If all nine essential amino acids are in a substance, it is a(n) .

The essential amino acids, which are also referred to as indispensable, are the ones you must get through the foods you eat because your body can’t make them. Nine out of the 20 amino acids are essential, but adults only need to obtain eight of them: valine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine and tryptophan. The ninth amino acid -- histidine -- is only essential for infants. Your body doesn’t store amino acids, so it needs a regular daily supply of these essential building blocks.