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【营养百科】必需氨基酸(Essential amino acid)
2010-07-20 08:17 wikipedia  发布范围:公开

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

Essential  Nonessential
Isoleucine Arginine*
Leucine Aspartate
Lysine Cysteine*
Methionine Glutamate
Phenylalanine Glutamine*
Threonine Glycine*
Tryptophan Valine Proline*
Histidine* Serine* 
Tyrosine* Asparagine*
Selenocysteine** Pyrrolysine**
Alanine

(*) 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.

 "Newer" Common Amino Acids
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.

Recommended daily amounts
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 3900 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.

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[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

Essential Amino Acid Deficiency

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.

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' 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. Due to repetition of letters, several amino acids have one letter abbreviations that are different from their first letter (e.g. lysine is K). Thus the complete list of essential amino acids using one-letter codes is M,I,L,K,F,R,H,T,V,W. It would help college students to have a one letter code mnemonic. One being: I Like Koala Vision Monday, Tuesday, Wednesday, and Friday. Another could be I Like Killing Vehicles Monday, Tuesday, Wednesday, and Friday. Yet another is M I L K FoR THe VW. And another is I'M LiKe WTF - High fiVe! A slightly easier mnemonic, due to its topical relevance, is "Must Ingest These, Vitamin-Like, From Wheat, Kale." One of the shortest and easiest seems to be "RH+ MILK FTW Val!" with the "+" implying conditional essentiality.

References
^ Fürst P, Stehle P (1 June 2004). "What are the essential elements needed for the determination of amino acid requirements in humans?". Journal of Nutrition 134 (6 Suppl): 1558S–1565S. PMID 15173430. http://jn.nutrition.org/cgi/content/full/134/6/1558S
^ Reeds PJ (1 July 2000). "Dispensable and indispensable amino acids for humans". J. Nutr. 130 (7): 1835S–40S. PMID 10867060. http://jn.nutrition.org/cgi/content/full/130/7/1835S
^ Young VR (1994). "Adult amino acid requirements: the case for a major revision in current recommendations". J. Nutr. 124 (8 Suppl): 1517S–1523S. PMID 8064412. http://jn.nutrition.org/cgi/reprint/124/8_Suppl/1517S.pdf
^ Imura K, Okada A (1998). "Amino acid metabolism in pediatric patients". Nutrition 14 (1): 143–8. doi:10.1016/S0899-9007(97)00230-X. PMID 9437700. http://jn.nutrition.org/cgi/content/full/130/7/1835S
^ a b FAO/WHO/UNU (2007). "PROTEIN AND AMINO ACID REQUIREMENTS IN HUMAN NUTRITION". WHO Press. http://whqlibdoc.who.int/trs/WHO_TRS_935_eng.pdf. , page 150
^ Fürst P, Stehle P (1 June 2004). "What are the essential elements needed for the determination of amino acid requirements in humans?". J. Nutr. 134 (6 Suppl): 1558S–1565S. PMID 15173430. http://jn.nutrition.org/cgi/content/full/134/6/1558S
^ Reeds PJ (1 July 2000). "Dispensable and indispensable amino acids for humans". J. Nutr. 130 (7): 1835S–40S. PMID 10867060. http://jn.nutrition.org/cgi/content/full/130/7/1835S
^ [1]
^ McGilvery, Robert W. Biochemistry, a Functional Approach 1979. Chapter 41, esp Page 787
^ "Nutrition for Everyone: Basics: Protein". Centers for Disease Control and Prevention. http://www.cdc.gov/nccdphp/dnpa/nutrition/nutrition_for_everyone/basics/protein.htm. Retrieved 2008-05-15. 
^ a b http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940015664_1994015664.pdf
^ Buckwheat Profile
^ hempnutrition.qxd
^ "Amaranth: Composition, Properties, and Applications of a Rediscovered Food Crop". http://eap.mcgill.ca/CPAT_1.htm. Retrieved 2010-03-07. 
^ Position of the American Dietetic Association and Dietitians of Canada: Vegetarian diets. JADA, 2003; 103(6) 748 – 765.
^ Mnemonic at medicalmnemonics.com 442 128
^ Essential amino acids Essential amino acids, Mnemonic.
^ Williams, R.A.D.; Eliot, J.C. (1989). Basic and Applied Dental Biochemistry. Elsevier Health Sciences. pp. 149. ISBN 0443031444. 

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