Essential amino acid

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An essential amino acid or indispensible 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. Nutritional essentiality is characteristic of the species, not the nutrient. Nine amino acids are generally regarded as essential for humans. They are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. In addition, the amino acids arginine, cysteine, glycine 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). Patients 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. The following table lists the recommended daily amounts for essential amino acids in humans, together with their standard one-letter abbreviations. (A three-letter abbreviation scheme also exists.)

Name	Recommended daily intake in human Adults mg per Kg body weight WHO	for 70Kg human (mg)
F Phenylalanine	14 (sum with Tyrosine)	980
L Leucine	14	980
M Methionine	13 (sum with Cysteine)	910
K Lysine	12	840
I Isoleucine	10	700
V Valine	10	700
T Threonine	7	490
W Tryptophan	3	245
H Histidine	unknown, 28 in infants (? sum with arginine)	(? 1960)
R Arginine	unknown, required for infants, maybe seniors	(?)

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.

1 Use of essential amino acids
2 Mnemonics
3 See also
4 External links

[edit] 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, 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.

Protein source	Limiting amino acid
Wheat	lysine
Rice	lysine
Maize	lysine and tryptophan
Pulses	methionine (or cysteine)
Beef	phenylalanine (or tyrosine)
Egg, chicken	none; the reference for absorbable protein
Milk or Whey, bovine	methionine (or cysteine)

[edit] Mnemonics

Using the one letter designation shown above, mnemonic devices have been developed for students wanting or needing to memorize the essential amino acids. None are useful without first understanding the one-letter chemical designation for the amino acids, and most were developed in a time when arginine was recognized as the only conditionally essential amino acid. With this in mind, the mnemonic words and phrases that have been used in the past are: PVT TIM HALL, or VP MATT HILL. 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."