Essential fatty acid

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Types of Fats in Food
Unsaturated fat Monounsaturated fat Polyunsaturated fat Trans fat Omega: 3, 6, 9 Saturated fat
See Also
Fatty acid Essential fatty acid

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Essential fatty acids, or EFAs, are fatty acids that are required in the human diet but which must be obtained from food as the body has no way of producing them internally. There are two closely related families of EFAs: ω-3 (or omega-3 or n-3) and ω-6 (omega 6, n-6.) Only one substance in each of these families is truly 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. Around 1930, it was realized that they are better classified with the fats than with the vitamins.

1 Functions
2 Nomenclature and terminology
- 2.1 What is "essential"?
3 Examples
4 Food sources
5 See also
6 References
7 External links

[edit] Functions

The biological effects of the ω-3 and ω-6 fatty acids are largely 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 eicosanoids (affecting inflammation and many other cellular functions)
- the endogenous cannabinoids (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, isoprostanes, epoxyeicosatrienoic acids (EETs) and neuroprotectin D
They form lipid rafts (affecting cellular signaling)<ref name="WStillwell">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.</ref>

They act on DNA (activating or inhibiting transcription factors for NFκB, a pro-inflammatory cytokine)<ref name="PCalder">Calder PC (2004). "n-3 fatty acids, inflammation, and immunity--relevance to postsurgical and critically ill patients". LIPIDS 39 (12): 1147-1161. PMID 15736910.</ref>

[edit] 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 greek letter, even though is is almost never in the 23rd position. 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) 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:

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[edit] 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. See (Cunnane 2003)<ref name="SCunnane">Cunnane SC (2003). "Problems with essential fatty acids: time for a new paradigm?". PROGRESS IN LIPID RESEARCH 42 (6): 544-568. PMID 14559071.</ref> for a discussion of the current status of the term 'essential'.

Do not confuse EFAs with essential oils, which are 'essential' in the sense of being a concentrated essence.

[edit] Examples

The essential fatty acids are:

Alpha-Linolenic acid (18:3) - ω-3
Linoleic acid (18:2) - ω-6

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 polyunsaturates:

ω-3 fatty acids:
- alpha-linolenic acid or LNA (18:3)
- stearidonic acid or SDA (18:4)
- eicosatetraenoic acid or ETA (20:4)
- eicosapentaenoic acid or EPA (20:5)
- docosahexaenoic acid or DHA (22:6)
ω-6 fatty acids:
- linoleic acid or LA (18:2)
- gamma-linolenic acid or GLA (18:3)
- dihomo-gamma-linolenic acid or DGLA (20:3)
- arachidonic acid or AA (20:4)

ω-9 fatty acids are not essential in humans, because humans possess all the enzymes required for their synthesis.

[edit] Food sources

Some of the food sources of ω-3 and ω-6 fatty acids are fish and shellfish, flaxseed (linseed), soya oil, canola (rapeseed) oil, hemp 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 IUPAC Lipid HandbookPDF 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. Stuchlik and Zak, 'Vegetable Lipids as Components of Functional Food'PDF 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.

[edit] See also

[edit] References

G.O. Burr and M.M. Burr (1930). "On the nature and role of the fatty acids essential in nutrition". J. Biol. Chem. 86:587
A.E. Hansen et al (1963). "Role of linoleic acid in infant nutrition". Pediatrics 31:171