Omega-6 fatty acid

Omega-6 fatty acids (also referred to as ω-6 fatty acids or n-6 fatty acids) are a family of polyunsaturated 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]

For an explanation of n and numerical nomenclature (such as n-6 or 18:2), see Fatty acid § Nomenclature. For the 1920s French automobile, see Oméga-Six.
Not to be confused with trans fatty acids or omega-3 fatty acid.
Types of fats in food
See also
The chemical structure of linoleic acid, a common omega-6 fatty acid found in many nuts, seeds and vegetable oils.

Biochemistry

Linoleic acid (18:2, n−6), the shortest-chained omega-6 fatty acid, is categorized as an essential fatty acid because the human body cannot synthesize it. Mammalian cells lack the enzyme omega-3 desaturase and therefore cannot convert omega-6 fatty acids to omega-3 fatty acids. Closely related omega-3 and omega-6 fatty acids act as competing substrates for the same enzymes.[2] This outlines the importance of the proportion of omega-3 to omega-6 fatty acids in a diet.[2]

Omega-6 fatty acids are precursors to endocannabinoids, lipoxins, and specific eicosanoids.

Medical research on humans found a correlation (though correlation does not imply causation) between the high intake of omega-6 fatty acids from vegetable oils and disease in humans. However, biochemistry research has concluded that air pollution, heavy metals, smoking, passive smoking, lipopolysaccharides, lipid peroxidation products (found mainly in vegetable oils, roasted/rancid nuts and roasted/rancid oily seeds) and other exogenous toxins initiate the inflammatory response in the cells which leads to the expression of the COX-2 enzyme and subsequently to the temporary production of inflammatory promoting prostaglandins from arachidonic acid for the purpose of alerting the immune system of the cell damage and eventually to the production of anti-inflammatory molecules (e.g. lipoxins & prostacyclin) during the resolution phase of inflammation, after the cell damage has been repaired.[3][4][5][6][7][8][9][10][11][12][13][14]

Pharmacology

The conversion of cell membrane arachidonic acid (20:4n-6) to omega-6 prostaglandin and omega-6 leukotriene eicosanoids during the inflammatory cascade provides many targets for pharmaceutical drugs to impede the inflammatory process in atherosclerosis,[15] 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).

Health effects

Some medical research suggests that excessive levels of omega-6 fatty acids from seed oils relative to certain omega-3 fatty acids may increase the probability of a number of diseases.[16][17][18] However, consumption of non-rancid nuts, which are high in omega 6, is associated with a lower risk for some diseases, such as cardiovascular diseases[19] including coronary heart disease (CHD), cancer, stroke, heart attacks, and lower rates of premature death.[19][20][21][22]

Modern Western diets typically have ratios of omega-6 to omega-3 in excess of 10, some as high as 30; the average ratio of omega-6 to omega-3 in the Western diet is 15–16.7.[15] 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,[15] although some sources suggest ratios as low as 1.[23] A ratio of 2–3 omega-6 to omega-3 helped reduce inflammation in patients with rheumatoid arthritis.[15] A ratio of 5-to-1 had a beneficial effect on patients with asthma but a ratio of 10-to-1 had a negative effect.[15] A ratio of 2.5-to-1 reduced rectal cell proliferation in patients with colorectal cancer, whereas a ratio of 4-to-1 had no effect.[15]

Excess omega-6 fatty acids from vegetable oils 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.[24]

Chronic excessive production of omega-6 eicosanoids is correlated with arthritis, inflammation, and cancer. Many of the medications used to treat and manage these conditions work by blocking the effects of the COX-2 enzyme.[25] Many steps in formation and action of omega-6 prostaglandins 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.[26] 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.[27] (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.[28][29] Many of the anti-mania medications used to treat bipolar disorder work by targeting the arachidonic acid cascade in the brain.[30]

A high consumption of oxidized polyunsaturated fatty acids (PUFAs), which are found in most types of vegetable oil, may increase the likelihood that postmenopausal women will develop breast cancer.[31] Similar effect was observed on prostate cancer, but the study was performed on mice.[32] 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".[33]

