Essential fatty acid

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Essential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the body requires them for good health but cannot synthesize them.[1] Those not essential are non-essential fatty acids.

The term "essential fatty acid" refers to fatty acids required for biological processes but does not include the fats that only act as fuel. Essential fatty acids should not be confused with essential oils, which are "essential" in the sense of being a concentrated essence.

Only two fatty acids are known to be essential for humans: alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid).[2] Some other fatty acids are sometimes classified as "conditionally essential," meaning that they can become essential under some developmental or disease conditions; examples include docosahexaenoic acid (an omega-3 fatty acid) and gamma-linolenic acid (an omega-6 fatty acid).

When the two EFAs were discovered in 1923, they were designated "vitamin F", but in 1929, research on rats showed that the two EFAs are better classified as fats rather than vitamins.[3]

Functions[edit]

The biological effects of the ω-3 and ω-6 fatty acids are mediated by their mutual interactions, see Essential fatty acid interactions for detail.

In the body, essential fatty acids serve multiple functions. In each of these, the balance between dietary ω-3 and ω-6 strongly affects function.

Nomenclature and terminology[edit]

Fatty acids are straight chain hydrocarbons possessing a carboxyl (COOH) group at one end. The carbon next to the carboxylate is known as α, the next carbon β, and so forth. Since biological fatty acids can be of different lengths, the last position is labelled as a "ω", the last letter in the Greek alphabet. The physiological properties of unsaturated fatty acids largely depend on the position of the first unsaturation relative to the end position and not the carboxylate. For example, the term ω-3 signifies that the first double bond exists as the third carbon-carbon bond from the terminal CH3 end (ω) of the carbon chain. The number of carbons and the number of double bonds are also listed.

ω-3 18:4 (stearidonic acid) or 18:4 ω-3 or 18:4 n−3 indicates an 18-carbon chain with 4 double bonds, and with the first double bond in the third position from the CH3 end. Double bonds are cis and separated by a single methylene (CH2) group unless otherwise noted. So in free fatty acid form, the chemical structure of stearidonic acid is:

Chemical structure of stearidonic acid showing physiological (red) and chemical (blue) numbering conventions

Examples[edit]

For complete tables of ω-3 and ω-6 essential fatty acids, see Polyunsaturated fatty acids.

The essential fatty acids start with the short chain polyunsaturated fatty acids (SC-PUFA):

These two fatty acids cannot be synthesized by humans because humans lack the desaturase enzymes required for their production.

They form the starting point for the creation of longer and more desaturated fatty acids, which are also referred to as long-chain polyunsaturated fatty acids (LC-PUFA):

ω-9 fatty acids are not essential in humans because they can be synthesized from carbohydrates or other fatty acids.

Essential fatty acids[edit]

Mammals lack the ability to introduce double bonds in fatty acids beyond carbon 9 and 10, hence ω-6 linoleic acid (18:2,9,12), abbreviated LA (18:2n-6), and the ω-3 linolenic acid (18:3,9,12,15), abbreviated ALA (18:3n-3), are essential for humans in the diet. In humans, arachidonic acid (20:4,5,8,11,14, abbreviated 20:4n-6) can be synthesized from LA by alternative desaturation and chain elongation.

Humans can convert both LA and ALA to docosapentaenoic acid (22:5n-6) and docosahexaenoic acid (22:6n-3; DHA) respectively, although the conversion to DHA is limited, resulting in lower blood levels of DHA than through direct ingestion. This is illustrated by studies in vegans and vegetarians.[6] If there is relatively more LA than ALA in the diet it favors the formation of docosapentaenoic acid (22:5n-6) from LA rather than docosahexaenoic acid (22:6n-3) from ALA. This effect can be altered by changing the relative ratio of LA:ALA, but is more effective when total intake of polyunsaturated fatty acids is low. However, the capacity to convert LA to AA and ALA to DHA in the preterm infant is limited, and preformed AA and DHA may be required to meet the needs of the developing brain. Both AA and DHA are present in breastmilk and contribute along with the parent fatty acids LA and ALA to meeting the requirements of the newborn infant. Many infant formulas have AA and DHA added to them with an aim to make them more equivalent to human milk.

Essential nutrients are defined as those that cannot be synthesized de novo in sufficient quantities for normal physiological function. This definition is met for LA and ALA but not the longer chain derivatives in adults.[7] The longer chain derivatives particularly, however, have pharmacological properties that can modulate disease processes, but this should not be confused with dietary essentiality.

