Protein is an essential component for every type of cell in the body, including muscles, bones, organs, tendons, and ligaments. Protein is ... needed in the formation of enzymes, antibodies, hormones, blood-clotting factors, and blood-transport proteins. The body is constantly undergoing renewal and repair of tissues. The amount of protein needed to build new tissue or maintain structure and function depends on the rate of renewal or the stage of growth and development. For example, the intestinal tract is renewed every couple of days, whereas blood cells have a life span of 60 to 120 days. Furthermore, an infant will utilize as much as one-third of the dietary protein for the purpose of building new connective and muscle tissues.