Is eicosapentaenoic acid (EPA) omega 3 or that's just a acid that comes with a omega 3? Same question for docosahexaenoic acid (DHA).

Just want to know are both of those two acid omega 3 despite being different acids.

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    $\begingroup$ No, $\omega$-3 is an umbrella term, much like "acid". How can they both be called acids, despite being different molecules? Well, just like that. $\endgroup$ – Ivan Neretin Jul 8 '19 at 11:06

There exists a range of ω-3 acids.

Fatty acids are made of a chain of carbon atoms (generally linear), with a carboxylic acid at one end. Usually the carbon atoms are linked with a single-bond, but in unsaturated acids one or more of these are double-bonds. In order to describe the positioning of these double bonds, the carbon-atoms are numbered, starting either from the carboxylic acid end, also called the α-end, or from the methyl-end (the other end), which is called the ω-end. Therefore, an ω-3 acid is a fatty acid with a double-bond from carbon number 3 to 4, counted from the methyl/ω-end.

Fatty acids can have several double-bonds in its chain, making it a polyunsaturated acid. An example is Eicosapentaenoic acid, which has 5 double bonds. Edit: As pointed out in comments, it is the position of the first double-bond that determines if the acid is called an ω-3 acid. Likewise: if the first double bond is from carbon number 6 to 7, the acid is an ω-6 acid.

Wikipedia has some very informative articles about both ω-3 acids and fatty acids in general.

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    $\begingroup$ Actually, the ω designation refers to the position of the first double bond from the methyl end. Thus, since the first double bond from the methyl end in EPA is at carbon 3, it is an ω3 fatty acid, not an ω3,6,9,12,15 fatty acid (the other double bonds aren't the first from the methyl end, and are thus not used in determining the ω notation). $\endgroup$ – theorist Jul 9 '19 at 1:29
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    $\begingroup$ I'll add the reason the ω notation is used is that organisms typically elongate fatty acids from the carboxylic acid end, so the ω-number of a fatty acid stays constant through this process. Thus if you eat an 18:3ω3 fatty acid, your body can convert it to EPA (20:5ω3) or DHA (22:6ω3) by elongating it and adding double bonds. But your body can't make DHA from, say, 18:1ω9 or 18:2ω6. $\endgroup$ – theorist Jul 9 '19 at 1:34
  • $\begingroup$ Thank you theorist, you are off course quite right when pointing out that the ω-nomeclature only refers to the first double bond. I stand corrected :-) $\endgroup$ – FrankS Jul 9 '19 at 5:51
  • $\begingroup$ @FrankS So is Eicosapentaenoic acid a ω-3 instead of ω-3,6,9,12,15 acid? (If yes, could you edit?) $\endgroup$ – Volker Siegel Jul 9 '19 at 12:55
  • $\begingroup$ @Volker: That is correct. I have added an edit to the answer. $\endgroup$ – FrankS Jul 11 '19 at 4:27

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