Omega-6 consumption

Industry-sponsored studies have suggested that omega-6 fatty acids should be consumed in a 1:1 ratio to omega-3,[34] though it has been observed that the diet of many individuals today is at a ratio of about 16:1, mainly from vegetable oils.[34] Omega-6 and omega-3 are essential fatty acids that are metabolized by some of the same enzymes, and therefore an imbalanced ratio can affect how the other is metabolized.[35] In a study performed by Ponnampalam,[36] it was noticed that feeding systems had a great effect on nutrient content on the meat sold to consumers. Cynthia Doyle conducted an experiment to observe the fatty acid content of beef raised through grass feeding versus grain feeding; she concluded that grass fed animals contain an overall omega-6:omega-3 ratio that is preferred by nutritionists.[35] In today's modern agriculture, the main focus is on production quantity, which has decreased the omega-3 content, and increased the omega-6 content, due to simple changes such as grain-feeding cattle.[15] In grain-feeding cattle, this is a way to increase their weight and prepare them for slaughter much quicker compared to grass-feeding. This modern way of feeding animals may be one of many indications as to why the omega-6:omega-3 ratio has increased.

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, ARA) 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
Osbond 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

The melting point of the fatty acids increase as the number of carbons in the chain increases.

Dietary linoleic acid requirement

Adding more controversy to the omega-6 fat issue is that the dietary requirement for linoleic acid has been questioned, because of a significant methodology error proposed by University of Toronto scientist Stephen Cunnane.[37] Cunnane proposed 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
The evening primrose flower (O. biennis) produces an oil containing a high content of γ-linolenic acid, a type of omega-6 fatty acid.

Vegetable oils are a major source of omega-6 linoleic acid. Worldwide, more than 100 million metric tons of vegetable oils are extracted annually from palm fruits, soybean seeds, rape seeds, and sunflower seeds, providing more than 32 million metric tons of omega-6 linoleic acid and 4 million metric tons of omega-3 alpha-linolenic acid.[38][39]

Dietary sources of omega-6 fatty acids include:[40]

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See also

Notes and references
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  2. Bibus, Doug; Lands, Bill (April 18, 2015). "Balancing proportions of competing omega-3 and omega-6 highly unsaturated fatty acids (HUFA) in tissue lipids". Prostaglandins Leukot. Essent. Fatty Acids. 99: 19–23. doi:10.1016/j.plefa.2015.04.005. PMID 26002802.
  3. Ricciotti, Emanuela; FitzGerald, Garret A. (2011). "Prostaglandins and inflammation". Arteriosclerosis, Thrombosis, and Vascular Biology. 31 (5): 986–1000. doi:10.1161/ATVBAHA.110.207449. PMC 3081099. PMID 21508345.
  4. Zhao, Yutong; Usatyuk, Peter V.; Gorshkova, Irina A.; He, Donghong; Wang, Ting; Moreno-Vinasco, Liliana; Geyh, Alison S.; Breysse, Patrick N.; et al. (2009). "Regulation of COX-2 Expression and IL-6 Release by Particulate Matter in Airway Epithelial Cells". American Journal of Respiratory Cell and Molecular Biology. 40 (1): 19–30. doi:10.1165/rcmb.2008-0105OC. PMC 5459547. PMID 18617679.
  5. Calderón-Garcidueñas, Lilian; Reed, William; Maronpot, Robert; Henriquez-Roldán, Carlos; Delgado-Chavez, Ricardo; Carlos Henriquez-Roldán, Ana; Dragustinovis, Irma; Franco-Lira, Maricela; et al. (2004). "Brain Inflammation and Alzheimer's-Like Pathology in Individuals Exposed to Severe Air Pollution". Toxicologic Pathology. 32 (6): 650–58. doi:10.1080/01926230490520232. PMID 15513908.
  6. Moraitis, Dimitrios; Du, Baoheng; De Lorenzo, Mariana S.; Boyle, Jay O.; Weksler, Babette B.; Cohen, Erik G.; Carew, John F.; Altorki, Nasser K.; et al. (2005). "Levels of Cyclooxygenase-2 Are Increased in the Oral Mucosa of Smokers: Evidence for the Role of Epidermal Growth Factor Receptor and Its Ligands". Cancer Research. 65 (2): 664–70. PMID 15695412.
  7. Yang, Chuen-Mao; Lee, I-Ta; Lin, Chih-Chung; Yang, Ya-Lin; Luo, Shue-Fen; Kou, Yu Ru; Hsiao, Li-Der (2009). "Cigarette smoke extract induces COX-2 expression via a PKCα/c-Src/EGFR, PDGFR/PI3K/Akt/NF-κB pathway and p300 in tracheal smooth muscle cells". American Journal of Physiology. Lung Cellular and Molecular Physiology. 297 (5): L892–902. doi:10.1152/ajplung.00151.2009. PMID 19717552.
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