First, what is an amino acid? Amino Acids are chemical substances that make up protein. There are 20 amino acids that are proteinogenic - meaning they are used by organisms to build proteins from-, of those there are 8 essential amino acids. 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.

(Technically, the "20 proteinogenic amino acids" figure is a bit vexing. You also find selenocysteine, a selenium-containing derivate, in a few proteins, adding another one to the score, but according to chemical nomenclature rules, proline, due to its structure, is really an imino acid - its side chain is linked to the backbone at two points forming a ring rather than simply branching off. Thus, there are indeed 20 proteinogenic amino acids, but strictly speaking, two of them are not those that are normally given. Additionally, amino acids can be chemically altered after being incorporated into a protein, but these derivates do not constitute proteinogenic amino acids as they are only created after polymerization of the protein.)

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 into each other, but neither can be synthesized from scratch in humans. Cysteine can be made from homocysteine, but it cannot be synthesized from scratch either. 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.

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:

• tryptophan
• lysine
• methionine
• phenylalanine
• threonine
• valine
• leucine
• isoleucine

The question of which amino acids are essential varies from species to species, as different metabolisms are able (or unable) to synthesize different substances. For instance, taurine (which is not, by strict definition, an amino acid) is essential for the cat, but not for the dog; thus, taurine is added to commercial cat food, but not to dog food - and therefore, dog food is not nutritionally sufficient for cats.

External links

• Amino Acid Profiles of Some Common Feeds (http://www.dasc.vt.edu/nutritioncc/9729.html)