Essential fatty acids

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

. Essential fatty acids, or EFAs, are fatty acids that are required in the human diet; they must be obtained from food as human cells have no biochemical pathways capable of producing them internally. 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.

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, hepoxilins, epoxyeicosatrienoic acids (EETs) and Neuroprotectin D
• They form lipid rafts (affecting cellular signaling)
• They act on DNA (activating or inhibiting transcription factors for NFκB, a pro-inflammatory cytokine)

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:

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

Examples

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 fats (SC-PUFA):

• α-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 (LC-PUFA):

• ω-3 fatty acids:
  • eicosapentaenoic acid or EPA (20:5)
  • docosahexaenoic acid or DHA (22:6)
• ω-6 fatty acids:
  • 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.

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) for a discussion of the current status of the term 'essential'. In scientific writing, common usage is that the term essential fatty acid comprises all the ω-3 or -6 fatty acids. Authorative 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. Some writers therefore hold that the LC-PUFA are not essential. But is not how the field has generally used the term.

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

Food sources

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), 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.

Role in human health

For discussion how essential fatty acids affect cardiovascular health, see Diet and heart disease.

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.

See also

• Polyunsaturated fat
• Omega-3 fatty acids
• Omega-6 fatty acids
• Essential fatty acid interactions
• Eicosanoid
  • Prostaglandin
  • Leukotriene
  • Thromboxane
• Endogenous Cannabinoid
• Essential nutrient
• Essential amino acids
• Fatty acid metabolism
• Oily fish

References

• A.E. Hansen et al (1963). "Role of linoleic acid in infant nutrition". Pediatrics 31:171

External links

• 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. URL accessed on 2006-03-14.
• Stillwell W, Wassall SR.. Docosahexaenoic acid: membrane properties of a unique fatty acid.. URL accessed on February 8, 2006. Chem Phys Lipids. 2003 Nov;126(1):1-27

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