Protein makes up the structure of all cells and tissues in the body, including muscle tissue, internal organs, tendons, skin, hair and nails. The 20 amino acids are the building blocks of proteins. 12 of these amino acids can be made in the body and are called non-essential amino acids, while the other 8 are known as essential amino acids, which must be supplied in the diet. Arginine is a non-essential amino acid.