Editing Fatty acid

{{short description|Carboxylic acid}}
{{Fats}}
[[Image:rasyslami.jpg|thumb|right|300px|Three-dimensional representations of several fatty acids. [[Saturated and unsaturated compounds|Saturated]] fatty acids have perfectly straight chain structure. [[Unsaturated compound|Unsaturated]] ones are typically bent, unless they have a [[#Unsaturated fatty acids|trans]] configuration.]]
In [[chemistry]], particularly in [[biochemistry]], a '''fatty acid''' is a [[carboxylic acid]] with an [[aliphatic]] chain, which is either [[saturated and unsaturated compounds#Organic chemistry|saturated or unsaturated]]. Most naturally occurring fatty acids have an [[Branched chain fatty acids|unbranched chain]] of an even number of carbon atoms, from 4 to 28.<ref name="iupac">{{cite journal |url=http://goldbook.iupac.org/F02330.html|title=IUPAC Compendium of Chemical Terminology|journal=Pure and Applied Chemistry|volume=67|issue=8–9|publisher=International Union of Pure and Applied Chemistry|year=1997 |pages=1307–1375|doi=10.1351/pac199567081307|access-date=2007-10-31|last1=Moss|first1=G. P.|last2=Smith|first2=P. A. S.|last3=Tavernier|first3=D.|s2cid=95004254|doi-access=free}}</ref> Fatty acids are a major component of the lipids (up to 70% by weight) in some species such as microalgae<ref>{{cite journal |last1=Chen |first1=Lin |title=Biodiesel production from algae oil high in free fatty acids by two-step catalytic conversion |journal=Bioresource Technology |date=2012 |volume=111  |pages=208–214 |doi=10.1016/j.biortech.2012.02.033 |pmid=22401712 |bibcode=2012BiTec.111..208C }}</ref> but in some other organisms are not found in their standalone form, but instead exist as three main classes of [[ester]]s: [[triglyceride]]s, [[phospholipid]]s, and [[cholesteryl ester]]s. In any of these forms, fatty acids are both important [[diet (nutrition)|dietary]] sources of fuel for animals and important structural components for [[cell (biology)|cells]].

==History==
The concept of fatty acid (''acide gras'') was introduced in 1813 by [[Michel Eugène Chevreul]],<ref>{{cite journal |last=Chevreul |first=M. E. |date=1813 |title=Sur plusieurs corps gras, et particulièrement sur leurs combinaisons avec les alcalis |journal=Annales de Chimie |volume=88 |pages=225–261 |publisher=H. Perronneau |location=Paris |url=http://gallica.bnf.fr/ark:/12148/bpt6k65741176/f225.item |via=Gallica}}</ref><ref>{{cite book |last=Chevreul |first=M. E. |title=Recherches chimiques sur les corps gras d'origine animale |publisher=Levrault |location=Paris |date=1823 |url=https://archive.org/details/rechercheschimi00chevgoog |via=Internet Archive}}</ref><ref>{{cite web |last=Leray |first=Claude |title=Chronological history of lipid science |website=Cyberlipid Center |date=11 November 2017 |url=http://www.cyberlipid.org/history/history1.htm |url-status=dead |archive-url=https://web.archive.org/web/20171006012012/http://www.cyberlipid.org/history/history1.htm |archive-date=2017-10-06 }}</ref> though he initially used some variant terms: ''graisse acide'' and ''acide huileux'' ("acid fat" and "oily acid").<ref>{{cite encyclopedia |editor-last=Menten |editor-first=P. |title=Dictionnaire de chimie: Une approche étymologique et historique |publisher=De Boeck |location=Bruxelles |date=2013 |isbn=978-2-8041-8175-8 |url=https://books.google.com/books?id=NKTKDgAAQBAJ}}</ref>

==Types of fatty acids==
[[Image:Isomers of oleic acid.svg|thumb|300px|right|Comparison of the [[Cis–trans isomerism|''trans'' isomer]] [[elaidic acid]] (top) and the ''cis'' isomer [[oleic acid]] (bottom)]]
Fatty acids are classified in many ways: by length, by saturation vs unsaturation, by even vs odd carbon content, and by linear vs branched.

===Length of fatty acids===
* [[Short-chain fatty acids]] (SCFAs) are fatty acids with [[aliphatic]] tails of five or fewer [[carbon]]s (e.g. [[butyric acid]]).<ref>{{cite book|title=Foodomics: Advanced Mass Spectrometry in Modern Food Science and Nutrition|isbn=978-1-118-16945-2|editor-last= Cifuentes|editor-first=Alejandro|publisher=John Wiley & Sons, 2013|chapter=Microbial Metabolites in the Human Gut|date=2013-03-18}}</ref>
* Medium-chain fatty acids (MCFAs) are fatty acids with aliphatic tails of 6 to 12<ref name=e-medicine>{{Cite journal|last=Roth|first=Karl S.|date=2013-12-19|url=http://emedicine.medscape.com/article/946755-overview |title=Medium-Chain Acyl-CoA Dehydrogenase Deficiency|website=Medscape}}</ref> carbons, which can form [[medium-chain triglyceride]]s.
* Long-chain fatty acids (LCFAs) are fatty acids with aliphatic tails of 13 to 21 carbons.<ref name=lipidworld>{{Cite journal | doi = 10.1186/1476-511X-2-10| year = 2003| last1 = Beermann | first1 = C. | journal = Lipids in Health and Disease| volume = 2| page = 10|title=Short term effects of dietary medium-chain fatty acids and ''n''−3 long-chain polyunsaturated fatty acids on the fat metabolism of healthy volunteers| last2 = Jelinek | first2 = J. | last3 = Reinecker | first3 = T. | last4 = Hauenschild | first4 = A. | last5 = Boehm | first5 = G. | last6 = Klör | first6 = H.-U. | pmc = 317357 | pmid=14622442| doi-access = free}}</ref>
* [[Very long chain fatty acids]] (VLCFAs) are fatty acids with aliphatic tails of 22 or more carbons.

===Saturated fatty acids===
{{Main|Saturated fat}}
{{Main list|List of saturated fatty acids}}
Saturated fatty acids have no C=C double bonds. They have the formula CH{{sub|3}}(CH{{sub|2}}){{sub|n}}COOH, for different ''n''. An important saturated fatty acid is [[stearic acid]] (''n''&nbsp;=&nbsp;16), which when neutralized with [[sodium hydroxide]] is the most common form of [[soap]].

