Bodies need protein to support a wide range of physiological functions. Some examples include the formation and repair of cells, tissue, bones, skin and muscles.
Protein is needed for the creation of nearly every type of cell in the body as well as for support and regulation of bodily functions. Protein contains nine amino acids that can't be synthesized by the body and that are not found in other types of nutrition. These amino acids all have different functions but are collectively referred to as the essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Different protein sources have differing amounts of these essential amino acids, but all nine are required in the human diet on a regular basis. Protein can also contain a range of other nonessential amino acids that are beneficial to the body but can be synthesized from other sources if they aren't taken in directly as part of a regular diet.
Collectively the nine essential amino acids are needed by the central nervous system, the digestive system, the kidneys and the immune system for regular function. They are also particularly important in muscle glucose and protein synthesis and for the regulation of cognitive function, mood and body nitrogen levels.