Between 1930 and 1950, arachidonic acid and linolenic acid were termed 'essential' because each was more or less able to meet the growth requirements of rats given fat-free diets. However, they were yet to be recognized as essential nutrients for humans. In the 1950s Arild Hansen showed that infants fed skimmed milk developed essential fatty acid deficiency. It was characterized by an increased food intake, poor growth, and a scaly dermatitis, and was cured by the administration of corn oil.

Later work by Hansen randomized 426 children, mainly black, to four treatments: modified cow's milk formula, skimmed milk formula, skimmed milk formula with coconut oil, or cow's milk formula with corn oil. The infants who received the skimmed milk formula or the formula with coconut oil developed essential fatty acid deficiency signs and symptoms. This could be cured by administration of ethyl linoleate (the ethyl ester of linoleic acid) with about 1% of the energy intake.[8]

Collins et al. 1970[9] were the first to demonstrate linoleic acid deficiency in adults. They found that patients undergoing intravenous nutrition with glucose became isolated from their fat supplies and rapidly developed biochemical signs of essential fatty acid deficiency (an increase in 20:3n-9/20:4n-6 ratio in plasma) and skin symptoms. This could be treated by infusing lipids, and later studies showed that topical application of sunflower oil would also resolve the dermal symptoms.[10] Linoleic acid has a specific role in maintaining the skin water-permeability barrier, probably as constituents of acylglycosylceramides. This role cannot be met by any ω-3 fatty acids or by arachidonic acid.

The main physiological requirement for ω-6 fatty acids is attributed to arachidonic acid. Arachidonic acid is the major precursor of prostaglandins, leukotrienes, and anandamides that play a vital role in cell signaling. Metabolites from the ω-3 pathway, mainly from eicosapentaenoic acid, are mostly inactive, and this explains why ω-3 fatty acids do not correct the reproductive failure in rats where arachidonic is needed to make active prostaglandins that cause uterine contraction.[11] To some extent, any ω-3 or ω-6 can contribute to the growth-promoting effects of EFA deficiency, but only ω-6 fatty acids can restore reproductive performance and correct the dermatitis in rats. Particular fatty acids are still needed at critical life stages (e.g. lactation) and in some disease states.

In nonscientific writing, common usage is that the term essential fatty acid comprises all the ω-3 or -6 fatty acids. Conjugated fatty acids like calendic acid are not considered essential. Authoritative sources include the whole families, but generally only make dietary recommendations for LA and ALA with the exception of DHA for infants under the age of 6 months. Recent reviews by WHO/FAO in 2009 and the European Food Safety Authority[12] have reviewed the evidence and made recommendations for minimal intakes of LA and ALA and have also recommended intakes of longer chain ω-3 fatty acids based on the association of oily fish consumption with a lower risk of cardiovascular disease. Some earlier review lumped all polyunsaturated fatty acids together without qualification whether they were short or long-chain PUFA or whether they were ω-3 and ω-6 PUFA.[13][14][15]

Conditionally essential[edit]

Traditionally speaking, the LC-PUFAs are not essential. Because the LC-PUFA are sometimes required, they may be considered conditionally essential fatty acids, or not essential to healthy adults.[16]

Food sources[edit]

Some of the food sources of ω-3 and ω-6 fatty acids are fish and shellfish, flaxseed (linseed) and flaxseed oil, hemp seed, olive oil, soya oil, canola (rapeseed) oil, chia seeds, pumpkin seeds, sunflower seeds, leafy vegetables, and walnuts.

Essential fatty acids play a part in many metabolic processes, and there is evidence to suggest that low levels of essential fatty acids, or the wrong balance of types among the essential fatty acids, may be a factor in a number of illnesses, including osteoporosis.[17]

Fish is the main source of the longer omega-3 fats; eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Some plant-based foods contain omega-3 in the form of alpha-linolenic acid (ALA), which appears to have a modest benefit for cardiovascular health.[18] The human body can (and in case of a purely vegetarian diet often must unless certain algae or supplements derived from them are consumed) convert ALA to EPA and subsequently DHA. This elongation of ALA is inefficient. Interestingly, conversion to DHA is higher in women than in men; this is thought to reflect the need to provide DHA to the fetus and infant during pregnancy and breast feeding.[19]

