Omega-6 fatty acid

Description

Omega-6 fatty acids are one of two groups of essential fatty acids (EFAs) that are required in human nutrition. (The other is the omega-3 fatty acid group.) Omega-6 fatty acids include linoleic acid and its derivatives. Essential means that these fatty acids must be consumed in the diet because humans cannot manufacture them from other dietary fats or nutrients, nor can they be stored in the body. They must be consumed daily to meet the body's requirements. They are macronutrients, required in amounts of grams per day (compared to micronutrients such as vitamins, which are required in milligrams per day). EFAs provide energy and are also components of nerve cells, cellular membranes, and are converted to hormone-like substances known as prostaglandins.

In the body, prostaglandins and EFAs are necessary for normal physiology, including:

• producing steroids and synthesizing hormones
• regulating pressure in the eye, joints, and blood vessels
• mediating immune response
• regulating bodily secretions and their viscosity
• dilating or constricting blood vessels
• regulating collateral circulation
• directing endocrine hormones to their target cells
• regulating smooth muscles and autonomic reflexes
• being primary constituents of cell membranes
• regulating the rate of cell division
• maintaining the fluidity and rigidity of cellular membranes
• regulating the inflow and out-flux of substances into and out of cells
• transporting oxygen from red blood cells to the tissues
• maintaining proper kidney function and fluid balance
• keeping saturated fats mobile in the blood stream
• preventing blood cells from clumping together (conglomeration, which is the cause of atherosclerotic plaque, and blood clots, can be a cause of stroke)
• mediating the release of inflammatory substances from cells that may trigger allergic conditions
• regulating nerve transmission and communication
• being the primary energy source for the heart muscle

EFAs are therapeutic, and clinical trials have shown that EFAs protect against such conditions as heart disease; cancer; autoimmune diseases, including rheumatoid arthritis and multiple sclerosis; skin diseases, including acne, atopic eczema, and psoriasis; and may protect against stroke. The prevalence of heart disease in populations has been shown to be inversely proportional to the relative concentration of linoleic acid in the diet.

Both linoleic acid and its derivatives are obtained from plant and animal sources. Plant sources include unprocessed, unheated vegetable oils such as corn, sunflower seed, safflower, soy, sesame, and cottonseed oils. They are also found in plant materials such as evening primrose, black currant seeds, and gooseberry oils as well as in raw nuts and seeds, legumes, and leafy greens. Animal sources of omega-6 fatty acids (although in smaller amounts than in plants) are lean meats, organ meats, and breast milk.

Linoleic acid is an 18-carbon long polyunsaturated fatty acid containing two double bonds. Its first double bond occurs at the sixth carbon from the omega end, classifying it as an omega-6 oil. As linoleic acid is absorbed and metabolized in the human body, it is converted into a derivative fatty acid, gamma linoleic acid (GLA), which is converted into di-homo-gamma linoleic acid (DGLA) and arachidonic acid (AA). The DGLA and AA are then converted into two types of prostaglandins by adding two carbon molecules and removing hydrogen molecules. There are three families of prostaglandins, PGE1, PGE2, and PGE3. DGLA is converted to PGE1, while AA is converted into PGE2. PGE3 is made by the conversion of omega-3 fatty acids. Both PGE1 and PGE3, anti-inflammatory agents, protect against coronary disease by keeping blood platelets slippery and flowing, thus preventing blood clotting. PGE2 is inflammatory and increases platelet stickiness and blood clotting. All three forms must be present to ensure a functioning clotting system. There must be enough PGE2 to ensure healthy clotting, but enough PGE1 and PGE3 to protect against too much clotting, which can lead to hardening of the arteries, heart attack, and stroke. Likewise, PGE1 appears to act as a diuretic, while PGE2 aids in the retention of water and salts in the kidneys. PGE2 also is required for healthy brain and synapse functioning. The three types of prostaglandins serve as a system of checks and balances within the body.

However, if AA and its derivative, PGE2, are over-produced or imbalanced with PGE1 and PGE3, they can cause illness or disease. The over-consumption of land-based meats and the under-consumption of cold-water fish and unprocessed oils can lead to an over-production of inflammation-producing PGE2 and an under-production of anti-inflammatory agents PGE1 and PGE3. The goal is to consume omega-6 fatty acids in a balance with omega-3 fatty acids. A suggested optimal ratio of omega-6 fatty acids to omega-3 fatty acids that should be consumed is four parts omega-6 to one part omega-3. Ratios of healthy populations range from 2.5:1 in Inuit diets to 6:1 in other traditional diets.