[[File:Arachidic formula representation.svg|thumb|300px|[[Arachidic acid]], a saturated fatty acid]]

{| class="wikitable"
|+ Examples of saturated fatty acids
|-
! Common name || Chemical structure || ''C'':''D''{{efn|name=c:d}}
|-
| [[Caprylic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|6}}COOH || 8:0
|-
| [[Capric acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|8}}COOH || 10:0
|-
| [[Lauric acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|10}}COOH || 12:0
|-
| [[Myristic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|12}}COOH || 14:0
|-
| [[Palmitic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|14}}COOH || 16:0
|-
| [[Stearic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|16}}COOH || 18:0
|-
| [[Arachidic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|18}}COOH || 20:0
|-
| [[Behenic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|20}}COOH || 22:0
|-
| [[Lignoceric acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|22}}COOH || 24:0
|-
| [[Cerotic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|24}}COOH || 26:0
|}

===Unsaturated fatty acids===
{{main|Unsaturated fat}}
{{Main list|List of unsaturated fatty acids}}
Unsaturated fatty acids have one or more C=C [[double bond]]s. The C=C double bonds can give either [[Cis-trans isomerism|''cis'' or ''trans'']] isomers.

; ''cis'' :A ''cis'' configuration means that the two hydrogen atoms adjacent to the double bond stick out on the same side of the chain. The rigidity of the double bond freezes its conformation and, in the case of the ''cis'' isomer, causes the chain to bend and restricts the conformational freedom of the fatty acid. The more double bonds the chain has in the ''cis'' configuration, the less flexibility it has. When a chain has many ''cis'' bonds, it becomes quite curved in its most accessible conformations. For example, [[oleic acid]], with one double bond, has a "kink" in it, whereas [[linoleic acid]], with two double bonds, has a more pronounced bend. [[α-Linolenic acid]], with three double bonds, favors a hooked shape. The effect of this is that, in restricted environments, such as when fatty acids are part of a phospholipid in a lipid bilayer or triglycerides in lipid droplets, cis bonds limit the ability of fatty acids to be closely packed, and therefore can affect the melting temperature of the membrane or of the fat. Cis unsaturated fatty acids, however, increase cellular membrane fluidity, whereas trans unsaturated fatty acids do not.
; ''trans'' : A ''trans'' configuration, by contrast, means that the adjacent two hydrogen atoms lie on ''opposite'' sides of the chain. As a result, they do not cause the chain to bend much, and their shape is similar to straight saturated fatty acids.

In most naturally occurring unsaturated fatty acids, each double bond has three ([[omega-3 fatty acid|n−3]]), six ([[omega-6 fatty acid|n−6]]), or nine ([[omega-9 fatty acid|n−9]]) carbon atoms after it, and all double bonds have a cis configuration. Most fatty acids in the ''trans'' configuration ([[trans fat]]s) are not found in nature and are the result of human processing (e.g., [[hydrogenation]]). Some trans fatty acids also occur naturally in the milk and meat of [[ruminant]]s (such as cattle and sheep). They are produced, by fermentation, in the rumen of these animals. They are also found in [[dairy product]]s from milk of ruminants, and may be also found in [[breast milk]] of women who obtained them from their diet.

The geometric differences between the various types of unsaturated fatty acids, as well as between saturated and unsaturated fatty acids, play an important role in biological processes, and in the construction of biological structures (such as cell membranes).

{| class="wikitable"
|+ Examples of Unsaturated Fatty Acids
|-
! Common name || Chemical structure || Δ{{sup|''x''}}{{efn|Each double bond in the fatty acid is indicated by Δx, where the double bond is located on the xth carbon–carbon bond, counting from the carboxylic acid end.}} || ''C'':''D''{{efn|name=c:d|1="C:D" is the numerical symbol: total amount of (C)arbon atoms of the fatty acid, and the number of (D)ouble (''unsaturated'') bonds in it; if D > 1 it is assumed that the double bonds are separated by one or more [[methylene bridge]](s).}} || IUPAC<ref name="IUPAClipid"/> || ''n''−''x''{{efn|name=omega-x|1=In ''n minus x'' (also ω−x or omega-x) nomenclature a double bond of the fatty acid is located on the xth carbon–carbon bond, counting from the terminal methyl carbon (designated as n or ω) toward the carbonyl carbon.}}
|-
| colspan="6" |'''Omega−3:'''
|-
|[[Eicosapentaenoic acid]]
|CH{{sub|3}}CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''(CH{{sub|2}}){{sub|3}}COOH
|''cis'',''cis'',''cis'',''cis'',''cis''-Δ{{sup|5}},Δ{{sup|8}},Δ{{sup|11}},Δ{{sup|14}},Δ{{sup|17}}
|20:5
|20:5(5,8,11,14,17)
|[[omega-3 fatty acid|''n''−3]]
|-
|[[α-Linolenic acid]]
|CH{{sub|3}}CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''(CH{{sub|2}}){{sub|7}}COOH
|''cis'',''cis'',''cis''-Δ{{sup|9}},Δ{{sup|12}},Δ{{sup|15}}
|18:3
|18:3(9,12,15)
|[[omega-3 fatty acid|''n''−3]]
|-
|[[Docosahexaenoic acid]]
|CH{{sub|3}}CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''(CH{{sub|2}}){{sub|2}}COOH
|''cis'',''cis'',''cis'',''cis'',''cis'',''cis''-Δ{{sup|4}},Δ{{sup|7}},Δ{{sup|10}},Δ{{sup|13}},Δ{{sup|16}},Δ{{sup|19}}
|22:6
|22:6(4,7,10,13,16,19)
|[[omega-3 fatty acid|''n''−3]]
|-
| colspan="6" |'''Omega−6:'''
|-
|[[Arachidonic acid]]
|CH{{sub|3}}(CH{{sub|2}}){{sub|4}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''(CH{{sub|2}}){{sub|3}}COOH<sup>[http://webbook.nist.gov/cgi/cbook.cgi?Name=Arachidonic+Acid&Units=SI NIST]</sup>
|''cis'',''cis'',''cis'',''cis''-Δ{{sup|5}}Δ{{sup|8}},Δ{{sup|11}},Δ{{sup|14}}
|20:4
|20:4(5,8,11,14)
|[[omega-6 fatty acid|''n''−6]]
|-
|[[Linoleic acid]]
|CH{{sub|3}}(CH{{sub|2}}){{sub|4}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''(CH{{sub|2}}){{sub|7}}COOH
|''cis'',''cis''-Δ{{sup|9}},Δ{{sup|12}}
|18:2
|18:2(9,12)
|[[omega-6 fatty acid|''n''−6]]
|-
|[[Linoelaidic acid]]
|CH{{sub|3}}(CH{{sub|2}}){{sub|4}}'''CH=CH'''CH{{sub|2}}'''CH=CH'''(CH{{sub|2}}){{sub|7}}COOH
|''trans'',''trans''-Δ{{sup|9}},Δ{{sup|12}}
|18:2
|18:2(9t,12t)
|[[omega-6 fatty acid|''n''−6]]
|-
| colspan="6" |'''Omega−9:'''
|-
|[[Oleic acid]]|| CH{{sub|3}}(CH{{sub|2}}){{sub|7}}'''CH=CH'''(CH{{sub|2}}){{sub|7}}COOH || ''cis''-Δ{{sup|9}}|| 18:1 || 18:1(9) || [[omega-9 fatty acid|''n''−9]]
|-
|[[Elaidic acid]]|| CH{{sub|3}}(CH{{sub|2}}){{sub|7}}'''CH=CH'''(CH{{sub|2}}){{sub|7}}COOH || ''trans''-Δ{{sup|9}}|| 18:1 || 18:1(9t) || [[omega-9 fatty acid|''n''−9]]
|-
|[[Erucic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|7}}'''CH=CH'''(CH{{sub|2}}){{sub|11}}COOH || ''cis''-Δ{{sup|13}}|| 22:1 || 22:1(13) || [[omega-9 fatty acid|''n''−9]]
|-
| colspan="6" |'''Omega−5, 7, and 10:'''
|-
|[[Myristoleic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|3}}'''CH=CH'''(CH{{sub|2}}){{sub|7}}COOH || ''cis''-Δ{{sup|9}} || 14:1 || 14:1(9) || ''n''−5
|-
|[[Palmitoleic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|5}}'''CH=CH'''(CH{{sub|2}}){{sub|7}}COOH || ''cis''-Δ{{sup|9}} || 16:1 || 16:1(9) || ''n''−7
|-
|[[Vaccenic acid]]
|CH{{sub|3}}(CH{{sub|2}}){{sub|5}}'''CH=CH'''(CH{{sub|2}}){{sub|9}}COOH
|''trans''-Δ{{sup|11}}
|18:1
|18:1(11t)
|''n''−7
|-
|[[Sapienic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|8}}'''CH=CH'''(CH{{sub|2}}){{sub|4}}COOH || ''cis''-Δ{{sup|6}} || 16:1 || 16:1(6) || ''n''−10
|}

