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In nutrition, polyunsaturated fat is an abbreviation of polyunsaturated fatty acid. That is a fatty acid in which more than one double bond exists within the representative molecule. That is, the molecule has two or more points on its structure capable of supporting hydrogen atoms not currently part of the structure. Polyunsaturated fatty acids can assume a cis or trans conformation depending on the geometry of the double bond.
The lack of the extra hydrogen atoms on the molecule's surface typically reduces the strength of the compound's intermolecular forces, thus causing the melting point of the compound to be significantly lower. This property can be observed by comparing predominately unsaturated vegetable oils, which remain liquid even at relatively low temperatures, to much more saturated fats such as butter or lard which are mainly solid at room temperature. Trans fats are more similar to saturated fat than are cis fats in many respects, including the fact that they solidify at a lower temperature.
A fatty acid has a carboxylic acid at one end and a methyl group at the other end. Carbon atoms in a fatty acid are identified by Greek letters on the basis of their distance from the carboxylic acid. The carbon atom closest to the carboxylic acid is the alpha carbon, the next adjacent carbon is the beta carbon, etc. In a long-chain fatty acid the carbon atom in the methyl group is called the omega carbon because omega is the last letter of the Greek alphabet.
Omega-3 fatty acids have a double bond three carbons away from the methyl carbon, whereas omega-6 fatty acids have a double bond six carbons away from the methyl carbon. The illustration below shows the omega-6 fatty acid, linoleic acid.
Polyunsaturated fat, along with monounsaturated fat are "healthy fats," the amount of which in one's daily diet should be near 25 g (in a 2000 calorie-per-day diet). Polyunsaturated fat can be found mostly in grain products, fish and sea food (herring, salmon, mackerel, halibut), soybeans, and fish oil. Foods like mayonnaise and soft margarine may also be good sources, but nutritional facts can vary by style and brand. Omega-3 fatty acids in fish oil, fish and seafood lower the total amount of fat in the blood, which can lower blood pressure and decrease the risk of cardiovascular diseases. Omega-6 fatty acids in sunflower oil and safflower oil also reduce the risk of cardiovascular disease, but can contribute to allergies and inflammation. Polyunsaturated fat may lower LDL and HDL cholesterol. 
Foods containing polyunsaturated fat
Polyunsaturated fat may lower LDL and increase levels of HDL cholesterol
Relation to cancer
Some studies have shown that consuming high amounts of polyunsaturated fat may increase the risk of an individual developing cancer. This is thought to be because polyunsaturated fat is prone to oxidation, which leads to the generation of free radicals. Studies on animals have shown a link between polyunsaturated fat and the incidence of tumours. In some of these studies the incidence of tumours increased with increasing intake of polyunsaturated fat, up to about 5% of total energy, near to the middle of the current dietary intake in humans. However, studies in humans have found little evidence of an association between polyunsaturated fat and the risk on cancer. It is advised that the level of polyunsaturated fats in the diet be regulated, although the effect on health may be more beneficial than harmful in terms of its effect on reducing cholesterol levels.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Polyunsaturated_fat". A list of authors is available in Wikipedia.