Essential amino acid

An essential amino acid for an organism is an amino acid that cannot be synthesized by the organism from other available resources, and therefore must be supplied as part of its diet.

Eight amino acids are generally regarded as essential for humans: tryptophan, lysine, methionine, phenylalanine, threonine, valine, leucine, and isoleucine. Two others, histidine and arginine are essential only in children and possibly seniors. A good mnemonic device for remembering these is "Private Tim Hall", abbreviated as: PVT TIM HALL. Since there are multiple amino acids that start with the letters 'A' and 'T' you can use the mnemonic "PVT TIM HALL always Argues, never Tires". This tells you that the 'A' in the mnemonic stands for arginine (Always Argues) and the 'T' does NOT stand for Tyrosine (Never Tires):

Taurine may be necessary to preserve arterial and collagen pliability at 2 mg/kg/day, small but needed (142mg/day per 70Kg human).

Which amino acids are essential, varies from species to species, as different metabolisms are able to synthesize different substances. For instance, taurine (which is not, by strict definition, an amino acid) is essential for cats, but not for dogs. Thus, dog food is not nutritionally sufficient for cats, and taurine is added to commercial cat food, but not to dog food.

The distinction between essential and non-essential amino acids is somewhat unclear, as some amino acids can be produced from others. The sulfur-containing amino acids, methionine and homocysteine, can be converted into each other but neither can be synthesized de novo in humans. Likewise, cysteine can be made from homocysteine, but cannot be synthesized on its own. So, for convenience, sulfur-containing amino acids are sometimes considered a single pool of nutritionally-equivalent amino acids. Likewise arginine, ornithine, and citrulline, which are interconvertible by the urea cycle, are considered a single group.

Use of essential amino acids
Foodstuffs that lack essential amino acids are poor sources of protein equivalents, as the body tends to deaminate the amino acids obtained, converting proteins into fats and carbohydrates. 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.

It is important to note that all essential amino acids may be obtained from plant sources, and that even strict vegetarian diets can provide all dietary requirements, though careful monitoring of nutrient levels is important, as limiting factors become significant when no meat is present in the diet.

The net protein utilization is profoundly affected by the limiting amino acid content (the essential amino acid found in the smallest quantity in the foodstuff), and somewhat affected by salvage of essential amino acids in the body. 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.