Essential amino acid

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Essential amino acids are supplied by a healthy diet.

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.


[edit] 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.[1] Cysteine (or sulphur-containing amino acids), tyrosine (or aromatic amino acids), histidine and arginine are additionally required by infants and growing children.[2][3] 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.[4][5] 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.

[edit] 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.[3]

Amino acid 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

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.

[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[6]. 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.[7] Complete proteins are also found in some plant sources such as quinoa,[8] buckwheat,[9] hempseed,[10] and amaranth,[citation needed] among others. Soya is near-complete[11] although this is contested[8][12][13], while rice protein is a complete protein.[8][14][15][16] 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 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
Legumes tryptophan
Maize lysine and tryptophan
Pulses methionine (or cysteine)
Egg, chicken none; the reference for absorbable protein

[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. 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 TM HILL.[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 than their first letter (e.g. lysine is K). Thus the complete list of essential amino acids utilizing 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!

[edit] See also

[edit] References

  1. ^ 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. 
  2. ^ 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. 
  4. ^ Fürst P, Stehle P (01 Jun 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. 
  5. ^ Reeds PJ (01 Jul 2000). "Dispensable and indispensable amino acids for humans". J. Nutr. 130 (7): 1835S–40S. PMID 10867060. 
  6. ^ McGilvery, Robert W. Biochemistry, a Functional Approach 1979. Chapter 41, esp Page 787
  7. ^ "Nutrition for Everyone: Basics: Protein". Centers for Disease Control and Prevention. Retrieved on 2008-05-15. 
  8. ^ a b c
  9. ^ Buckwheat Profile
  10. ^ hempnutrition.qxd
  11. ^ The Scoop on Protein Powders By Sofia Segounis, Nutritionist
  13. ^ Protein Means Power and a Whole Lot More
  14. ^ The Scoop on Protein Powders By Sofia Segounis, Nutritionist
  16. ^ Protein Means Power and a Whole Lot More
  17. ^ Mnemonic at 442 128
  18. ^ Williams, R.A.D.; Eliot, J.C. (1989). Basic and Applied Dental Biochemistry. Elsevier Health Sciences. pp. 149. ISBN 0443031444. 

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