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. Biochemistry. Linoleic acid (18:2, "n"−6), the shortest-chain of the common omega-6 fatty acids in the human diet, is categorized as an essential fatty acid because the human body cannot synthesize it. Omega-6 fatty acids are precursors to endocannabinoids, lipoxins, and specific eicosanoids. Mammalian cells lack the enzyme omega-3 desaturase and  therefore cannot convert omega-6 fatty acids to omega-3 fatty acids, which is why certain omega-3 fatty acids are also essential. 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, asthma, arthritis, vascular disease, thrombosis, immune-inflammatory processes (e.g. in immune-mediated inflammatory diseases), 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). List of  omega-6 fatty acids. The melting point of the fatty acids increases as the number of carbons in the chain increases. Nutrition. 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. Ratio of omega-6 to omega-3. 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. 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. Both omega-3 and omega-6 are metabolized by the same enzymes, meaning an imbalanced ratio can affect how the other is metabolized. 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 including coronary heart disease (CHD), cancer, stroke, heart attacks, and lower rates of premature death. 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 and mainly from vegetable oils. 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, although some sources suggest ratios as low as 1. A ratio of 2–3 omega-6 to omega-3 helped reduce inflammation in patients with rheumatoid arthritis. 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. 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. In a study performed by Ponnampalam, 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 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. Feeding cattle grain is a way to increase their weight and prepare them for slaughter much more quickly. This modern way of feeding animals may be one of many indications as to why the omega-6:omega-3 ratio has increased. 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 cells. This 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. Eventually anti-inflammatory molecules (e.g. lipoxins & prostacyclin) are produced during the resolution phase of inflammation, after the cell damage has been repaired."" Dietary sources. Dietary sources of omega-6 fatty acids include: Vegetable oils. 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.