===Even- vs odd-chained fatty acids===
Most fatty acids are even-chained, e.g. stearic (C18) and oleic (C18), meaning they are composed of an even number of carbon atoms. Some fatty acids have odd numbers of carbon atoms; they are referred to as odd-chained fatty acids (OCFA). The most common OCFA are the saturated C15 and C17 derivatives, [[pentadecanoic acid]] and [[heptadecanoic acid]] respectively, which are found in dairy products.<ref>{{cite journal|doi=10.3945/an.115.011387|pmid=27422507|pmc=4942867|title=Pentadecanoic and Heptadecanoic Acids: Multifaceted Odd-Chain Fatty Acids|year=2016|last1=Pfeuffer|first1=Maria|last2=Jaudszus|first2=Anke|journal=Advances in Nutrition|volume=7|issue=4|pages=730–734}}</ref><ref>{{cite journal|doi=10.1096/fasebj.8.15.8001737|title=The Animal Fatty Acid Synthase: One Gene, One Polypeptide, Seven Enzymes|year=1994|last1=Smith|first1=S.|journal=The FASEB Journal|volume=8|issue=15|pages=1248–1259|doi-access=free |pmid=8001737|s2cid=22853095}}</ref> On a molecular level, OCFAs are biosynthesized and metabolized slightly differently from the even-chained relatives.

===Branching===
Most common fatty acids are [[straight-chain compound]]s, with no additional carbon atoms bonded as [[side group]]s to the main hydrocarbon chain. [[Branched-chain fatty acid]]s contain one or more [[methyl groups]] bonded to the hydrocarbon chain.

==Nomenclature==

===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]].

===Naming of fatty acids===
The following table describes the most common systems of naming fatty acids.

{{Clear}}
{| class="wikitable"
|-
!Nomenclature
!Examples
!Explanation
|-
!Trivial
|[[Palmitoleic acid]]
|'''[[Trivial name]]s''' (or '''common names''') are non-systematic historical names, which are the most frequent naming system used in literature. Most common fatty acids have trivial names in addition to their ''systematic names'' (see below). These names frequently do not follow any pattern, but they are concise and often unambiguous.
|-
!Systematic
|[[Oleic acid|cis-9-octadec-9-enoic acid]]<br />[[Oleic acid|(9''Z'')-octadec-9-enoic acid]]
|'''[[Systematic name]]s''' (or '''[[International Union of Pure and Applied Chemistry|IUPAC]] names''') derive from the standard ''[[IUPAC nomenclature of organic chemistry|IUPAC Rules for the Nomenclature of Organic Chemistry]]'', published in 1979,<ref name="nomenclature-1979">{{cite book |title=Nomenclature of Organic Chemistry |last1=Rigaudy|first1= J. |last2=Klesney|first2=S. P. |publisher=[[Pergamon]] |year=1979 |isbn=978-0-08-022369-8 |oclc=5008199}}</ref> along with a recommendation published specifically for lipids in 1977.<ref name="nomenclature-1977">{{cite journal |year=1977 |title=The Nomenclature of Lipids. Recommendations, 1976 |volume=79 |issue=1 |pages=11–21 |doi=10.1111/j.1432-1033.1977.tb11778.x  |journal=[[European Journal of Biochemistry]]|doi-access=free }}</ref> [[#Numbering of the carbon atoms in a fatty acid|Carbon atom numbering]] begins from the [[carboxylic group|carboxylic]] end of the molecule backbone. [[Double bond]]s are labelled with [[cis-trans isomerism|''cis''-/''trans''-]] notation or ''[[E-Z notation|E]]''-/''[[E-Z notation|Z]]''- notation, where appropriate. This notation is generally more verbose than common nomenclature, but has the advantage of being more technically clear and descriptive.
|-
!Δ<sup>''x''</sup>
|[[Linoleic acid|''cis''-Δ<sup>9</sup>, ''cis''-Δ<sup>12</sup> octadecadienoic acid]]
|In '''Δ<sup>''x''</sup>''' (or '''delta-''x''''') '''nomenclature''', each double bond is indicated by Δ<sup>''x''</sup>, where the double bond begins at the ''x''th carbon–carbon bond, [[#Numbering of the carbon atoms in a fatty acid|counting]] from [[carboxylic group|carboxylic]] end of the molecule backbone. Each double bond is preceded by a ''[[cis-trans isomerism|cis]]''- or ''[[cis-trans isomerism|trans]]''- prefix, indicating the configuration of the molecule around the bond. For example, [[linoleic acid]] is designated "''cis''-Δ<sup>9</sup>, ''cis''-Δ<sup>12</sup> octadecadienoic acid". This nomenclature has the advantage of being less verbose than systematic nomenclature, but is no more technically clear or descriptive.{{citation needed|reason=citation needed for this verbose delta notation, sometimes in chemical it's seen simpler notation with single delta, locants in superscript comma-separated, for various types of unsaturated natural organic compounds |date=September 2018}}
|-
!''n''−''x'' <br />(or ω−''x'')
|[[Omega-3 fatty acid|''n''−3]]<br />(or [[Omega-3 fatty acid|ω−3]])
|'''''n''−''x''''' ('''''n'' minus ''x'''''; also '''ω−''x''''' or '''omega−''x''''') '''nomenclature''' both provides names for individual compounds and classifies them by their likely biosynthetic properties in animals. A double bond is located on the ''x''<sup>th</sup> carbon–carbon bond, [[#Numbering of the carbon atoms in a fatty acid|counting]] from the [[Methyl group|methyl]] end of the molecule backbone. For example, [[α-Linolenic acid|α-linolenic acid]] is classified as a [[omega-3 fatty acid|''n''−3]] or [[omega-3|omega−3]] fatty acid, and so it is likely to share a biosynthetic pathway with other compounds of this type. The ω−''x'', omega−''x'', or "omega" notation is common in popular nutritional literature, but [[IUPAC nomenclature|IUPAC]] has deprecated it in favor of ''n''−''x'' notation in technical documents.<ref name="nomenclature-1979" /> The most commonly researched fatty acid biosynthetic pathways are [[omega-3 fatty acid|''n''−3]] and [[omega-6 fatty acid|''n''−6]].
|-
!Lipid numbers
|18:3<br />[[α-Linolenic acid|18:3n3]]<br />[[α-Linolenic acid|18:3,&nbsp;''cis'',''cis'',''cis''-Δ<sup>9</sup>,Δ<sup>12</sup>,Δ<sup>15</sup>]]<br />[[α-Linolenic acid|18:3(9,12,15)]]
|'''Lipid numbers''' take the form ''C'':''D'',{{efn|name=c:d}} where ''C'' is the number of carbon atoms in the fatty acid and ''D'' is the number of double bonds in the fatty acid. If D is more than one, the double bonds are assumed to be interrupted by [[methylene bridge|{{chem|CH|2}} units]], ''i.e.'', at intervals of 3 carbon atoms along the chain. For instance, [[α-Linolenic acid|α-linolenic acid]] is an 18:3 fatty acid and its three double bonds are located at positions Δ<sup>9</sup>, Δ<sup>12</sup>, and Δ<sup>15</sup>. This notation can be ambiguous, as some different fatty acids can have the same ''C'':''D'' numbers. Consequently, when ambiguity exists this notation is usually paired with either a Δ<sup>''x''</sup> or ''n''−''x'' term.<ref name="nomenclature-1979" /> For instance, although [[α-Linolenic acid|α-linolenic acid]] and [[gamma-Linolenic acid|γ-linolenic acid]] are both 18:3, they may be unambiguously described as 18:3n3 and 18:3n6 fatty acids, respectively. For the same purpose, IUPAC recommends using a list of double bond positions in parentheses, appended to the C:D notation.<ref name="IUPAClipid">{{cite web |title=IUPAC Lipid nomenclature: Appendix A: names of and symbols for higher fatty acids |url=http://www.sbcs.qmul.ac.uk/iupac/lipid/appABC.html#appA |website=www.sbcs.qmul.ac.uk}}</ref> For instance, IUPAC recommended notations for α- and γ-linolenic acid are 18:3(9,12,15) and 18:3(6,9,12), respectively.
|}

