Essential fatty acid
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Essential fatty acids, or EFAs, are fatty acids that cannot be constructed within an organism from other components (generally all references are to humans) by any known chemical pathways; and therefore must be obtained from the diet. The term refers to those involved in biological processes, and not fatty acids which may just play a role as fuel. As many of the compounds created from essential fatty acids can be taken directly in the diet, it is possible that the amounts required in the diet (if any) are overestimated. It is also possible they can be underestimated as organisms can still survive in unideal, malnourished conditions.
There are two families of EFAs: ω-3 (or omega-3 or n-3) and ω-6 (omega-6, n-6.) Fats from each of these families are essential, as the body can convert one omega-3 to another omega-3, for example, but cannot create an omega-3 from scratch. They were originally designated as Vitamin F when they were discovered as essential nutrients in 1923. In 1930, work by Burr, Burr and Miller showed that they are better classified with the fats than with the vitamins.
- 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.
- They are modified to make
- the classic eicosanoids (affecting inflammation and many other cellular functions)
- the endocannabinoids (affecting mood, behavior and inflammation)
- the lipoxins from ω-6 EFAs and resolvins from ω-3 (in the presence of aspirin, downregulating inflammation.)
- the isofurans, neurofurans, isoprostanes, hepoxilins, epoxyeicosatrienoic acids (EETs) and Neuroprotectin D
- They form lipid rafts (affecting cellular signaling)
- They act on DNA (activating or inhibiting transcription factors such as NFκB, which is linked to pro-inflammatory cytokine production ) 
Nomenclature and terminology
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 ω, the last letter in the Greek alphabet. Since 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, the position is signified by (ω minus n). 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 is 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:
- For a 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 synthesised by humans, as 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-3 fatty acids:
- ω-6 fatty acids:
ω-9 fatty acids are not essential in humans, because humans possess all the enzymes required for their synthesis. The public is sometimes perceived as ignorant of this, as many supplement companies market Omega 3-6-9 blends.
What is "essential"?
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. Further research has shown that human metabolism requires both ω-3 and ω-6 fatty acids. To some extent, any ω-3 and any ω-6 can relieve the worst symptoms of fatty acid deficiency. 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. Authoritative sources include the whole families, without qualification.   The human body can make some long-chain PUFA (arachidonic acid, EPA and DHA) from lineolate or lineolinate.
Traditionally speaking the LC-PUFA are not essential.See (Cunnane 2003) for a discussion of the current status of the term 'essential'. Because the LC-PUFA are sometimes required, they may be considered "conditionally essential", or not essential to healthy adults.
Mary Enig, PhD, has pointed out numerous studies showing the need for omega-3 and omega-6 essential fatty acids in mammalians  A 2005 study has shown evidence that gamma-linolenic acid, GLA, a product of omega-6, has been shown to inhibit the breast cancer promoting gene of Her2/neu. 
Biologist Ray Peat, PhD, has pointed out flaws in the studies purportedly showing the need for n-3 and n-6 fats. He notes that so-called EFA deficiencies have sometimes been reversed by adding B vitamins or a fat-free liver extract to the diet. In his view, 'the optional dietary level of the "essential fatty acids" might be close to zero, if other dietary factors were also optimized.' 
Essential fatty acids should not be confused with essential oils, which are "essential" in the sense of being a concentrated essence.
Almost all the polyunsaturated fat in the human diet is from EFA. Some of the food sources of ω-3 and ω-6 fatty acids are fish and shellfish, flaxseed (linseed), hemp 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.
Plant sources of ω-3 do not contain eicosapentaenoic acid and docosahexaenoic acid. This is thought to be the reason that absorption of essential fatty acids is much greater from animal rather than plant sources (see Fish and plants as a source of Omega-3 for more).
The PDF (370 KiB) provides a very large and detailed listing of fat contents of animal and vegetable fats, including ω-3 and -6 oils. The National Institutes of Health's EFA Education group publishes 'Essential Fats in Food Oils.' This lists 40 common oils, more tightly focused on EFAs and sorted by n-6:3 ratio. PDF (139 KiB) list notable vegetable sources of EFAs as well as commentary and an overview of the biosynthetic pathways involved. Users can interactively search at Nutrition Data for the richest food sources of particular EFAs or other nutrients. 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.
Role in human health
- For discussion how essential fatty acids affect cardiovascular health, see Diet and heart disease.
- Polyunsaturated fat
- Omega-3 fatty acid
- Omega-6 fatty acid
- Essential fatty acid interactions
- Nonclassic eicosanoid
- Endogenous Cannabinoid
- Essential nutrient
- Essential amino acid
- Fatty acid metabolism
- Fatty acid synthase
- Oily fish
- 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.
- 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): 559–579. PMID 16188208.
- Calder PC (2004). "n-3 fatty acids, inflammation, and immunity--relevance to postsurgical and critically ill patients". LIPIDS. 39 (12): 1147–1161. PMID 15736910.
- Or at least the polyunsaturated, straight-chain methylene-interrupted ones; conjugated fatty acids like calendic acid are not.
Heather Hutchins, MS, RD (10/19/2005). "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."
- Nugent K, Spigelman A, Phillips R (1996). "Tissue prostaglandin levels in familial adenomatous polyposis patients treated with sulindac". Dis Colon Rectum. 39 (6): 659–62. PMID 8646953.
- "Arachidonic acid is an essential fatty acid..."
- Carlstedt-Duke J, Brönnegård M, Strandvik B (1986). "Pathological regulation of arachidonic acid release in cystic fibrosis: the putative basic defect". Proc Natl Acad Sci U S A. 83 (23): 9202–6. PMID 3097647.
- "[T]he turnover of essential fatty acids is increased (7). Arachidonic acid is one of the essential fatty acids affected."
- Cunnane SC (2003). "Problems with essential fatty acids: time for a new paradigm?". PROGRESS IN LIPID RESEARCH. 42 (6): 544–568. PMID 14559071.
- Enig, M (2001), Know Your Fats: The Complete Primer for Understanding the Nutrition of Fats, Oils and Cholesterol (First Edition ed.), Silver Spring, MD: Bethesda Press, ISBN 0-9678126-0-7*
- Menendez JA, Vellon R, Colomer R, Lupu R (2005). "Effect of gamma-linolenic acid on the transcriptional activity or Her2/neu (Erb-2) oncogene". J Natl Cancer Inst. 97 (2): 1611–1615. PMID 16264182.*
- "External blockade...by polyunsaturated fatty acids". PMID 43279. - see page 1 of this link
- "Antiarrythmic effects of omega-3 fatty acids". PMID 16919517.
- "Alpha-linolenic acid, cardiovascular disease and sudden death". PMID 17086218.
- "Omega-3 and health". PMID 17091903.
- A.E. Hansen et al (1963). "Role of linoleic acid in infant nutrition". Pediatrics 31:171
- Fats You Need -- Essential Fatty Acids
- USDA (public domain resource, and a source for this article)
- Cunnane Stephen C (June, 2005). "Essential Fatty Acids: Time for a New Paradigm?". PUFA Newsletter. Retrieved 2006-03-14.
- Stillwell W, Wassall SR. "Docosahexaenoic acid: membrane properties of a unique fatty acid.". Retrieved February 8. Chem Phys Lipids. 2003 Nov;126(1):1-27