ω−3 fatty acids (commonly spelled omega-3 fatty acids) are a family of polyunsaturated fatty acids which have in common a carbon-carbon double bond in the ω−3 position.Important nutritionally essential ω−3 fatty acids are: α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). For a more complete list, see list of ω−3 fatty acids. The human body cannot synthesize ω−3 fatty acids de novo, but it can form 20- and 22-carbon unsaturated ω−3 fatty acids from the eighteen-carbon ω−3 fatty acid, α-linolenic acid. These conversions occur competitively with ω−6 fatty acids, which are essential closely related chemical analogues that are derived from linoleic acid. Both the ω−3 α-linolenic acid and ω−6 linoleic acid are essential nutrients which must be obtained from food. Synthesis of the longer ω−3 fatty acids from linolenic acid within the body is competitively slowed by the ω−6 analogues. Thus accumulation of long-chain ω−3 fatty acids in tissues is more effective when they are obtained directly from food or when competing amounts of ω−6 analogs do not greatly exceed the amounts of ω−3.