Fatty acids are a simple lipid made up of a hydrophilic carboxylate group attached to a long hydrocarbon chain. These fatty acids are categorized as either saturated or unsaturated, depending on if all the carbons in the chain are fully bonded to hydrogen atoms (it is saturated with hydrogen) or if some carbons share bonds with each other (it is unsaturated). Polyunsaturated fatty acids, often abbreviated PUFAs, are fatty acids that have multiple (poly) double bonds in a cis configuration. These double bonds cause kinks in the normally straight hydrocarbon chain, which prevents the fatty acids from packing tightly together. The bends lowers the melting point of the fatty acid and causes it to be liquid at room temperature. Additional double bonds help to further lower the melting point.
Unlike monounsaturated and saturated fatty acids, the major polyunsaturated fatty acids cannot be synthesized by the body. They are therefore called "essential fatty acids" and must be consumed. The two major types of polyunsaturated fatty acids are linoleic acid and linolenic acid. Linoleic is an omega-6 fatty acid, meaning one of its double bonds is between the sixth and seventh carbon on the chain. It is the most abundant polyunsaturated fatty acid in human tissues and can be found in soybeans and in most vegetable oils including corn, sunflower, and safflower oil. Linolenic acid is an omega-3 fatty acid, meaning one of its double bonds is between the third and fourth carbons. This type is present in walnuts, soybeans, wheat germ, and flax, hemp, and pumpkin seeds.
There are also several types of important polyunsaturated fatty acid that can be synthesized from one of the two major fatty acids described above. Linoleic acid can be turned into arachidonic acid, another omega-6 fatty acid. This fatty acid is also found in animal products such as liver and egg yolks. Linolenic acid can be converted into eicospentaenoic acid (EPA) and docosahexaenoic acid (DHA), both omega-3 fatty acids. Both can also be found in the oils of fish, especially salmon.
Polyunsaturated fatty acids are much more chemically reactive than monounsaturated or saturated fatty acids. Because of this oils that contain high levels of polyunsaturated fatty acids, such as soybean oil and corn oil, are much more prone to oxidation and spoilage. Manufacturers have been able to overcome this problem by hydrogenating the oil. Hydrogenation stabilizes the oil, increases its shelf life, and makes it solid at room temperature. This process forcibly adds hydrogen atoms to the double bonds in the carbon chain and converts the healthy unsaturated fatty acid back to an unhealthy saturated fatty acid. It also creates trans fatty acids, which are thought to be especially unhealthy.
Polyunsaturated fatty acids have a variety of functions in the body. They have been shown to effectively lower plasma cholesterol levels and are a common component of cell membranes. They also are important for the function of brain cells, nerves, adrenal glands, and certain hormones. Arachidonic acid is the building block for hormone-like molecules called eicosanoids. These molecules include prostaglandins, thromboxanes, prostacyclins and leukotrienes. They are involved in a wide variety of bodily functions including muscle movement, blood clotting, and the immune response. EPA helps to prevent the formation of blood clots that can lead to clogged arteries and also appears to lower levels of triglycerides, a risk factor in heart disease. DHA is important for lipid formation in the brain and the retina and is also converted into prostaglandins.
http://www.medicinalfoodnews.com/vol01/issue7/sat_fat.htm
http://www.lordsday.org/Fats.htm
http://www.metametrix.com/Publications/Fatty%20Acid%20Profiling/
http://www.canola-council.org/pubs/oilprop.html | 