=== Free fatty acids ===
{{See also|Deep frying#Oil deterioration and chemical changes}}
When [[circulatory system|circulating]] in the [[blood plasma|plasma]] (plasma fatty acids), not in their [[ester]], fatty acids are known as non-esterified fatty acids (NEFAs) or free fatty acids (FFAs). FFAs are always bound to a [[transport protein]], such as [[albumin]].<ref name="Dorlands">{{cite book|title=Dorland's Illustrated Medical Dictionary |publisher=[[Elsevier]] |url=http://dorlands.com/}}</ref>

FFAs also form from [[triglyceride]] food oils and fats by hydrolysis, contributing to the characteristic [[Rancidification|rancid]] odor.<ref>{{Cite journal|last1=Mariod|first1=Abdalbasit|last2=Omer|first2=Nuha|last3=Al|first3=El Mugdad|first4=Mohammed|last4=Mokhtar |date=2014-09-09|title=Chemical Reactions Taken Place During deep-fat Frying and Their Products: A review|url=https://www.researchgate.net/publication/270727167|journal=Sudan University of Science & Technology SUST Journal of Natural and Medical Sciences|volume=Supplementary issue|pages=1–17}}</ref> An analogous process happens in [[biodiesel]] with risk of part corrosion.

==Production==

===Industrial===
Fatty acids are usually produced industrially by the [[hydrolysis]] of [[triglyceride]]s, with the removal of [[glycerol]] (see [[oleochemical]]s). [[Phospholipid]]s represent another source. Some fatty acids are produced synthetically by [[carbonylation|hydrocarboxylation]] of alkenes.<ref name="Ullmann Fatty Acids">{{Ullmann|doi=10.1002/14356007.a10_245.pub2|title=Fatty Acids|date=2006|last1=Anneken|first1=David J.|last2=Both|first2=Sabine|last3=Christoph|first3=Ralf|last4=Fieg|first4=Georg|last5=Steinberner|first5=Udo|last6=Westfechtel|first6=Alfred}}</ref>

===By animals===
{{main|Fatty acid synthesis}}
In animals, fatty acids are formed from carbohydrates predominantly in the [[liver]], [[adipose tissue]], and the [[mammary gland]]s during lactation.<ref name=stryer>{{cite book |last1= Stryer |first1= Lubert | title=Biochemistry |chapter= Fatty acid metabolism. |edition= 4th |location= New York |publisher= W. H. Freeman and Company|date= 1995 |pages= 603–628 |isbn= 978-0-7167-2009-6 }}</ref>

Carbohydrates are converted into [[Pyruvic acid|pyruvate]] by [[glycolysis]] as the first important step in the conversion of carbohydrates into fatty acids.<ref name=stryer /> Pyruvate is then decarboxylated to form [[acetyl-CoA]] in the [[mitochondrion]]. However, this acetyl CoA needs to be transported into [[cytosol]] where the synthesis of fatty acids occurs. This cannot occur directly. To obtain cytosolic acetyl-CoA, [[Citric acid|citrate]] (produced by the condensation of acetyl-CoA with [[Oxaloacetic acid|oxaloacetate]]) is removed from the [[citric acid cycle]] and carried across the inner mitochondrial membrane into the cytosol.<ref name=stryer /> There it is cleaved by [[ATP citrate lyase]] into acetyl-CoA and oxaloacetate. The oxaloacetate is returned to the mitochondrion as [[malate]].<ref name= ferre>{{cite journal | doi = 10.1159/000100426 | title = SREBP-1c Transcription Factor and Lipid Homeostasis: Clinical Perspective | journal = Hormone Research | year = 2007 | first1 = P. | last1 = Ferre |first2=F. |last2=Foufelle | volume = 68 | issue = 2 | pages = 72–82| doi-broken-date = 2024-09-11 | pmid = 17344645 | quote = this process is outlined graphically in page 73| doi-access = free }}</ref> The cytosolic acetyl-CoA is carboxylated by [[acetyl-CoA carboxylase]] into [[malonyl-CoA]], the first committed step in the synthesis of fatty acids.<ref name= ferre /><ref name=Voet>{{cite book |last1=Voet |first1=Donald |first2=Judith G. |last2=Voet |first3=Charlotte W. |last3=Pratt |title=Fundamentals of Biochemistry |edition=2nd |publisher=John Wiley and Sons |year=2006 |pages=[https://archive.org/details/fundamentalsofbi00voet_0/page/547 547, 556] |isbn=978-0-471-21495-3 |url-access=registration |url=https://archive.org/details/fundamentalsofbi00voet_0/page/547 }}</ref>

