View source for Triglyceride

{{Short description|Any ester of glycerol having all three hydroxyl groups esterified with fatty acids}}
{{Fats}}
[[Image:Fat triglyceride shorthand formula.PNG|thumb|upright=1.35|Example of an unsaturated fat triglyceride (C<sub>55</sub>H<sub>98</sub>O<sub>6</sub>). Left part: [[glycerol]]; right part, from top to bottom: [[palmitic acid]], [[oleic acid]], [[alpha-linolenic acid]].]]

A '''triglyceride''' (from ''[[wikt:tri-#Prefix|tri-]]'' and ''[[glyceride]]''; also '''TG''', '''triacylglycerol''', '''TAG''', or '''triacylglyceride''') is an [[ester]] derived from [[glycerol]] and three [[fatty acids]].<ref>
{{cite web
  |title=Nomenclature of Lipids
  |publisher=IUPAC-IUB Commission on Biochemical Nomenclature (CBN)
  |url=http://www.chem.qmul.ac.uk/iupac/lipid/
  |access-date=2007-03-08
}}</ref>
Triglycerides are the main constituents of [[body fat]] in humans and other vertebrates as well as [[vegetable fat]].<ref>
{{cite book
  |last1=Nelson
  |first1=D. L.
  |last2=Cox
  |first2=M. M.
  |title=Lehninger, Principles of Biochemistry
  |edition=3rd
  |publisher=Worth Publishing
  |location=New York
  |date=2000
  |isbn=1-57259-153-6
  |url-access=registration
  |url=https://archive.org/details/lehningerprincip01lehn
}}</ref>
They are also present in the blood to enable the bidirectional transference of [[Adipose tissue|adipose]] fat and blood glucose from the liver and are a major component of [[sebum|human skin oils]].<ref>
{{cite journal
  |title = Human stratum corneum lipids: characterization and regional variations
  |journal = J. Lipid Res.
  |year = 1983
  |first1 = M. A.
  |last1 = Lampe
  |first2=A. L.
  |last2= Burlingame
  |first3=J.
  |last3= Whitney
  |first4=M. L.
  |last4= Williams
  |first5=B. E.
  |last5=Brown
  |first6=E.
  |last6=Roitman
  |first7=M.
  |last7=Elias
  |volume = 24
  |issue = 2
  |pages = 120–130
  |doi = 10.1016/S0022-2275(20)38005-6
 |pmid=6833889
|doi-access= free
  }}</ref>

Many types of triglycerides exist.  One specific classification focuses on [[Saturated and unsaturated compounds|saturated and unsaturated]] types. [[Saturated fat]]s have ''no'' C=C groups; [[unsaturated fat]]s feature one or more C=C groups. Unsaturated fats tend to have a lower [[melting point]] than saturated analogues; as a result, they are often [[liquid]] at room temperature.

== Chemical structure ==
[[File:Unsaturated Triglyceride Structural Formula V1.svg|thumb|upright=1.35|Example of a natural mixed triglyceride with residues of three different fatty acids. The first fatty acid residue is saturated (<span style="color:blue;">'''blue'''</span> highlighted), the second fatty acid residue contains one [[double bond]] within the carbon chain (<span style="color:green;">'''green'''</span> highlighted). The third fatty acid residue (a polyunsaturated fatty acid residue, highlighted in <span style="color:red;">'''red'''</span>) contains three [[double bond]]s within the carbon chain. All carbon-carbon double bonds shown are ''cis'' isomers.]]

The three fatty acids [[substituents]] can be the same, but they are usually different. The positions of the three fatty acids are specified using [[stereospecific numbering]] as sn-1, sn-2, and sn-3. The compositions of many fats and oils have been determined.<ref name=Alfieri>{{cite journal |display-authors=etal|last1=Andreina Alfieri |title=Effects of Plant Oil Interesterified Triacylglycerols on Lipemia and Human Health |journal=Int J Mol Sci |date=Dec 2017 |volume=19 |issue=1 |page=104 |doi=10.3390/ijms19010104 |doi-access=free |pmid=29301208 |pmc=5796054 }}</ref> Many triglycerides are known because many [[fatty acid]]s are known. The chain lengths of the fatty acid groups vary in naturally occurring triglycerides,  Those containing 16, 18, or 20 [[carbon]] atoms are defined as '''long-chain triglycerides''', while [[medium-chain triglycerides]] contain shorter fatty acids. Animals [[fatty acid synthesis|synthesize]] even-numbered fatty acids, but bacteria possess the ability to synthesise odd- and branched-chain fatty acids. As a result, [[ruminant]] animal fat contains odd-numbered fatty acids, such as 15, due to the action of [[bacteria]] in the [[rumen]].  Many fatty acids are unsaturated; some are polyunsaturated (e.g., those derived from [[linoleic acid]]).<ref name=Ullmann/>