Daily consumption of omega-6 fatty acids by many people may be excessive, due to the presence of omega-6 fatty acids in common cooking vegetable oils and processed foods. The ratio of omega-6 to omega-3 fatty acid consumption can often reach 20:1. To achieve a more desirable ratio, an approach is to eliminate sources of omega-6 fatty acids, especially those hidden in processed foods and to increase the amount of omega-3 fatty acids consumed through fish oil or flaxseed supplements. In addition, to convert the omega-6 fatty acids present in oils (such as corn, safflower, or soybean) to GLA requires that the oils be unprocessed and unheated and in the natural form (cis form). In oils that have undergone processing (heating and/or hydrogenation) to prolong shelf life (e.g., many store bought oils) or to form a solid at room temperature (e.g., shortening and margarine), the fatty acid structure has been changed to the trans form, and the conversion process of omega-6 fatty acids to GLA may be inhibited.

General Use

Most people receive sufficient amounts of omega-6 fatty acids in their diet. Deficiencies are considered to be rare and limited to people with severe malabsorption, short bowel syndrome, or for people on an extremely low fat diet. For those who are unable to convert LA to GLA, dietary supplements containing GLA can be taken to increase the production of prostaglandins. Evening primrose, black currant, and borage oil all contain GLA. For individuals with diabetes, GLA supplementation can improve nerve function and help prevent diabetic nerve disease. Long term exclusive or excessive use of flaxseed oil, which contains large amounts of omega-3 fatty acids, can result in omega-6 fatty acid deficiency and require the addition of oils containing omega-6 fatty acids to the diet.

Preparations

Omega-6 fatty acids may be consumed either as linoleic acid in oils that contain high levels of linoleic acid, or in the converted form, GLA, in dietary supplements. Oils high in linoleic acid include soybean, peanut, corn, sunflower seed, cottonseed, soy, sesame, and safflower. There is no official recommended daily dose for omega-6 fatty acids. Taken in oil form, approximately 1–2% of the daily calorie intake, or 1 tsp (3–6 g per day), should prevent signs of deficiency in most adults. An optimal dose is around 3–6% of daily calorie intake, or 1 tbsp (9–18 g daily). Some researchers suggest that 10% of daily calorie intake as linoleic acid is required to maintain optimal function.

For GLA supplementation, primrose oil, borage oil, and black currant seed oil are available in capsule form.

Precautions

Stress, alcohol consumption, and prescription medicines can interfere with the conversion of linoleic acid to its derivatives. Therefore, those with such conditions may benefit from the use of GLA supplementation to improve the production of prostaglandins. In addition, to improve prostaglandin production, the use of unprocessed, unheated omega-6 oils in the cis form is recommended.

Side Effects

Overconsumption of omega-6 oils in relation to consumption of omega-3 oils may lead to an overproduction of inflammation-producing prostagladins (PGE2s) and a scarcity of anti-inflammatory prostaglandins (PGE1s and PGE2s), which may lead to a variety of health problems.

Linoleic acid appears to have at least one negative effect on the human body. It appears to increase a person's risk of developing age-related macular degeneration (ARMD), a disease of the eye that leads to a progressive loss of vision and eventual blindness.

Interactions

Nutrients essential for the use of omega-6 fatty acids in the body include magnesium, selenium, zinc, and vitamins A, carotene, B₃, B₆, C, and E.

Resources

Books

Gittleman, Ann Louise. Eat Fat, Lose Weight. Keats Publishing, 1999.

Murray, Michael T., and Jade Beutler. Understanding Fats and Oils: Your Guide to Healing with Essential Fatty Acids. Apple Publishing Co. Ltd., 2000.

Periodicals

"Harvard Study Outlines Role of Fats in Blinding Eye Disease." Angiogenesis Weekly (October 12, 2001).

Majumder, Barun, Klaus J. Wahle, Susan Moir, and Steven D. Heys. "Conjugated Linoleic Acid Reduces Breast Tumor Growth Both by P53-Dependent and P53-Independent Pathways." Journal of Nutrition 131 (November 2001): 3140S.

[Article by: Judith Sims; Rebecca J. Frey, PhD]

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

Types of fats in food
Unsaturated fat Monounsaturated fat Polyunsaturated fat Trans fat Omega fatty acids: ω−3 ω−6 ω−9 Saturated fat Interesterified fat
See also
Fatty acid Essential fatty acid

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 from the end of the fatty acid.

