Omega-3 fatty acids (also called ω-3 fatty acids or n-3 fatty acids) are polyunsaturated fatty acids (PUFAs) with a double bond (C=C) at the third carbon atom from the end of the carbon chain. The fatty acids have two ends, the carboxylic acid (-COOH) end, which is considered the beginning of the chain, thus "alpha", and the methyl (CH3) end, which is considered the "tail" of the chain, thus "omega." The nomenclature of the fatty acid is taken from the location of the first double bond, counted from the methyl end, that is, the omega (ω-) or the n- end.The three types of omega-3 fatty acids involved in human physiology are ALA (found in plant oils), EPA, and DHA (both commonly found in marine oils). Marine algae and phytoplankton are primary sources of omega-3 fatty acids. Common sources of plant oils containing the omega 3 ALA fatty acid include walnut, edible seeds, clary sage seed oil, algal oil, flaxseed oil, Sacha Inchi oil, Echium oil, and hemp oil, while sources of animal omega-3 EPA and DHA fatty acids include fish oils, egg oil, squid oils, and krill oil. Supplementation with omega-3 fatty acids does not appear to affect the risk of death, cancer or heart disease.Omega-3 fatty acids are important for normal metabolism. Mammals have a limited ability to synthesize omega-3 fats when the diet includes the shorter-chain omega-3 fatty acid ALA (α-linolenic acid, 18 carbons and 3 double bonds) to form the more important long-chain omega-3 fatty acids, EPA (eicosapentaenoic acid, 20 carbons and 5 double bonds) and then from EPA, the most crucial, DHA (docosahexaenoic acid, 22 carbons and 6 double bonds) with even greater inefficiency. The ability to make the longer-chain omega-3 fatty acids from ALA may also be impaired in aging. In foods exposed to air, unsaturated fatty acids are vulnerable to oxidation and rancidity.