So why is it that so many bodybuilders take protein shakes? Well, protein is used for the production of muscles. Proteins are also used to manufacture hormones, enzymes, cellular messengers, nucleic acids, and immune-system components. Without adequate protein, our bodies can't put together the structures that make up every cell, tissue, and organ, nor can it generate the biochemical substances needed for cardiovascular function, muscle contraction, growth, and healing. Without an adequate amount of protein our muscles wouldn't heal up as quickly and could therefore lead to overtraining your muscle which could lead to injury.