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, 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 1 Key n−6 fatty acids 2 Negative health effects 3 Dietary linoleic acid requirement 4 Dietary sources 5 List of n−6 fatty acids 6 See also 7 References 7.1 Additional sources 8 Further reading

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 n−6 fatty acids, relative to n−3 fatty acids, may increase the probability of a number of diseases and depression.^[1][2][3]

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.^[4][5]

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.^[6]

Chronic excessive production of n−6 eicosanoids is associated with heart attacks, thrombotic stroke, arrhythmia, arthritis, osteoporosis, inflammation, mood disorders, obesity, and cancer.^[7] Many of the medications used to treat and manage these conditions work by blocking the effects of the potent n−6 fat, arachidonic acid.^[8] 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.^[9] 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.^[10] (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.^[11][12] Many of the anti-mania medications used to treat bipolar disorder work by targeting the arachidonic acid cascade in the brain.^[13]

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^[14]. Similar effect was observed on prostate cancer^[15]. 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^[16].

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.^[17] 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 LA 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 ^[18]

• poultry
• avocado
• eggs
• cereals
• whole-grain breads
• baked goods
• nuts
• most vegetable oils
• evening primrose oil
• borage oil
• blackcurrant seed oil
• flax/linseed oil
• hemp oil
• soybean oil
• cottonseed oil
• sunflower seed oil
• corn oil
• safflower oil
• pumpkin seeds
• acai berry

List of n−6 fatty acids

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

See also

• Essential fatty acid interactions
• Essential nutrients
• n−3 fatty acid
• n−9 fatty acid

References

1. ^ 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. 
2. ^ Hibbeln, Joseph R. (01 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. 
3. ^ 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 (Karger) 96 (Prevention of Coronary Heart Disease): 83–103. doi:10.1159/000097809. ISBN 3805581793. PMID 17167282. 
4. ^ 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. 
5. ^ 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. 
6. ^ 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 (Karger) 92 (Omega-6/Omega-3 Essential Fatty Acid Ratio: The Scientific Evidence): 1–174. doi:10.1159/000073788. ISBN 3805576404. PMID 14579680. 
7. ^ Calder, Philip C. (01 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. 
8. ^ 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. 
9. ^ Wada, M. (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. 
10. ^ 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. PMID 16542466. http://arthritis-research.com/content/8/1/202. 
11. ^ 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. 
12. ^ Broughton, K. Shane; Johnson, Cody S.; Pace, Bobin K.; Liebman, Michael; Kleppinger, Kent M. (01 April 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. 
13. ^ 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. 
14. ^ Emily Sonestedt, Ulrika Ericson, Bo Gullberg, Kerstin Skog, Håkan Olsson, Elisabet Wirfält (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. 
15. ^ 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. doi:10.1172/JCI31494. PMID 1890998. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1890998. Retrieved 2008-11-30. 
16. ^ Valeria Pala, Vittorio Krogh, Paola Muti, Véronique Chajès, Elio Riboli, Andrea Micheli, Mitra Saadatian, Sabina Sieri, Franco Berrino (18 Jul 2001). "Erythrocyte Membrane Fatty Acids and Subsequent Breast Cancer: a Prospective Italian Study". JNCL 93 (14): 1088. doi:10.1093/jnci/93.14.1088. PMID 11459870. http://jnci.oxfordjournals.org/cgi/content/full/93/14/1088. Retrieved 2008-11-30. 
17. ^ 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. 
18. ^ "Omega-6 fatty acids". WholeHealthMD. http://209.196.51.230/ME2/dirmod.asp?sid=17E09E7CFFF640448FFB0B4FC1B7FEF0&nm=Reference+Library&type=AWHN_Supplements&mod=Supplements&mid=&id=BD5CF1DF38044FBFB58C5BFB72B262D1&tier=2. Retrieved 2008-03-23. 

Additional sources

• Tokar, Steve (2005-09-02). "Omega-6 fatty acids cause prostate tumor cell growth in culture". Medical News Today (MediLexicon International). http://www.medicalnewstoday.com/articles/29976.php. Retrieved 2008-03-23. 
• "Brain fatty acid levels linked to depression". News-Medical.Net (AZoNetwork). 2005-05-25. http://www.news-medical.net/?id=10398. Retrieved 2008-03-23. 
• Tribole, E.F. (2006-03-27). "Excess Omega-6 Fats Thwart Health Benefits from Omega-3 Fats". British Medical Journal Rapid Responses to Hooper, et al., 2006. http://www.bmj.com/cgi/eletters/332/7544/752#130637. Retrieved 2008-03-23. 

Further reading

• Erasmus, Udo. Fats That Heal, Fats That Kill. 3rd ed. Burnaby (BC): Alive Books; 1993.

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