Essential amino acid
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/* Use of essential amino acids */ "Spirulina" are bacteria, not plants.
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An '''essential amino acid''' or '''indispensable amino acid''' is an [[amino acid]] that cannot be synthesized ''[[de novo synthesis|de novo]]'' by the organism (usually referring to humans), and therefore must be supplied in the diet.
==Essentiality vs. conditional essentiality in humans==
Eight amino acids are generally regarded as essential for humans: [[phenylalanine]], [[valine]], [[threonine]], [[tryptophan]], [[isoleucine]], [[methionine]], [[leucine]], and [[lysine]].<ref>{{cite journal |author=Young VR |title=Adult amino acid requirements: the case for a major revision in current recommendations |journal=J. Nutr. |volume=124 |issue=8 Suppl |pages=1517S–1523S |year=1994 |pmid=8064412 |url=http://jn.nutrition.org/cgi/reprint/124/8_Suppl/1517S.pdf}}</ref> [[Cysteine]] (or sulphur-containing amino acids), [[tyrosine]] (or aromoatic amino acids), [[histidine]] and [[arginine]] are additionally required by infants and growing children.<ref>{{cite journal |author=Imura K, Okada A |title=Amino acid metabolism in pediatric patients |journal=Nutrition |volume=14 |issue=1 |pages=143–8 |year=1998 |pmid=9437700 |url=http://jn.nutrition.org/cgi/content/full/130/7/1835S |doi=10.1016/S0899-9007(97)00230-X}}</ref><ref name="WHO"/> Essential amino acids are so called 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.<ref>{{cite journal |author=Fürst P, Stehle P |title=What are the essential elements needed for the determination of amino acid requirements in humans? |journal=J. Nutr. |volume=134 |issue=6 Suppl |pages=1558S–1565S |year=2004 |pmid=15173430 |url=http://jn.nutrition.org/cgi/content/full/134/6/1558S}}</ref><ref>{{cite journal |author=Reeds PJ |title=Dispensable and indispensable amino acids for humans |journal=J. Nutr. |volume=130 |issue=7 |pages=1835S–40S |year=2000 |pmid=10867060 |url=http://jn.nutrition.org/cgi/content/full/130/7/1835S}}</ref> 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.
==Recommended daily amounts==
Estimating the daily requirement for the indispensible amino acids has proven to be difficult; these numbers have undergone considerable revision over the last 20 years. It was known for years that infants were unable to synthesis histidine, but only relatively recently has it been determined that adults are also unable to do this. 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.<ref name="WHO">{{cite web|url=http://whqlibdoc.who.int/trs/WHO_TRS_935_eng.pdf|title=PROTEIN AND AMINO ACID REQUIREMENTS IN HUMAN NUTRITION|author=FAO/WHO/UNU|publisher=WHO Press|date=2007}}, page 150</ref>
{| class="wikitable"
! Amino acid
! mg per kg body weight
! mg per 70 kg
|-
| '''H''' [[Histidine]]
| 10
| 700
|-
| '''I''' [[Isoleucine]]
| 20
| 1400
|-
| '''L''' [[Leucine]]
| 39
| 2730
|-
| '''K''' [[Lysine]]
| 30
| 2100
|-
| '''M''' [[Methionine]]
+ '''C''' [[Cysteine]]
| 15 (total)
| 1050
|-
| '''F''' [[Phenylalanine]]
+ '''Y''' [[Tyrosine]]
| 25 (total)
| 1750
|-
| '''T''' [[Threonine]]
| 15
| 1050
|-
| '''W''' [[Tryptophan]]
| 4
| 280
|-
| '''V''' [[Valine]]
| 26
| 1820
|}
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.
