Omega-6 fatty acid

Omega-6 fatty acid
For an explanation of n and numerical nomenclature (such as n−6 or 18:2), see Fatty acid#Nomenclature.
Types of fats in food
See also
The chemical structure of linoleic acid, a common n−6 fatty acid found in many vegetable oils.

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

Contents

Key n−6 fatty acids

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

Negative health effects

Some medical research suggests that excessive levels of certain n−6 fatty acids, relative to certain n−3 (Omega-3) fatty acids, may increase the probability of a number of diseases.[3][4][5]

Modern Western diets typically have ratios of n−6 to n−3 in excess of 10 to 1, some as high as 30 to 1. The optimal ratio is thought to be 4 to 1 or lower.[6][7]

Excess n−6 fats interfere with the health benefits of n−3 fats, in part because they compete for the same rate-limiting enzymes. A high proportion of n−6 to n−3 fat in the diet shifts the physiological state in the tissues toward the pathogenesis of many diseases: prothrombotic, proinflammatory and proconstrictive.[8]

Chronic excessive production of n−6 eicosanoids is associated with heart attacks, thrombotic stroke, arrhythmia, arthritis, osteoporosis, inflammation, mood disorders, obesity, and cancer.[9] Many of the medications used to treat and manage these conditions work by blocking the effects of the potent n−6 fat, arachidonic acid.[10] Many steps in formation and action of n-6 hormones from n-6 arachidonic acid proceed more vigorously than the corresponding competitive steps in formation and action of n-3 hormones from n-3 eicosapentaenoic acid.[11] 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.[12] (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.[13][14] Many of the anti-mania medications used to treat bipolar disorder work by targeting the arachidonic acid cascade in the brain.[15]

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.[16] Similar effect was observed on prostate cancer.[17] Other "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".[18]

Dietary linoleic acid requirement

Adding more controversy to the n−6 fat issue is that the dietary requirement for linoleic acid (the key n−6 fatty acid), has been seriously questioned, because of a significant methodology error discovered by University of Toronto scientist Stephen Cunnane.[19] 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 n−3 fats. The n−3 deficiency was not taken into account. The n−6 oils added back systematically to correct the deficiency also contained trace amounts of n−3 fats. Therefore the researchers were inadvertently correcting the n−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

The evening primrose flower (O. biennis) produces an oil containing a high content of γ-linolenic acid, a type of n−6 fatty acid.

Four major food oils (palm, soybean, rapeseed, and sunflower) provide more than 100 million metric tons annually, providing more than 32 million metric tons of n-6 linoleic acid and 4 million metric tons of n-3 alpha-linolenic acid.[20]

Dietary sources of n−6 fatty acids include:[21]

List of n−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
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
Calendic acid 18:3 (n−6) 8E,10E,12Z-octadecatrienoic acid

