Essential fatty acid

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Essential fatty acids, or EFAs, are fatty acids that are required in the human diet. This means they cannot be synthesized by the body from other fatty acids and must be obtained from food. There are two closely related families of EFAs: ω-3 (or omega-3 or n-3) and ω-6 (omega 6, n-6.) 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

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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 funtions. 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.) (Stillwell and Wassal, 2003)
They act on DNA (activating or inhibiting transcription factors for NFκB, a pro-inflammatory cytokine.) (Calder, 2004)

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Nomenclature and terminology

Fatty acids are straight chain hydrocarbons posessing a carboxyl (COOH) group at one end (α) and (usually) a methyl group at the other (ω) end. In physiology, EFAs are named by the position of the first double bond from the ω end. 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: Image:SAnumbering.png

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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 2005) 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 essense.

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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 (18:3)
- stearidonic acid (18:4)
- eicosatetraenoic acid (20:4)
- eicosapentaenoic acid or EPA (20:5)
- docosahexaenoic acid or DHA (22:6)
ω-6 fatty acids:
- linoleic acid (18:2)
- gamma-linolenic acid (18:3)
- dihomo-gamma-linolenic acid (20:3)
- arachidonic acid or ARA (20:4)

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

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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, 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.

The IUPAC Lipid Handbook (in .pdf format) 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' (in .pdf format) 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.

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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

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