An essential amino acid is one that cannot be synthesized from other available resources, and therefore must be supplied as part of the diet. Not all amino acids need to be supplied. Alanine can be synthesized from pyruvate in humans, but humans cannot synthesize phenylalanine and hence it is an essential amino acid.
The boundary between an essential amino acid and one that is not can sometimes be unclear. Methionine and homocysteine, sulfur containing amino acids, can be converted one to the other, but neither can be synthesized from scratch in humans. Cysteine can be made from homocysteine, but it also cannot be synthesized from scratch. So, for convenience, people will sometimes count the sulfur containing amino acids as a single pool. Likewise, because of the urea cycle, arginine, ornithine, and citrulline are interconvertible, and therefore form a single pool of nutritionally equivalent amino acids.
Foodstuffs that are lacking essential amino acids are poor sources of protein equivalents, as the body will tend to deaminate the amino acids obtained and convert proteins into fats and carbohydrates instead. Therefore, a balance of essential amino acids is necessary for a high degree of net protein utilization, which is the mass ratio of amino acids converted to proteins to amino acids supplied. This figure is somewhat affected by salvage of essential amino acids in the body, but otherwise is profoundly affected by the limiting amino acid content, which is the essential amino acid found in the smallest quantity in the foodstuff.
| Protein source | Limiting amino acid |
| wheat | lysine |
| rice | lysine and threonine |
| maize | tryptophan and lysine |
| pulses | methionine |
| beef | methionine and cysteine |
| whey | none |
It is therefore a good idea to mix foodstuffs that have different weaknesses in their essential amino acid distributions. This limits the loss of nitrogen through deamination and increases overall net protein utilization.
8 amino acids are generally regarded as essential, with two others, histidine and arginine, essential only in children:
In mammals other than humans, amino acids regarded as essential can be considerably different. For example, cats lack an enzyme that allows them to synthesize taurine, so taurine is essential in cats.