The term applies to members of a group of non-crystallisable nitrogenous substances widely distributed in the animal and vegetable kingdoms, and forming the characteristic materials of their tissues and fluids. They are essentially combinations of AMINO ACIDS. Typical examples of protein substances are white of egg and gelatin.

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. To be absorbed, or digested, proteins have to be broken down into their constituent amino acids. The adult human body can maintain equilibrium on a mixture of eight amino acids, which are therefore known as the INDISPENSABLE AMINO ACIDS (or essential amino acids). They are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. In addition, infants require histidine.