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 ratio of amino acid converted to proteins to the ratio of 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.
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.
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.