Essential Amino Acids are amino acids that your body does not have the ability to synthesize. Hundreds of different amino acids exist in nature, and about two dozen of them are important to human nutrition. Nine of these–histidine, isoleucine, leucine, lysine, methionine, phenylalanine, theronine, tryptophan, and valine–are considered essential, since they must be supplied by your diet.

While the essential amino acids requirements vary among different individuals, the Food and Agricultural organisation of the United Nations (FAO) has proposed a standard that specifies the minimum amount of each essential amino acid that should be supplied per gramme of protein consumed. This standard is the reference by which protein quality is determined. By comparing the limiting (i.e., lowest level) amino acid in a food with this standard, the amino acid score is determined. A score of 100 or above indicates a complete or high-quality protein; a score below 100 indicates a lower-quality protein.