Proteins are continually synthesised in the cells as they are the principal component required for growth. Proteins are composed of amino acids which are derived largely from the diet and partly from the breakdown of protein available in the body. Generally all proteins are made from about 20 different amino acids in various combinations.
However, it is not necessary to supply all the 20 amino acids. Some can be formed in the body, using other amino acids but others have to be supplied through diet because they are not formed in the body. The amino acids that are synthesised in the body are called non-essential amino acids while those that have to be supplied through diet are known as essential amino acids.