==Use of essential amino acids==
Foodstuffs that lack essential amino acids are poor sources of [[protein]] equivalents, as the body tends to [[deamination|deaminate]] the amino acids obtained, converting proteins into [[fat]]s and [[carbohydrate]]s<ref>McGilvery, Robert W. ''Biochemistry, a Functional Approach'' 1979. Chapter 41, esp Page 787</ref>. 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, provided they are based on a variety of whole plant foods. 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.{{Fact|date=August 2007}}
[[Complete protein]]s 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.<ref>{{cite web|url=http://www.cdc.gov/nccdphp/dnpa/nutrition/nutrition_for_everyone/basics/protein.htm|title=Nutrition for Everyone: Basics: Protein|publisher=Centers for Disease Control and Prevention|accessdate=2008-05-15}}</ref> Complete proteins are also found in some plant sources such as [[quinoa]],<ref name="NASA">http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940015664_1994015664.pdf</ref> [[buckwheat]],<ref>http://www.agmrc.org/agmrc/commodity/specialitycrops/buckwheat/buckwheatprofile.html</ref> [[hempseed]],<ref>http://www.drbronner.com/pdf/hempnutrition.pdf</ref> and [[amaranth]],{{Fact|date=July 2008}} among others. [[Soybeans]] are considered by some to be a source of complete protein<ref>http://www.truestarhealth.com/members/cm_archives12ML3P1A8.html The Scoop on Protein Powders By Sofia Segounis, Nutritionist</ref> although this is contested.<ref>[http://www.nutrition4health.org/NOHAnews/NNF01SoyBeatrice.htm THE DOWNSIDE OF SOYBEAN CONSUMPTION]</ref><ref>[http://life.familyeducation.com/nutrition-and-diet/weight/46761.html Protein Means Power and a Whole Lot More]</ref><ref name="NASA"/>
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.
{| class="wikitable"
! '''Protein source'''
! '''Limiting amino acid'''
|-
| [[Wheat]]
| [[lysine]]
|-
| [[Rice]]
| [[lysine]]
|-
| [[Legumes]]
| [[tryptophan]]
|-
| [[Maize]]
| [[lysine]] and [[tryptophan]]
|-
| [[Pulses]]
| [[methionine]] (or [[cysteine]])
|-
| [[egg (food)|Egg]], chicken
| none; the reference for absorbable protein
|}
==Mnemonics==
Using the one letter designation shown above, [[mnemonic device]]s 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''.<ref>{{MedicalMnemonics|442|128||}}</ref>
Another method uses the first letter of each essential amino acid to begin each word in a phrase, such as: "''A''ny ''H''elp ''I''n ''L''earning ''T''hese ''L''ittle ''M''olecules ''P''roves ''T''ruly ''V''aluable."<ref>{{cite book
| last = Williams
| first = R.A.D.
| authorlink =
| coauthors = Eliot, J.C.
| title = Basic and Applied Dental Biochemistry
| publisher = Elsevier Health Sciences
| date = 1989
| location =
| pages = p.149
| url =
| doi =
| id =
| isbn = 0443031444}}</ref> 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.{{Fact|date=July 2008}}
==See also==
*[[Essential nutrient]]
*[[Essential fatty acid]]
*[[Biological Value|Biological Value (BV)]]
*[[List of standard amino acids]]
*[[Edible protein per unit area of land]]
*[[PDCAAS|Protein Digestibility Corrected Amino Acid Score]]
*[[Protein]]
==References==
{{reflist}}
==External links==
*[http://www.veganhealth.org/articles/protein Amino acid content of some vegetarian foods] - In a meal, make sure at least one food has a low value in each row of the "Amount to meet RDA" column, at veganhealth.org
*[http://www.dasc.vt.edu/extension/nutritioncc/9729.html Amino Acid Profiles of Some Common Feeds] at [[Virginia Tech]]
* [http://micro.magnet.fsu.edu/aminoacids/index.html Molecular Expressions: The Amino Acid Collection] - Has detailed information and crystal photographs of each amino acid. at [[Florida State University]]
{{AminoAcids}}
[[Category:Amino acids]]
[[Category:Essential nutrients]]
[[Category:Nitrogen metabolism]]
[[Category:Nutrition]]
[[ca:Aminoàcid essencial]]
[[cs:Esenciální aminokyselina]]
[[da:Essentiel aminosyre]]
[[de:Essentielle Aminosäure]]
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