The IUPAC Lipid Handbook provides a very large and detailed listing of fat contents of animal and vegetable fats, including ω-3 and -6 oils.[20] The National Institutes of Health's EFA Education group publishes Essential Fats in Food Oils.[21] This lists 40 common oils, more tightly focused on EFAs and sorted by n-6:3 ratio. Vegetable Lipids as Components of Functional Food lists notable vegetable sources of EFAs as well as commentary and an overview of the biosynthetic pathways involved.[22] Careful readers will note that these sources are not in excellent agreement. EFA content of vegetable sources varies with cultivation conditions. Animal sources vary widely, both with the animal's feed and that the EFA makeup varies markedly with fats from different body parts.

Human health[edit]

Almost all the polyunsaturated fats in the human diet are EFAs. Essential fatty acids play an important role in the life and death of cardiac cells.[23][24][25][26]

Reference intake values[edit]

Reference intake values for as published by the Panel on Dietetic Products, Nutrition and Allergies of the European Food Safety Authority (EFSA).[27]

Common name Type Reference intake values
alpha-Linolenic acid (ALA) Omega-3 2g
Linoleic acid (LA) Omega-6 10g

In the United States, the recommendations for omega-3 fatty acids covers ALA, EPA and DHA, and for adults the values are 1.6 g/day for men and 1.1. g/day for women. The recommendation for omega-6 fatty acids is for linoleic acid: 17 g/day for younger men, dropping to 14 g/day for older men; for women 12 and 11 g/day.[citation needed]

Essential fatty acid deficiency[edit]

Essential fatty acid deficiency results in a dermatitis similar to that seen in zinc or biotin deficiency.[28]:485

See also[edit]

References[edit]

