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Omega-6 fatty acids

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Title: Omega-6 fatty acids  
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Subject: Unsaturated fat, Vegetarian nutrition, Eicosatetraenoic acid
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Omega-6 fatty acids

For an explanation of n and numerical nomenclature (such as n−6 or 18:2), see Fatty acid#Nomenclature.

Omega-6 fatty acids (also referred to as ω−6 fatty acids or n−6 fatty acids) are a family of unsaturated fatty acids that have in common a final carbon–carbon double bond in the n−6 position, that is, the sixth bond, counting from the methyl end.[1]

The biological effects of the omega−6 fatty acids are largely mediated by their conversion to omega-6 eicosanoids that bind to diverse receptors found in every tissue of the body. The conversion of tissue arachidonic acid (20:4n-6) to omega-6 prostaglandin and omega-6 leukotriene hormones provides many targets for pharmaceutical drug development and treatment to diminish excessive omega-6 actions in atherosclerosis,[2] asthma, arthritis, vascular disease, thrombosis, immune-inflammatory processes, and tumor proliferation. Competitive interactions with the omega-3 fatty acids affect the relative storage, mobilization, conversion and action of the omega-3 and omega-6 eicosanoid precursors (see Essential fatty acid interactions).

Key omega−6 fatty acids

Linoleic acid (18:2, n−6), the shortest-chained omega−6 fatty acid, is an essential fatty acid. Arachidonic acid (20:4) is a physiologically significant omega−6 fatty acid and is the precursor for prostaglandins and other physiologically active molecules.

Suggested negative health effects

Some medical research suggests that excessive levels of certain omega−6 fatty acids relative to certain omega-3 fatty acids may increase the probability of a number of diseases.[3][4][5] However, scientific research indicates that air pollution, smoking, second-hand smoke, and other exogenous toxins in conjunction with the excessive intake of n−6 fatty acids leads to inflammation and the overexpression of the COX-2 enzyme, and not solely the excessive intake of n−6 fatty acids.[6][7][8][9][10]

Modern Western diets typically have ratios of omega−6 to omega−3 in excess of 10 to 1, some as high as 30 to 1; the average ratio of omega-6 to omega-3 in the Western diet is 15/1–16.7/1.[2] Humans are thought to have evolved with a diet of a 1-to-1 ratio of omega-6 to omega-3 and the optimal ratio is thought to be 4 to 1 or lower,[2][11] and it is even better if there is more omega−3 than omega−6 (especially healthy ratio of omega−6 to omega−3 is from 1:1 to 1:4).[12] A ratio of 2–3/1 omega 6 to omega 3 helped reduce inflammation in patients with rheumatoid arthritis.[2] A ratio of 5/1 had a beneficial effect on patients with asthma but a 10/1 ratio had a negative effect.[2] A ratio of 2.5/1 reduced rectal cell proliferation in patients with colorectal cancer, whereas a ratio of 4/1 had no effect.[2]

Excess omega−6 fats interfere with the health benefits of omega−3 fats, in part because they compete for the same rate-limiting enzymes. A high proportion of omega−6 to omega−3 fat in the diet shifts the physiological state in the tissues toward the pathogenesis of many diseases: prothrombotic, proinflammatory and proconstrictive.[13]

Chronic excessive production of omega−6 eicosanoids is associated with arthritis, inflammation, and cancer. Many of the medications used to treat and manage these conditions work by blocking the effects of the potent omega−6 fat, arachidonic acid.[14] Many steps in formation and action of omega-6 hormones from omega-6 arachidonic acid proceed more vigorously than the corresponding competitive steps in formation and action of omega-3 hormones from omega-3 eicosapentaenoic acid.[15] The COX-1 and COX-2 inhibitor medications, used to treat inflammation and pain, work by preventing the COX enzymes from turning arachidonic acid into inflammatory compounds.[16] (See Cyclooxygenase for more information.) The LOX inhibitor medications often used to treat asthma, work by preventing the LOX enzyme from converting arachidonic acid into the leukotrienes.[17][18] Many of the anti-mania medications used to treat bipolar disorder work by targeting the arachidonic acid cascade in the brain.[19]

A high consumption of omega-6 polyunsaturated fatty acids (PUFAs), which are found in most types of vegetable oil, may increase the likelihood that postmenopausal women will develop breast cancer.[20] Similar effect was observed on prostate cancer, but the study was performed on mice.[21] Another "analysis suggested an inverse association between total polyunsaturated fatty acids and breast cancer risk, but individual polyunsaturated fatty acids behaved differently [from each other]. [...] a 20:2 derivative of linoleic acid [...] was inversely associated with the risk of breast cancer".[22]

Dietary linoleic acid requirement

Adding more controversy to the omega−6 fat issue is that the dietary requirement for linoleic acid (the key omega−6 fatty acid), has been seriously questioned, because of a significant methodology error discovered by University of Toronto scientist Stephen Cunnane.[23] Cunnane discovered that the seminal research used to determine the dietary requirement for linoleic acid was based on feeding animals linoleic acid-deficient diets, which were simultaneously deficient in omega−3 fats. The omega−3 deficiency was not taken into account. The omega−6 oils added back systematically to correct the deficiency also contained trace amounts of omega−3 fats. Therefore the researchers were inadvertently correcting the omega−3 deficiency as well. Ultimately, it took more oil to correct both deficiencies. According to Cunnane, this error overestimates linoleic acid requirements by 5 to 15 times.

Dietary sources

Four major food oils (palm, soybeans, rapeseed, and sunflower) provide more than 100 million metric tons annually, providing more than 32 million metric tons of omega-6 linoleic acid and 4 million metric tons of omega-3 alpha-linolenic acid.[24]

Dietary sources of omega−6 fatty acids include:[25]

List of omega−6 fatty acids

Common name Lipid name Chemical name
Linoleic acid (LA) 18:2 (n−6) all-cis-9,12-octadecadienoic acid
Gamma-linolenic acid (GLA) 18:3 (n−6) all-cis-6,9,12-octadecatrienoic acid
Calendic acid 18:3 (n−6) 8E,10E,12Z-octadecatrienoic acid
Eicosadienoic acid 20:2 (n−6) all-cis-11,14-eicosadienoic acid
Dihomo-gamma-linolenic acid (DGLA) 20:3 (n−6) all-cis-8,11,14-eicosatrienoic acid
Arachidonic acid (AA) 20:4 (n−6) all-cis-5,8,11,14-eicosatetraenoic acid
Docosadienoic acid 22:2 (n−6) all-cis-13,16-docosadienoic acid
Adrenic acid 22:4 (n−6) all-cis-7,10,13,16-docosatetraenoic acid
Docosapentaenoic acid 22:5 (n−6) all-cis-4,7,10,13,16-docosapentaenoic acid
Tetracosatetraenoic acid 24:4 (n−6) all-cis-9,12,15,18-tetracosatetraenoic acid
Tetracosapentaenoic acid 24:5 (n−6) all-cis-6,9,12,15,18-tetracosapentaenoic acid

See also

Notes and references

Additional sources

External links

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