Omega-6 fatty acid
- For the 1920s French automobile - see Oméga-Six
|Types of fats in food|
Omega-6 fatty acids (also referred to as ω-6 fatty acids or n-6 fatty acids) are a family of pro-inflammatory 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. Some medical research suggests that eating a lot of certain omega−6 fatty acids may lead to some diseases.
The biological effects of the omega-6 fatty acids are largely produced during physical activity and inflammation by their conversion to omega-6 eicosanoids that bind to diverse receptors found in every tissue of the body. The conversion of cell membrane arachidonic acid (20:4n-6) to omega-6 prostaglandin and omega-6 leukotriene eicosanoids during inflammation provides many targets for pharmaceutical drug development to diminish the inflammation-promoting actions of omega-6 fatty acids in atherosclerosis, 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).
Linoleic acid (18:2, n−6), the shortest-chained omega−6 fatty acid, is one of many essential fatty acids and 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 to omega-3.
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. However, scientific research indicates that air pollution, smoking, second-hand smoke, and other exogenous toxins cause the initial inflammation in the cells which leads to the overexpression of the COX-2 enzyme and subsequently to the overproduction 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 prostaglandins during the resolution phase of inflammation, after the cell damage has been repaired.
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. 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, 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). A ratio of 2–3/1 omega 6 to omega 3 helped reduce inflammation in patients with rheumatoid arthritis. A ratio of 5/1 had a beneficial effect on patients with asthma but a 10/1 ratio had a negative effect. A ratio of 2.5/1 reduced rectal cell proliferation in patients with colorectal cancer, whereas a ratio of 4/1 had no effect.
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.
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. 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. 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. (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. Many of the anti-mania medications used to treat bipolar disorder work by targeting the arachidonic acid cascade in the brain.
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. Similar effect was observed on prostate cancer, but the study was performed on mice. 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".
Studies have indicated that omega-6 should be consumed in a 1:1 ratio to omega-3, though it has been observed that the diet of many individuals today is at a ratio of about 16:1. 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. Why has the ratio changed so dramatically? In a study performed by Pannampalam, 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. In todays 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. Grain-feeding cattle is a way to increase their weight, to 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 immensely.
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|
It is interesting to note that 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. 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.
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 omega-6 linoleic acid and 4 million metric tons of omega-3 alpha-linolenic acid.
Dietary sources of omega−6 fatty acids include:
- durum wheat
- whole-grain breads
- most vegetable oils
- evening primrose oil
- borage oil
- blackcurrant seed oil
- flax/linseed oil
- rapeseed or canola oil
- hemp oil
- soybean oil
- cottonseed oil
- sunflower seed oil
- corn oil
- safflower oil
- pumpkin seeds
- acai berry
- pine nuts
- spirulina
- Essential fatty acid interactions
- Essential nutrients
- Linolenic acid
- Omega-3 fatty acid
- Omega-7 fatty acid
- Omega-9 fatty acid
- Wheat germ oil
- Lipid peroxidation
- Cattle feeding
Notes and references
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