Proteins are highly complex molecules comprised of linked amino acids. Amino acids are simple compounds containing carbon, hydrogen, oxygen, nitrogen and occasionally sulphur. Proteins are broken down into their constituent amino acids during digestion which are then absorbed and used to make new proteins in the body. Certain amino acids can be made by the human body. However, the essential amino acids cannot be made and so they must be supplied in the diet. The eight essential amino acids required by humans are: leucine, isoleucine, valine, threonine, methionine, phenylalanine, tryptophan, and lysine. For children, histidine is also considered to be an essential amino acid. Proteins are essential for growth and repair. They play a crucial role in virtually all biological processes in the body. All enzymes are proteins and are vital for the body's metabolism. Muscle contraction, immune protection, and the transmission of nerve impulses are all dependent on proteins. Proteins in skin and bone provide structural support. Many hormones are proteins. Protein can also provide a source of energy. Generally the body uses carbohydrate and fat for energy but when there is excess dietary protein or inadequate dietary fat and carbohydrate, protein is used.