Malonyl-CoA is then involved in a repeating series of reactions that lengthens the growing fatty acid chain by two carbons at a time. Almost all natural fatty acids, therefore, have even numbers of carbon atoms. When synthesis is complete the free fatty acids are nearly always combined with glycerol (three fatty acids to one glycerol molecule) to form [[triglyceride]]s, the main storage form of fatty acids, and thus of energy in animals. However, fatty acids are also important components of the [[phospholipid]]s that form the [[phospholipid bilayers]] out of which all the membranes of the cell are constructed (the [[cell wall]], and the membranes that enclose all the [[organelle]]s within the cells, such as the [[Cell nucleus|nucleus]], the [[Mitochondrion|mitochondria]], [[endoplasmic reticulum]], and the [[Golgi apparatus]]).<ref name=stryer />

The "uncombined fatty acids" or "free fatty acids" found in the circulation of animals come from the breakdown (or [[lipolysis]]) of stored triglycerides.<ref name=stryer /><ref>{{cite journal | last1 = Zechner | first1 = R. | last2 = Strauss | first2 = J. G. | last3 = Haemmerle | first3 = G. | last4 = Lass | first4 = A. | last5 = Zimmermann | first5 = R. | year = 2005 | title = Lipolysis: pathway under construction | journal = Curr. Opin. Lipidol. | volume = 16 | issue = 3| pages = 333–340 | doi = 10.1097/01.mol.0000169354.20395.1c | pmid = 15891395 | s2cid = 35349649 }}</ref> Because they are insoluble in water, these fatty acids are transported bound to plasma [[albumin]]. The levels of "free fatty acids" in the blood are limited by the availability of albumin binding sites. They can be taken up from the blood by all cells that have mitochondria (with the exception of the cells of the [[central nervous system]]). Fatty acids can only be broken down in mitochondria, by means of [[beta-oxidation]] followed by further combustion in the [[citric acid cycle]] to CO{{sub|2}} and water. Cells in the central nervous system, although they possess mitochondria, cannot take free fatty acids up from the blood, as the [[blood–brain barrier]] is impervious to most free fatty acids,{{citation needed|date=June 2016}} excluding [[short-chain fatty acid]]s and [[medium-chain fatty acid]]s.<ref name="SCFA MCT-mediated BBB passage - 2005 review">{{cite journal | vauthors = Tsuji A | title = Small molecular drug transfer across the blood–brain barrier via carrier-mediated transport systems | journal = NeuroRx | volume = 2 | issue = 1 | pages = 54–62 | year = 2005 | pmid = 15717057 | pmc = 539320 | doi = 10.1602/neurorx.2.1.54 | quote = Uptake of valproic acid was reduced in the presence of medium-chain fatty acids such as hexanoate, octanoate, and decanoate, but not propionate or butyrate, indicating that valproic acid is taken up into the brain via a transport system for medium-chain fatty acids, not short-chain fatty acids.&nbsp;... Based on these reports, valproic acid is thought to be transported bidirectionally between blood and brain across the BBB via two distinct mechanisms, monocarboxylic acid-sensitive and medium-chain fatty acid-sensitive transporters, for efflux and uptake, respectively.}}</ref><ref name="SCFA MCT-mediated BBB passage - 2014 review">{{cite journal | vauthors = Vijay N, Morris ME | title = Role of monocarboxylate transporters in drug delivery to the brain | journal = Curr. Pharm. Des. | volume = 20 | issue = 10 | pages = 1487–98 | year = 2014 | pmid = 23789956 | pmc = 4084603 | doi = 10.2174/13816128113199990462| quote = Monocarboxylate transporters (MCTs) are known to mediate the transport of short chain monocarboxylates such as lactate, pyruvate and butyrate.&nbsp;... MCT1 and MCT4 have also been associated with the transport of short chain fatty acids such as acetate and formate which are then metabolized in the astrocytes [78].}}</ref> These cells have to manufacture their own fatty acids from carbohydrates, as described above, in order to produce and maintain the phospholipids of their cell membranes, and those of their organelles.<ref name=stryer />

====Variation between animal species====
Studies on the [[cell membrane]]s of [[mammal]]s and [[reptile]]s discovered that mammalian cell membranes are composed of a higher proportion of polyunsaturated fatty acids ([[docosahexaenoic acid|DHA]], [[omega-3 fatty acid|omega−3 fatty acid]]) than [[reptile]]s.<ref name=hulb1999/> Studies on bird fatty acid composition have noted similar proportions to mammals but with 1/3rd less omega−3 fatty acids as compared to [[omega-6 fatty acid|omega−6]] for a given body size.<ref name=hulb2002/> This fatty acid composition results in a more fluid cell membrane but also one that is permeable to various ions ({{chem2|H+}} & {{chem2|Na+}}), resulting in cell membranes that are more costly to maintain. This maintenance cost has been argued to be one of the key causes for the high metabolic rates and concomitant [[warm-blooded]]ness of mammals and birds.<ref name=hulb1999/> However polyunsaturation of cell membranes may also occur in response to chronic cold temperatures as well. In [[fish]] increasingly cold environments lead to increasingly high cell membrane content of both monounsaturated and polyunsaturated fatty acids, to maintain greater membrane fluidity (and functionality) at the lower [[temperature]]s.<ref name=hulb2003xa/><ref name=rayn1991/>

==Fatty acids in dietary fats==
The following table gives the fatty acid, [[vitamin E]] and [[cholesterol]] composition of some common dietary fats.<ref>
{{cite book
 | last1=McCann
 | last2=Widdowson
 | title=The Composition of Foods
 | chapter=Fats and Oils
 | author3=Food Standards Agency
 | publisher=Royal Society of Chemistry
 | year=1991
}}</ref><ref>{{cite web
 | url=http://www.efn.org/~sundance/fats_and_oils.html
 | title=More Than You Wanted To Know About Fats/Oils
 | last=Altar
 | first=Ted
 | access-date=2006-08-31
 | publisher=Sundance Natural Foods
 | archive-date=2010-12-05
 | archive-url=https://web.archive.org/web/20101205154008/http://www.efn.org/~sundance/fats_and_oils.html
 }}</ref>

