary to popular opinion, muscle size is not dependent on protein intake. If daily minimum intakes are met, muscle size will be dictated by the specific training demands and of course genetic potential. Protein can be supplied from animal food sources, such as lean meat, poultry, fish, eggs, milk, and cheese, or vegetable food sources such as dried beans and peas, peanut butter, nuts, seeds, and tofu.