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.
Contents |
| Essential | Nonessential |
|---|---|
| Isoleucine | Alanine |
| Leucine | Arginine* |
| Lysine | Aspartate |
| Methionine | Cysteine* |
| Phenylalanine | Glutamate |
| Threonine | Glutamine* |
| Tryptophan | Glycine* |
| Valine | Proline* |
| Histidine* | Serine* |
| Tyrosine* | Asparagine* |
| Selenocysteine** | Pyrrolysine** |
(*) Essential only in certain cases.[1][2]
(**) Truly unclassified. Added to sustain the 22 Numbers of Essential Amino Acids.
Eight amino acids are generally regarded as essential for humans: phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, leucine, and lysine.[3] Additionally, cysteine (or sulphur-containing amino acids), tyrosine (or aromatic amino acids), histidine and arginine are required by infants and growing children.[4][5] Essential amino acids are "essential" not because they are more important to life than the others, but because the body does not synthesize them, making it essential to include them in one's diet in order to obtain them. In addition, the amino acids arginine, cysteine, glycine, glutamine, histidine, proline, serine 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.[6][7] 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.
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 as are the aromatic amino acid pair, phenylalanine and tyrosine. Likewise arginine, ornithine, and citrulline, which are interconvertible by the urea cycle, are considered a single group.
The above list of 20 common amino acids is the traditional one, but there are more than 20 amino acids that are routinely found in proteins. The common 21 amino acids, in addition to pyrrolysine, found only in bacteria, are referred to as the proteinogenic amino acids[8]. In addition to the 22+ alpha-amino acids, scientists are finding far more beta-amino acids in bacteria with a variety of natural functions, such as contributing to antibiotic resistance.
Estimating the daily requirement for the indispensable amino acids has proven to be difficult; these numbers have undergone considerable revision over the last 20 years. The following table lists the WHO recommended daily amounts currently in use for essential amino acids in adult humans, together with their standard one-letter abbreviations.[5]
| Amino acid(s) | mg per kg body weight | mg per 70 kg | mg per 100 kg |
|---|---|---|---|
| I Isoleucine | 20 | 1400 | 2000 |
| L Leucine | 39 | 2730 | 3900 |
| K Lysine | 30 | 2100 | 3000 |
| M Methionine
+ C Cysteine |
10.4 + 4.1 (15 total) | 1050 | 1500 |
| F Phenylalanine
+ Y Tyrosine |
25 (total) | 1750 | 2500 |
| T Threonine | 15 | 1050 | 1500 |
| W Tryptophan | 4 | 280 | 400 |
| V Valine | 26 | 1820 | 2600 |
The recommended daily intakes for children aged three years and older is 10% to 20% higher than adult levels and those for infants can be as much as 150% higher in the first year of life.
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[9]. 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.
Complete proteins contain a balanced set of essential amino acids for humans. Animal sources such as meat, poultry, eggs, fish, milk, and cheese provide all of the essential amino acids.[10] Near-complete proteins are also found in some plant sources such as quinoa,[11] buckwheat,[12] hempseed,[13] and amaranth,[14] among others. Soya appears as lower in sulfur-containing amino acids (methionine and cysteine)[11], which instead are abundant in many other plant protein sources. It is not necessary to consume plant foods containing complete proteins as long as a reasonably varied diet is maintained. By consuming a wide variety of plant foods, a full set of essential amino acids will be supplied and the human body can convert the amino acids into proteins.
The net protein utilization of a human eating only one protein source (only wheat, for instance) is affected by the limiting amino acid content (the essential amino acid found in the smallest quantity in the foodstuff) of that source. Adequate protein utilization, however, will readily be obtained if a balanced variety of protein sources is eaten within 4-6 hours,[15] and/or the total (limited) protein consumed is greater than the requirement.
| Protein source | Limiting amino acid |
|---|---|
| Wheat | lysine |
| Rice | lysine |
| Legumes | tryptophan or methionine (or cysteine) |
| Maize | lysine and tryptophan |
| soy | methionine |
| egg | none; the reference for absorbable protein |
A chronic deficiency in the essential amino acids will lead to either a form of childhood oedema known as kwashiorkor or emaciation known as marasmus.
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' names, and in general did not include arginine which is not always essential. Mnemonic devices that can used are PVT TIM HALL[16] and TT HALL V(ery) IMP(ortant).[17]
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."[18] 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.
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