  1. ^ Robert S. Goodhart; Maurice E. Shils (1980). Modern Nutrition in Health and Disease (6th ed.). Philadelphia: Lea and Febinger. pp. 134–138. ISBN 0-8121-0645-8. 
  2. ^ Whitney Ellie; Rolfes SR (2008). Understanding Nutrition (11th ed.). California: Thomson Wadsworth. p. 154. 
  3. ^ Burr, G.O., Burr, M.M. and Miller, E. (1930). "On the nature and role of the fatty acids essential in nutrition" (PDF). J. Biol. Chem. 86 (587). Retrieved 2007-01-17. 
  4. ^ Stillwell W, Shaikh SR, Zerouga M, Siddiqui R, Wassall SR (2005). "Docosahexaenoic acid affects cell signaling by altering lipid rafts". Reproduction, Nutrition, Development. 45 (5): 559–79. PMID 16188208. doi:10.1051/rnd:2005046. 
  5. ^ Calder PC (December 2004). "n-3 fatty acids, inflammation, and immunity--relevance to postsurgical and critically ill patients". Lipids. 39 (12): 1147–61. PMID 15736910. doi:10.1007/s11745-004-1342-z. 
  6. ^ Sanders TA (2009). "DHA Status of vegetarians". Prostaglandins Leukotrienes Essential Fatty Acids. 81 (2–3): 137–41. PMID 19500961. doi:10.1016/j.plefa.2009.05.013. 
  7. ^ FAO/WHO Fats and fatty acids in human nutrition. Report of an expert consultation. FAO Food and Nutrition Paper 91, Rome 2011. ISSN 0254-4725
  8. ^ Wiese, H; Hansen, A; Adams, DJ (1958). "Text". Journal of Nutrition. 66 (3): 345–360. PMID 13611579. 
  9. ^ Collins FD, Sinclair AJ, Royle JP, Coats DA, Maynard AT, Leonard RF (1971). "Plasma lipids in human linoleic acid deficiency.". Nutr Metab. 13 (3): 150–67. PMID 5001758. doi:10.1159/000175332. 
  10. ^ Prottey, C; Hartop, PJ; Press, M (1975). "Correction of the cutaneous manifestations of essential fatty acid deficiency in man by application of sunflower-seed oil to the skin.". J Invest Dermatol. 64 (4): 228–34. PMID 1117180. doi:10.1111/1523-1747.ep12510667. 
  11. ^ Sanders T and Emery P. The Molecular Basis of Human Nutrition, Taylor Frances, London, 2003 ISBN 0-748-40753-7
  12. ^ Jones, A (2010). "EFSA Scientific Opinion on Dietary Reference Values for fats, including saturated fatty acids, polyunsaturated fatty acids, monounsaturated fatty acids, trans fatty acids and cholesterol". EFSA Journal. 8 (3): 1461. doi:10.2903/j.efsa.2010.1461. 
  13. ^ Heather Hutchins, MS, RD (2005-10-19). "Symposium Highlights -- Omega-3 Fatty Acids: Recommendations for Therapeutics and Prevention". Omega-3 fatty acids and their counterparts, n-6 fatty acids, are essential polyunsaturated fatty acids (PUFA) because they cannot be synthesized de novo in the body. 
  14. ^ Nugent KP, Spigelman AD, Phillips RK (June 1996). "Tissue prostaglandin levels in familial adenomatous polyposis patients treated with sulindac". Diseases of the Colon and Rectum. 39 (6): 659–62. PMID 8646953. doi:10.1007/BF02056946. Arachidonic acid is an essential fatty acid… 
  15. ^ Carlstedt-Duke J, Brönnegård M, Strandvik B (December 1986). "Pathological regulation of arachidonic acid release in cystic fibrosis: the putative basic defect". Proceedings of the National Academy of Sciences of the United States of America. 83 (23): 9202–6. PMC 387103Freely accessible. PMID 3097647. doi:10.1073/pnas.83.23.9202. [T]he turnover of essential fatty acids is increased (7). Arachidonic acid is one of the essential fatty acids affected. 
  16. ^ Cunnane SC (November 2003). "Problems with essential fatty acids: time for a new paradigm?". Progress in Lipid Research. 42 (6): 544–68. PMID 14559071. doi:10.1016/S0163-7827(03)00038-9. 
  17. ^ Kruger MC, Horrobin DF (September 1997). "Calcium metabolism, osteoporosis and essential fatty acids: a review". Progress in Lipid Research. 36 (2–3): 131–51. PMID 9624425. doi:10.1016/S0163-7827(97)00007-6. 
  18. ^ Pan A, Chen M, Chowdhury R, et al. (December 2012). "α-Linolenic acid and risk of cardiovascular disease: a systematic review and meta-analysis". Am. J. Clin. Nutr. (Systematic review). 96 (6): 1262–73. PMC 3497923Freely accessible. PMID 23076616. doi:10.3945/ajcn.112.044040. 
  19. ^ Burdge GC, Calder PC (September 2005). "Conversion of alpha-linolenic acid to longer-chain polyunsaturated fatty acids in human adults". Reprod. Nutr. Dev. 45 (5): 581–97. PMID 16188209. doi:10.1051/rnd:2005047. 
  20. ^ IUPAC Lipid Handbook
  21. ^ Essential Fats in Food Oils
  22. ^ Vegetable Lipids as Components of Functional Food, Stuchlik and Zak
  23. ^ Honoré E, Barhanin J, Attali B, Lesage F, Lazdunski M (March 1994). "External blockade of the major cardiac delayed-rectifier K+ channel (Kv1.5) by polyunsaturated fatty acids". Proceedings of the National Academy of Sciences of the United States of America. 91 (5): 1937–41. PMC 43279Freely accessible. PMID 8127910. doi:10.1073/pnas.91.5.1937. 
  24. ^ Reiffel JA, McDonald A (August 2006). "Antiarrhythmic effects of omega-3 fatty acids". The American Journal of Cardiology. 98 (4A): 50i–60i. PMID 16919517. doi:10.1016/j.amjcard.2005.12.027. 
  25. ^ Landmark K, Alm CS (November 2006). "[Alpha-linolenic acid, cardiovascular disease and sudden death]". Tidsskrift for Den Norske Lægeforening (in Norwegian). 126 (21): 2792–4. PMID 17086218. 
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  27. ^ European Food Safety Authority (EFSA) (2009-07-01). "Labelling reference intake values for n-3 and n-6 polyunsaturated fatty acids". EFSA Journal. 7 (7): n/a–n/a. ISSN 1831-4732. doi:10.2903/j.efsa.2009.1176. 
  28. ^ James, William; Berger, Timothy; Elston, Dirk (2005). Andrews' Diseases of the Skin: Clinical Dermatology. (10th ed.). Saunders. ISBN 0-7216-2921-0.