{| class="wikitable" |
|+
! !! Saturated !! Monounsaturated !! Polyunsaturated !! Cholesterol !! Vitamin E
|-
| || align="center" | g/100g || align="center" | g/100g || align="center" | g/100g || align="center" | mg/100g || align="center" | mg/100g
|-
| colspan="6" | '''''Animal fats'''''
|-
|[[Duck fat]]<ref name="usda">{{cite web
 |url=http://www.nal.usda.gov/fnic/foodcomp/search/
 |title=USDA National Nutrient Database for Standard Reference
 |access-date=2010-02-17
 |publisher=U.S. Department of Agriculture
 |archive-url=https://web.archive.org/web/20150303184216/http://www.nal.usda.gov/fnic/foodcomp/search/
 |archive-date=2015-03-03
}}
</ref>|| align="right" | 33.2 || align="right" | 49.3 || align="right" | 12.9 || align="right" | 100 || align="right" | 2.70
|-
| [[Lard]]<ref name="usda" /> || align="right" | 40.8 || align="right" | 43.8 || align="right" | 9.6 || align="right" | 93 || align="right" | 0.60
|-
|[[Tallow]]<ref name="usda" />|| align="right" | 49.8 || align="right" | 41.8 || align="right" | 4.0 || align="right" | 109 || align="right" | 2.70
|-
| [[Butter]] || align="right" | 54.0 || align="right" | 19.8 || align="right" | 2.6 || align="right" | 230 || align="right" | 2.00
|-
| colspan="6" | '''''Vegetable fats'''''
|-
| [[Coconut oil]] || align="right" |  85.2 || align="right" |  6.6 || align="right" |  1.7 || align="right" |  0 || align="right" |  .66
|-
| [[Cocoa butter]] || align="right" |  60.0 || align="right" |  32.9 || align="right" |  3.0 || align="right" |  0 || align="right" |  1.8
|-
| [[Palm kernel oil]] || align="right" |  81.5 || align="right" |  11.4 || align="right" |  1.6 || align="right" |  0 || align="right" |  3.80
|-
| [[Palm oil]] || align="right" |  45.3 || align="right" |  41.6 || align="right" |  8.3 || align="right" |  0 || align="right" |  33.12
|-
| [[Cottonseed oil]] || align="right" |  25.5 || align="right" |  21.3 || align="right" |  48.1 || align="right" |  0 || align="right" |  42.77
|-
| [[Wheat germ oil]] || align="right" |  18.8 || align="right" |  15.9 || align="right" |  60.7 || align="right" |  0 || align="right" |  136.65
|-
| [[Soybean oil]] || align="right" |  14.5 || align="right" |  23.2 || align="right" |  56.5 || align="right" |  0 || align="right" |  16.29
|-
| [[Olive oil]] || align="right" |  14.0 || align="right" |  69.7 || align="right" |  11.2 || align="right" |  0 || align="right" |  5.10
|-
| [[Corn oil]] || align="right" |  12.7 || align="right" |  24.7 || align="right" |  57.8 || align="right" |  0 || align="right" |  17.24
|-
| [[Sunflower oil]] || align="right" |  11.9 || align="right" |  20.2 || align="right" |  63.0 || align="right" |  0 || align="right" |  49.00
|-
| [[Safflower oil]] || align="right" |  10.2 || align="right" |  12.6 || align="right" |  72.1 || align="right" |  0 || align="right" |  40.68
|-
| [[Hemp oil]] || align="right" |  10 || align="right" |  15 || align="right" |  75 || align="right" |  0 || align="right" |  12.34
|-
| [[Canola|Canola/Rapeseed oil]] || align="right" |  5.3 || align="right" |  64.3 || align="right" |  24.8 || align="right" |  0 || align="right" |  22.21
|}

==Reactions of fatty acids==
Fatty acids exhibit reactions like other carboxylic acids, i.e. they undergo [[esterification]] and acid-base reactions.

===Acidity===
Fatty acids do not show a great variation in their acidities, as indicated by their respective [[Acid dissociation constant|p''K''<sub>a</sub>]]. [[Nonanoic acid]], for example, has a p''K''{{sub|a}} of 4.96, being only slightly weaker than acetic acid (4.76). As the chain length increases, the solubility of the fatty acids in water decreases, so that the longer-chain fatty acids have minimal effect on the [[pH]] of an aqueous solution. Near neutral pH, fatty acids exist at their conjugate bases, i.e. oleate, etc.

Solutions of fatty acids in [[ethanol]] can be [[titration|titrated]] with [[sodium hydroxide]] solution using [[phenolphthalein]] as an indicator. This analysis is used to determine the free fatty acid content of fats; i.e., the proportion of the triglycerides that have been [[hydrolysis|hydrolyze]]d.

Neutralization of fatty acids, one form of [[saponification]] (soap-making), is a widely practiced route to [[metallic soap]]s.<ref>{{Ullmann |last1=Schumann |first1=Klaus |last2=Siekmann |first2=Kurt |year=2000 |doi=10.1002/14356007.a24_247 |title=Soaps}}</ref>

===Hydrogenation and hardening===
[[Hydrogenation]] of unsaturated fatty acids is widely practiced. Typical conditions involve 2.0–3.0 MPa of H{{sub|2}} pressure, 150&nbsp;°C, and nickel supported on silica as a catalyst. This treatment affords saturated fatty acids. The extent of hydrogenation is indicated by the [[iodine number]]. Hydrogenated fatty acids are less prone toward [[rancidification]]. Since the saturated fatty acids are [[melting point|higher melting]] than the unsaturated precursors, the process is called hardening. Related technology is used to convert vegetable oils into [[margarine]]. The hydrogenation of triglycerides (vs fatty acids) is advantageous because the carboxylic acids degrade the nickel catalysts, affording nickel soaps. During partial hydrogenation, unsaturated fatty acids can be isomerized from ''cis'' to ''trans'' configuration.<ref name="Ullmann Fatty Acids"/>

More forcing hydrogenation, i.e. using higher pressures of H{{sub|2}} and higher temperatures, converts fatty acids into [[fatty alcohol]]s. Fatty alcohols are, however, more easily produced from fatty acid [[ester]]s.

In the [[Varrentrapp reaction]] certain unsaturated fatty acids are cleaved in molten alkali, a reaction which was, at one point of time, relevant to structure elucidation.

===Auto-oxidation and rancidity===
{{Main|Rancidification}}
Unsaturated fatty acids and their esters undergo [[auto-oxidation]], which involves replacement of a C-H bond with C-O bond. The process requires oxygen (air) and is accelerated by the presence of traces of metals, which serve as catalysts. Doubly unsaturated fatty acids are particularly prone to this reaction.  Vegetable oils resist this process to a small degree because they contain antioxidants, such as [[tocopherol]]. Fats and oils often are treated with [[chelation|chelating agents]] such as [[citric acid]] to remove the metal catalysts.

===Ozonolysis===
Unsaturated fatty acids are susceptible to degradation by ozone. This reaction is practiced in the production of [[azelaic acid]] ((CH{{sub|2}}){{sub|7}}(CO{{sub|2}}H){{sub|2}}) from [[oleic acid]].<ref name="Ullmann Fatty Acids"/>

==Circulation==

===Digestion and intake===
{{Main|Digestion#Fat digestion}}
[[Short-chain fatty acid|Short-]] and [[medium-chain fatty acids]] are absorbed directly into the blood via intestine capillaries and travel through the [[portal vein]] just as other absorbed nutrients do. However, [[long-chain fatty acids]] are not directly released into the intestinal capillaries. Instead they are absorbed into the fatty walls of the intestine [[Intestinal villus|villi]] and reassemble again into [[triglyceride]]s. The triglycerides are coated with [[cholesterol]] and protein (protein coat) into a compound called a [[chylomicron]].