See also

Notes & references

  1. ^ Chow, Ching Kuang (2001). Fatty Acids in Foods and Their Health Implications. New York: Routledge Publishing. http://worldcat.org/oclc/25508943&referer=brief_results. 
  2. ^ Simopoulos, A. P. (October 2002). "Polyunsaturated fatty acids in biology and diseases. The importance of the ratio of omega-6/omega-3 essential fatty acids". Biomedecine & Pharmacotherapy 56 (8): 365–379. doi:10.1016/S0753-3322(02)00253-6. PMID 12442909. http://www.sciencedirect.com/science/article/pii/S0753332202002536. 
  3. ^ Lands, William E.M. (December 2005). "Dietary fat and health: the evidence and the politics of prevention: careful use of dietary fats can improve life and prevent disease". Annals of the New York Academy of Sciences (Blackwell) 1055: 179–192. doi:10.1196/annals.1323.028. PMID 16387724. 
  4. ^ Hibbeln, Joseph R.; Nieminen, Levi R.G.; Blasbalg, Tanya L.; Riggs, Jessica A.; Lands, William E. M. (1 June 2006). "Healthy intakes of n−3 and n−6 fatty acids: estimations considering worldwide diversity". American Journal of Clinical Nutrition (American Society for Nutrition) 83 (6, supplement): 1483S–1493S. PMID 16841858. http://www.ajcn.org/cgi/content/full/83/6/S1483. 
  5. ^ Okuyama, Hirohmi; Ichikawa, Yuko; Sun, Yueji; Hamazaki, Tomohito; Lands, William E. M. (2007). "ω3 fatty acids effectively prevent coronary heart disease and other late-onset diseases: the excessive linoleic acid syndrome". World Review of Nutritional Dietetics. World Review of Nutrition and Dietetics (Karger) 96 (Prevention of Coronary Heart Disease): 83–103. doi:10.1159/000097809. ISBN 3805581793. PMID 17167282. 
  6. ^ Daley, C. A.; Abbott, A.; Doyle, P.; Nader, G.; and Larson, S. (2004). A literature review of the value-added nutrients found in grass-fed beef products. California State University, Chico (College of Agriculture). http://www.csuchico.edu/agr/grassfedbeef/health-benefits/index.html. Retrieved 2008-03-23. 
  7. ^ Simopoulos, Artemis P. (October 2002). "The importance of the ratio of omega-6/omega-3 essential fatty acids". Biomedicine & Pharmacotherapy 56 (8): 365–379. doi:10.1016/S0753-3322(02)00253-6. PMID 12442909. 
  8. ^ Simopoulos, Artemis P. (September 2003). "Importance of the ratio of omega-6/omega-3 essential fatty acids: evolutionary aspects". World Review of Nutrition and Dietetics. World Review of Nutrition and Dietetics (Karger) 92 (Omega–6/Omega–3 Essential Fatty Acid Ratio: The Scientific Evidence): 1–174. doi:10.1159/000073788. ISBN 3805576404. PMID 14579680. 
  9. ^ Calder, Philip C. (1 June 2006). "n−3 polyunsaturated fatty acids, inflammation, and inflammatory diseases". American Journal of Clinical Nutrition (American Society for Nutrition) 83 (6, supplement): 1505S–1519S. PMID 16841861. http://www.ajcn.org/cgi/content/full/83/6/S1505. 
  10. ^ Smith, William L. (January 2008). "Nutritionally essential fatty acids and biologically indispensable cyclooxygenases". Trends in Biochemical Sciences (Elsevier) 33 (1): 27–37. doi:10.1016/j.tibs.2007.09.013. PMID 18155912. 
  11. ^ Wada, M.; Delong, CJ; Hong, YH; Rieke, CJ; Song, I; Sidhu, RS; Yuan, C; Warnock, M et al. (August 3 2007). "Enzymes and receptors of prostaglandin pathways with arachidonic acid-derived versus eicosapentaenoic acid-derived substrates and products. Nutritionally essential fatty acids and biologically indispensable cyclooxygenases". J. Biol. Chem. (ASBMB) 282 (31): 22254–22266. doi:10.1074/jbc.M703169200. PMID 17519235. 
  12. ^ Cleland, Leslie G.; James, Michael J.; Proudman, Susanna M. (January 2006). "Fish oil: what the prescriber needs to know". Arthritis Research & Therapy (BioMed Central) 8 (1): 202. doi:10.1186/ar1876. PMC 1526555. PMID 16542466. http://arthritis-research.com/content/8/1/202. 
  13. ^ Mickleborough, Timothy D. (June 2005). "Dietary omega-3 polyunsaturated fatty acid supplementation and airway hyperresponsiveness in asthma". The Journal of Asthma (Informa Healthcare) 42 (5): 305–314. doi:10.1081/JAS-200062950. PMID 16036405. 
  14. ^ Broughton, K. Shane; Johnson, Cody S.; Pace, Bobin K.; Liebman, Michael; Kleppinger, Kent M. (April 1, 2005). "Reduced asthma symptoms with n−3 fatty acid ingestion are related to 5-series leukotriene production". American Journal of Clinical Nutrition (American Society for Nutrition) 65 (4): 1011–1017. PMID 9094887. http://www.ajcn.org/cgi/reprint/65/4/1011. 
  15. ^ Lee, H.J.; Rao, J.S.; Rapoport, S.I.; Bazinet, R.P. (November 2007). "Antimanic therapies target brain arachidonic acid signaling: lessons learned about the regulation of brain fatty acid metabolism". Prostaglandins, Leukotrienes and Essential Fatty Acids (Elsevier) 77 (5): 239–246. doi:10.1016/j.plefa.2007.10.018. PMID 18042366. 
  16. ^ Sonestedt, Emily; Ericson, Ulrika; Gullberg, Bo; Skog, Kerstin; Olsson, Håkan; Wirfält, Elisabet (2008). "Do both heterocyclic amines and omega-6 polyunsaturated fatty acids contribute to the incidence of breast cancer in postmenopausal women of the Malmö diet and cancer cohort?". The International Journal of Cancer (UICC International Union Against Cancer) 123 (7): 1637–1643. doi:10.1002/ijc.23394. PMID 10970215. http://www3.interscience.wiley.com/journal/120780752/abstract. Retrieved 2008-11-30. 
  17. ^ Yong Q. Chen, at al (2007). "Modulation of prostate cancer genetic risk by omega-3 and omega-6 fatty acids". The Journal of Clinical Investigation 117 (7): 1866–1875. doi:10.1172/JCI31494. PMC 1890998. PMID 17607361. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1890998. 
  18. ^ Pala, Valeria; Krogh, Vittorio; Muti, Paola; Chajès, Véronique; Riboli, Elio; Micheli, Andrea; Saadatian, Mitra; Sieri, Sabina et al. (18 July 2001). "Erythrocyte Membrane Fatty Acids and Subsequent Breast Cancer: a Prospective Italian Study". JNCL 93 (14): 1088–95. doi:10.1093/jnci/93.14.1088. PMID 11459870. http://jnci.oxfordjournals.org/cgi/content/full/93/14/1088. Retrieved 2008-11-30. 
  19. ^ Cunnane, Stephen C. (November 2003). "Problems with essential fatty acids: time for a new paradigm?". Progress in Lipid Research 42 (6): 544–568. doi:10.1016/S0163-7827(03)00038-9. PMID 14559071. 
  20. ^ Gunstone, Frank (December 2007) "Oilseed markets: Market update: Palm oil". INFORM (AOCS) 18(12): 835-836.
  21. ^ "Food sources of total omega 6 fatty acids". http://riskfactor.cancer.gov/diet/foodsources/fatty_acids/table2.html. Retrieved 2011-09-04. 

Additional sources

External links

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