Editing Fatty acid (section)

===Carbon atom numbering===
{{See also|Essential fatty acid#Nomenclature and terminology}}
[[File:Fatty acid carbon numbering.svg|thumb|upright=2|Numbering of carbon atoms. The systematic (IUPAC) C-''x'' numbers are in blue. The omega-minus "ω−''x''" labels are in red. The Greek letter labels are in green.{{efn|name=note.omega}} Note that [[#Unsaturated fatty acids|unsaturated fatty acids]] with a ''cis'' configuration are actually "kinked" rather than straight as shown here.]]

Most naturally occurring fatty acids have an [[branched chain fatty acids|unbranched chain]] of carbon atoms, with a [[carboxyl group]] (–COOH) at one end, and a [[methyl group]] (–CH3) at the other end.

The position of each carbon atom in the backbone of a fatty acid is usually indicated by counting from 1 at the −COOH end. Carbon number ''x'' is often abbreviated C-''x'' (or sometimes C''x''), with ''x'' = 1, 2, 3, etc. This is the numbering scheme recommended by the [[IUPAC]].

Another convention uses letters of the [[Greek alphabet]] in sequence, starting with the first carbon ''after'' the carboxyl group. Thus carbon α ([[alpha]]) is C-2, carbon β ([[beta]]) is C-3, and so forth.

Although fatty acids can be of diverse lengths, in this second convention the last carbon in the chain is always labelled as ω ([[omega]]), which is the last letter in the Greek alphabet. A third numbering convention counts the carbons from that end, using the labels "ω", "ω−1", "ω−2". Alternatively, the label "ω−''x''" is written "n−''x''", where the "n" is meant to represent the number of carbons in the chain.{{efn|name=note.omega|A common mistake is to say that the last carbon is "ω−1".<br />Another common mistake is to say that the position of a bond in omega-notation is the number of the carbon closest to the END.<br />For double bonds, these two mistakes happen to compensate each other; so that a "ω−3" fatty acid indeed has the double bond between the 3rd and 4th carbons from the end, counting the methyl as 1.<br />However, for substitutions and other purposes, they don't: a hydroxyl "at ω−3" is on carbon 15 (4th from the end), not 16. See for example this article. {{doi|10.1016/0005-2760(75)90089-2}}<br />Note also that the "−" in the omega-notation is a minus sign, and "ω−3" should in principle be read "omega minus three". However, it is very common (especially in non-scientific literature) to write it "ω-3" (with a hyphen/dash) and read it as "omega-three". See for example Karen Dooley (2008), [https://podcasts.ufhealth.org/omega-three-fatty-acids-and-diabetes/ Omega-three fatty acids and diabetes].}}

In either numbering scheme, the position of a [[double bond]] in a fatty acid chain is always specified by giving the label of the carbon closest to the '''carboxyl''' end.{{efn|name=note.omega}} Thus, in an 18 carbon fatty acid, a double bond between C-12 (or ω−6) and C-13 (or ω−5) is said to be "at" position C-12 or ω−6. The IUPAC naming of the acid, such as "octadec-12-enoic acid" (or the more pronounceable variant "12-octadecanoic acid") is always based on the "C" numbering.

The notation Δ<sup>''x'',''y'',...</sup> is traditionally used to specify a fatty acid with double bonds at positions ''x'',''y'',.... (The capital Greek letter "Δ" ([[Delta (letter)|delta]]) corresponds to [[Latin alphabet|Roman]] "D", for '''D'''ouble bond). Thus, for example, the 20-carbon [[arachidonic acid]] is Δ<sup>5,8,11,14</sup>, meaning that it has double bonds between carbons 5 and 6, 8 and 9, 11 and 12, and 14 and 15.

In the context of human diet and fat metabolism, unsaturated fatty acids are often classified by the position of the double bond closest between to the ω carbon (only), even in the case of [[polyunsaturated fatty acid|multiple double bonds]] such as the [[essential fatty acid]]s. Thus [[linoleic acid]] (18 carbons, Δ<sup>9,12</sup>), [[gamma-Linolenic acid|γ-linole'''n'''ic acid]] (18-carbon, Δ<sup>6,9,12</sup>), and arachidonic acid (20-carbon, Δ<sup>5,8,11,14</sup>) are all classified as "ω−6" fatty acids; meaning that their [[condensed structural formula|formula]] ends with –CH=CH–{{chem|CH|2}}–{{chem|CH|2}}–{{chem|CH|2}}–{{chem|CH|2}}–{{chem|CH|3}}.

Fatty acids with an [[odd number]] of carbon atoms are called [[odd-chain fatty acid]]s, whereas the rest are even-chain fatty acids. The difference is [[gluconeogenesis#Precursors|relevant to gluconeogenesis]].