From within the cell, the chylomicron is released into a [[lymphatic]] capillary called a [[lacteal]], which merges into larger lymphatic vessels. It is transported via the lymphatic system and the [[thoracic duct]] up to a location near the heart (where the arteries and veins are larger). The thoracic duct empties the chylomicrons into the bloodstream via the left [[subclavian vein]]. At this point the chylomicrons can transport the triglycerides to tissues where they are stored or metabolized for energy.

===Metabolism===
{{Main|Fatty acid metabolism}}
Fatty acids are broken down to CO{{sub|2}} and water by the intra-cellular [[mitochondria]] through [[beta oxidation]] and the [[citric acid cycle]]. In the final step ([[oxidative phosphorylation]]), reactions with oxygen release a lot of energy, captured in the form of large quantities of [[Adenosine triphosphate|ATP]]. Many cell types can use either [[glucose]] or fatty acids for this purpose, but fatty acids release more energy per gram. Fatty acids (provided either by ingestion or by drawing on triglycerides stored in fatty tissues) are distributed to cells to serve as a fuel for muscular contraction and general metabolism.

====Essential fatty acids====
{{Main|Essential fatty acid}}
Fatty acids that are required for good health but cannot be made in sufficient quantity from other substrates, and therefore must be obtained from food, are called essential fatty acids. There are two series of essential fatty acids: one has a double bond [[Omega-3 fatty acid|three carbon atoms]] away from the methyl end; the other has a double bond [[Omega-6 fatty acid|six carbon atoms]] away from the methyl end. Humans lack the ability to introduce double bonds in fatty acids beyond carbons 9 and 10, as counted from the carboxylic acid side.<ref>{{cite book|last=Bolsover|first=Stephen R.|title=Cell Biology: A Short Course|url=https://books.google.com/books?id=3a6p9pA5gZ8C&pg=PA42+|date=15 February 2004|publisher=John Wiley & Sons|isbn=978-0-471-46159-3|pages=42ff|display-authors=etal}}</ref> Two essential fatty acids are [[linoleic acid]] (LA) and [[α-Linolenic acid|alpha-linolenic acid]] (ALA). These fatty acids are widely distributed in plant oils. The human body has a limited ability to convert ALA into the longer-chain [[omega-3 fatty acid]]s — [[eicosapentaenoic acid]] (EPA) and [[docosahexaenoic acid]] (DHA), which can also be obtained from fish. Omega−3 and [[Omega-6 fatty acid|omega−6]] fatty acids are [[Biosynthesis|biosynthetic]] precursors to [[Cannabinoid#Endocannabinoids|endocannabinoids]] with [[Nociception|antinociceptive]], [[anxiolytic]], and [[Nervous system|neurogenic]] properties.<ref>{{Cite journal|last1=Ramsden|first1=Christopher E.|last2=Zamora|first2=Daisy|author-link3=Alexandros Makriyannis|last3=Makriyannis|first3=Alexandros|last4=Wood|first4=JodiAnne T.|last5=Mann|first5=J. Douglas|last6=Faurot|first6=Keturah R.|last7=MacIntosh|first7=Beth A.|last8=Majchrzak-Hong|first8=Sharon F.|last9=Gross|first9=Jacklyn R.|date=August 2015|title=Diet-induced changes in n-3 and n-6 derived endocannabinoids and reductions in headache pain and psychological distress|journal=The Journal of Pain|volume=16|issue=8|pages=707–716|doi=10.1016/j.jpain.2015.04.007|issn=1526-5900|pmc=4522350|pmid=25958314}}</ref>

===Distribution===
{{Main|Blood fatty acids}}
Blood fatty acids adopt distinct forms in different stages in the blood circulation. They are taken in through the intestine in [[chylomicron]]s, but also exist in [[very low density lipoprotein]]s (VLDL) and [[low density lipoprotein]]s (LDL) after processing in the liver. In addition, when released from [[adipocytes]], fatty acids exist in the blood as [[free fatty acids]].

It is proposed that the blend of fatty acids exuded by mammalian skin, together with [[lactic acid]] and [[pyruvic acid]], is distinctive and enables animals with a keen sense of smell to differentiate individuals.<ref>{{cite web
 | url=https://www.sciencedaily.com/releases/2009/07/090721091839.htm
 | title=Electronic Nose Created To Detect Skin Vapors
 | date=July 21, 2009
 | website=Science Daily
 | access-date=2010-05-18
}}</ref>

==Skin==
The [[stratum corneum]] {{ndash}} the outermost layer of the [[epidermis]] {{ndash}} is composed of terminally [[Cellular differentiation|differentiated]] and [[Enucleation (microbiology)|enucleated]] [[corneocyte]]s within a lipid matrix.<ref name=":0">{{cite journal | last1=Knox | first1=Sophie | last2=O'Boyle | first2=Niamh M. | title=Skin lipids in health and disease: A review | journal=Chemistry and Physics of Lipids | volume=236 | year=2021 | issn=0009-3084 | pmid=33561467 | doi=10.1016/j.chemphyslip.2021.105055 | page=105055| s2cid=231864420 | doi-access=free }}</ref> Together with [[cholesterol]] and [[ceramide]]s, free fatty acids form a water-impermeable barrier that prevents [[evaporation|evaporative water loss]].<ref name=":0"/> Generally, the epidermal lipid matrix is composed of an equimolar mixture of ceramides (about 50% by weight), cholesterol (25%), and free fatty acids (15%).<ref name=":0"/> Saturated fatty acids 16 and 18 carbons in length are the dominant types in the epidermis,<ref name=":0"/><ref name=":1">{{Cite journal|display-authors=3 |last1=Merleev |first1=Alexander A. |last2=Le |first2=Stephanie T. |last3=Alexanian |first3=Claire |last4=Toussi |first4=Atrin |last5=Xie |first5=Yixuan |last6=Marusina |first6=Alina I. |last7=Watkins |first7=Steven M. |last8=Patel |first8=Forum |last9=Billi |first9=Allison C. |last10=Wiedemann |first10=Julie |last11=Izumiya |first11=Yoshihiro |last12=Kumar |first12=Ashish |last13=Uppala |first13=Ranjitha |last14=Kahlenberg |first14=J. Michelle |last15=Liu |first15=Fu-Tong |date=2022-08-22 |title=Biogeographic and disease-specific alterations in epidermal lipid composition and single-cell analysis of acral keratinocytes |journal=JCI Insight |volume=7 |issue=16 |pages=e159762 |doi=10.1172/jci.insight.159762 |issn=2379-3708 |pmc=9462509 |pmid=35900871}}</ref> while unsaturated fatty acids and saturated fatty acids of various other lengths are also present.<ref name=":0"/><ref name=":1" /> The relative abundance of the different fatty acids in the epidermis is dependent on the body site the skin is covering.<ref name=":1" /> There are also characteristic epidermal fatty acid alterations that occur in [[psoriasis]], [[atopic dermatitis]], and other [[inflammation|inflammatory conditions]].<ref name=":0"/><ref name=":1" />

==Analysis==

The chemical analysis of fatty acids in lipids typically begins with an [[interesterification]] step that breaks down their original esters (triglycerides, waxes, phospholipids etc.) and converts them to [[methyl]] esters, which are then separated by gas chromatography<ref>{{cite journal | vauthors = Aizpurua-Olaizola O, Ormazabal M, Vallejo A, Olivares M, Navarro P, Etxebarria N, Usobiaga A | display-authors = 6 | title = Optimization of supercritical fluid consecutive extractions of fatty acids and polyphenols from Vitis vinifera grape wastes | journal = Journal of Food Science | volume = 80 | issue = 1 | pages = E101-7 | date = January 2015 | pmid = 25471637 | doi = 10.1111/1750-3841.12715 }}</ref> or analyzed by [[gas chromatography]] and mid-[[infrared spectroscopy]].

