Proteins constitute an essential part of the diet as a source of energy, and for the replacement of protein lost in the wear and tear of daily life. Their essential constituent from this point of view is the nitrogen which they contain. To be absorbed, or digested, proteins have to be broken down into their constituent amino acids. The adult human body can maintain nitrogenous equilibrium on a mixture of eight amino acids, which are therefore known as the essential (or indispensable) amino acids. They are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. In addition, infants require histidine.