好好读吧，姜还是老滴辣~~

Chemical structure of alpha-linolenic acid (ALA), an essential n−3 fatty acid, (18:3Δ9c,12c,15c, which means a chain of 18 carbons with 3 double bonds on carbons numbered 9, 12, and 15). Although chemists count from the carbonyl carbon (blue numbering), physiologists count from the n (ω) carbon (red numbering). Note that, from the n end (diagram right), the first double bond appears as the third carbon-carbon bond (line segment), hence the name "n−3". This is explained by the fact that the n end is almost never changed during physiologic transformations in the human body, as it is more energy-stable, and other carbohydrates compounds can be synthesized from the other carbonyl end, for example in glycerides, or from double bonds in the middle of the chain.