Separation of unsaturated isomers is possible by [[Argentation chromatography|silver ion complemented thin-layer chromatography]].<ref>{{Cite journal | doi = 10.1093/chromsci/25.7.302| pmid = 3611285| title = Separation of Fatty Acids or Methyl Esters Including Positional and Geometric Isomers by Alumina Argentation Thin-Layer Chromatography| journal = Journal of Chromatographic Science| volume = 25| issue = 7| pages = 302–6| year = 1987| last1 = Breuer | first1 = B.| last2 = Stuhlfauth | first2 = T.| last3 = Fock | first3 = H. P.}}</ref> Other separation techniques include [[high-performance liquid chromatography]] (with short columns packed with [[silica gel]] with bonded phenylsulfonic acid groups whose hydrogen atoms have been exchanged for silver ions). The role of silver lies in its ability to form complexes with unsaturated compounds.

==Industrial uses==
Fatty acids are mainly used in the production of [[soap]], both for cosmetic purposes and, in the case of [[metallic soap]]s, as lubricants. Fatty acids are also converted, via their methyl esters, to [[fatty alcohol]]s and [[fatty amine]]s, which are precursors to surfactants, detergents, and lubricants.<ref name="Ullmann Fatty Acids"/> Other applications include their use as [[Emulsion#Emulsifiers|emulsifiers]], texturizing agents, wetting agents, [[Defoamer|anti-foam agents]], or stabilizing agents.<ref name="buildingblocks">{{cite web| url=http://www.aciscience.org/docs/Fatty_Acids_Building_Blocks_for_Industry.pdf |archive-url=https://web.archive.org/web/20180423033611/http://www.aciscience.org/docs/Fatty_Acids_Building_Blocks_for_Industry.pdf |archive-date=2018-04-23 |url-status=live | title= Fatty Acids: Building Blocks for Industry | access-date=22 Apr 2018 |author=<!--Not stated--> |website=aciscience.org |publisher= American Cleaning Institute }}</ref>

Esters of fatty acids with simpler alcohols (such as methyl-, ethyl-, n-propyl-, isopropyl- and butyl esters) are used as emollients in cosmetics and other personal care products and as synthetic lubricants. Esters of fatty acids with more complex alcohols, such as [[sorbitol]], [[ethylene glycol]], [[diethylene glycol]], and [[polyethylene glycol]] are consumed in food, or used for personal care and water treatment, or used as synthetic lubricants or fluids for metal working.

<!--Content merged from [[saturated fat]]:
===Molecular description===
[[File:Myristic acid.svg|thumb|left|500px|Two-dimensional representation of the saturated fatty acid [[myristic acid]]]]
[[File:Myristic-acid-3D-vdW.png|thumb|left|500px|A [[space-filling model]] of the saturated fatty acid [[myristic acid]]]]  The two-dimensional illustration has implicit hydrogen atoms bonded to each of the carbon atoms in the polycarbon tail of the [[myristic acid]] molecule (there are 13 carbon atoms in the tail; 14 carbon atoms in the entire molecule).

Carbon atoms are also implicitly drawn, as they are  portrayed as [[Line-line intersection|intersections]] between two straight lines.  "Saturated", in general, refers to a maximum number of hydrogen atoms bonded to each carbon of the polycarbon tail as allowed by the [[Octet Rule]].  This also means that only [[single bond]]s ([[sigma bonds]]) will be present between adjacent carbon atoms of the tail.
-->

==See also==
{{Commons|Fatty acids}}
{{col div|colwidth=30em}}
* [[Fatty acid synthase]]
* [[Fatty acid synthesis]]
* [[Fatty aldehyde]]
* [[List of saturated fatty acids]]
* [[List of unsaturated fatty acids]]
* [[List of carboxylic acids]]
* [[Vegetable oil]]
*[[Lactobacillic acid]]
{{colend}}

==References==
{{notelist}}
{{reflist|refs=
<ref name=rayn1991>{{cite journal | vauthors = Raynard RS, Cossins AR | title = Homeoviscous adaptation and thermal compensation of sodium pump of trout erythrocytes | journal = The American Journal of Physiology | volume = 260 | issue = 5 Pt 2 | pages = R916–24 | date = May 1991 | pmid = 2035703 | doi = 10.1152/ajpregu.1991.260.5.R916 | s2cid = 24441498 }}</ref>

<ref name=hulb1999>{{cite journal | vauthors = Hulbert AJ, Else PL | title = Membranes as possible pacemakers of metabolism | journal = Journal of Theoretical Biology | volume = 199 | issue = 3 | pages = 257–74 | date = August 1999 | pmid = 10433891 | doi = 10.1006/jtbi.1999.0955 | bibcode = 1999JThBi.199..257H }}</ref>

<ref name=hulb2002>{{cite journal | vauthors = Hulbert AJ, Faulks S, Buttemer WA, Else PL | title = Acyl composition of muscle membranes varies with body size in birds | journal = The Journal of Experimental Biology | volume = 205 | issue = Pt 22 | pages = 3561–9 | date = November 2002 | doi = 10.1242/jeb.205.22.3561 | pmid = 12364409 }}</ref>

<ref name=hulb2003xa>{{cite journal | vauthors = Hulbert AJ | title = Life, death and membrane bilayers | journal = The Journal of Experimental Biology | volume = 206 | issue = Pt 14 | pages = 2303–11 | date = July 2003 | pmid = 12796449 | doi = 10.1242/jeb.00399 | doi-access = free }}</ref>
}}

==External links==
{{Scholia|chemical-class}}
* [http://lipidlibrary.aocs.org/ Lipid Library]
* [https://web.archive.org/web/20071012173913/http://intl.elsevierhealth.com/journals/plef/ ''Prostaglandins, Leukotrienes & Essential Fatty Acids'' journal]
* [https://web.archive.org/web/20110720155135/http://www.dmfpolska.eu/diagnostics.html Fatty blood acids ]

{{Fatty acids}}
{{Fatty-acid metabolism intermediates}}

{{Authority control}}

{{DEFAULTSORT:Fatty Acid}}
[[Category:Fatty acids| ]]
[[Category:Commodity chemicals]]
[[Category:E-number additives]]
[[Category:Edible oil chemistry]]