Essential amino acid

# Overview
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.

# Essentiality vs. conditional essentiality in humans
Nine amino acids are generally regarded as essential for humans: isoleucine, leucine, lysine, threonine, tryptophan, methionine, histidine, valine and phenylalanine.  In addition, the amino acids arginine, cysteine, glycine, glutamine and tyrosine are considered conditionally essential, meaning they are not normally required in the diet, but must be supplied exogenously to specific populations that do not synthesize it in adequate amounts. An example would be with the disease phenylketonuria (PKU). Individuals living with PKU must keep their intake of phenylalanine extremely low to prevent mental retardation and other metabolic complications. However, phenylalanine is the precursor for tyrosine synthesis. Without phenylalanine, tyrosine cannot be made and so tyrosine becomes essential in the diet of PKU patients.
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 when the base ingredients do not meet the requirements of the cat, 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.

# Recommended daily amounts
The following table lists the recommended daily amounts for essential amino acids in humans, together with their standard one-letter abbreviations.
In some cases, humans can use either of two amino acids, so only the total matters.
Taurine may be necessary to preserve arterial and collagen pliability
at 2 mg/kg/day, small but needed (142 mg/day per 70 kg human).

# 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.
All essential amino acids may be obtained from plant sources, and even strict vegetarian diets can provide all dietary requirements, although most vegetarians may not be so thorough. Some believe that careful monitoring of nutrient levels is important in strict vegetarian diets, but there are virtually no cases of protein-deficiency among populations consuming adequate calories. The only common cases of protein-deficiency occur among populations that are chronically undernourished.
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.

# Mnemonics
Using the one letter designation shown above, mnemonic devices have been developed for students wanting or needing to memorize the essential amino acids. Previous devices have utilized the first letter of the amino acids name, and in general did not include arginine which is not always essential. One mnemonic device that has been used in the past is PVT TIM HALL.
Another method uses the first letter of each essential amino acid to begin each word in a phrase, such as: "Any Help In Learning These Little Molecules Proves Truly Valuable." This method begins with the two amino acids that need some qualifications as to their requirements.
Note that these devices work by using the first letter of the actual amino acids name.  Due to repetition of letters, several amino acids have one letter abbreviations that are different than their first letter (e.g. lysine is K).  Thus the complete list of essential amino acids utilizing one-letter codes is MILKVWTHFR.
A mnemonic that involves only the true one-letter codes for each amino acid is: "I Have Received Much Kudos For Learning These Very Well," for  IHRMKFLTVW.