Amino acids can not be manufactured in sufficient quantities in the body without being consumed in the diet. Essential amino acids include: tryptophan, valine, threonine, isoleucine, leucine, lysine, phenylalanine, and methionine.
An amino acid that cannot be made in the body and has to be ingested as part of the diet. Of the 20 amino acids used in the body, 8 of them are ‘essential’.
Any of the nine amino acids that the human body cannot manufacture and that must be supplied by the diet, as they are necessary for growth and maintenance.
An essential amino acid or indispensable amino acid, is an amino acid that cannot be synthesized de novo by the organism (usually referring to humans), and therefore must be supplied in the diet. Nutritional essentiality is characteristic of the species, not the nutrient. Nine amino acids are generally regarded as essential for humans.