Most natural fats contain a complex mixture of individual triglycerides. Because of their heterogeneity, they melt over a broad range of temperatures. Cocoa butter is unusual in that it is composed of only a few triglycerides, derived from [[Palmitic acid|palmitic]], [[Oleic acid|oleic]], and [[stearic acid]]s in the 1-, 2-, and 3-positions of glycerol, respectively.<ref name=Ullmann/>

The simplest triglycerides are those where the three fatty acids are identical. Their names indicate the fatty acid: [[stearin]] derived from stearic acid, [[triolein]] derived from [[oleic acid]], [[palmitin]] derived from [[palmitic acid]], etc.  These compounds can be obtained in three crystalline forms ([[Polymorphism (materials science)|polymorphs]]): α, β, and β′, the three forms differing in their melting points.<ref name=Ullmann>
{{cite encyclopedia
  |author=Alfred Thomas
  |encyclopedia=[[Ullmann's Encyclopedia of Industrial Chemistry]]
  |publisher=[[Wiley-VCH]]
  |place=Weinheim
  |year=2002
  |doi=10.1002/14356007.a10_173
  |chapter=Fats and Fatty Oils
  |isbn=3527306730
}}</ref><ref name=spring>
{{Cite journal
  |doi = 10.1007/BF02643296
  |title = Thermal properties of fats and oils
  |journal = [[J. Am. Oil Chem. Soc.]]
  |volume = 24
  |issue = 5
  |pages = 140
  |year = 1947
  |last1 = Charbonnet
  |first1 = G. H.
  |last2 = Singleton
  |first2 = W. S.
|s2cid = 101805872
 }}</ref>

A triglyceride containing different fatty acids is known as a ''mixed triglyceride''.<ref>{{Cite web |title=Mixed triglyceride {{!}} chemical compound {{!}} Britannica |url=https://www.britannica.com/science/mixed-triglyceride |access-date=2023-02-13 |website=www.britannica.com |language=en}}</ref> These are more common in nature.

If all three fatty acids on the glycerol differ, then the mixed triglyceride is [[Chirality (chemistry)|chiral]].

==Physical properties==
Triglycerides are colorless, although degraded samples can appear yellowish.  Stearin, a simple, saturated, symmetrical triglyceride, is a solid near room temperature, but most examples are oils.  Their densities range from around 0.89 with very long-chain fatty acids, through about 0.93 to 0.98 with medium-chain, and above 1.0 for very-short-chain acids.<ref>{{cite web |title=Densities of Triglycerides |url=https://bionumbers.hms.harvard.edu/files/Densities%20of%20triglycerides%20at%2020%C2%B0C%20&%2040%C2%B0C.pdf |website=BioNumbers – The Database of Useful Biological Numbers |publisher=Harvard |access-date=25 February 2025}} table from Gouw, T. H., and J. C. Vlugter. "Physical properties of triglycerides. I. Density and refractive index." Fette, Seifen, Anstrichmittel 68, no. 7 (1966): 544-549.</ref>

==Biosynthesis==
Triglycerides are tri-[[ester]]s derived from the [[condensation reaction]] of [[glycerol]] with three [[fatty acid]]s. Their formation can be summarised by the following overall equation:

:{{chem2| CH(OH)(CH2OH)2  +  RCOOH  +  R'COOH  +  R"COOH   →   RC(O)OCH2\sCH(OC(O)R')\sCH2C(O)OR"  +  3H2O}}

In nature, the formation of triglycerides is not random; rather, specific fatty acids are selectively condensed with the hydroxyl functional groups of glycerol. Animal fats typically have unsaturated fatty acid residues on carbon atoms 1 and 3. Extreme examples of non-random fats are [[cocoa butter]] (mentioned above) and [[lard]], which contains about 20% triglyceride with [[palmitic acid]] on carbon 2 and [[oleic acid]] on carbons 1 and 3.<ref name=Ullmann/> An early step in the biosynthesis is the formation of the [[glycerol-1-phosphate]]:<ref name=Ullmann/>

:{{chem2| CH(OH)(CH2OH)2  +  H2PO4-  ->  HOCH2\sCH(OH)\sCH2\sOPO3H-  +  H2O}}

The three oxygen atoms in this phosphate ester are differentiated, setting the stage for regiospecific formation of triglycerides, as the [[diol]] reacts selectively with coenzyme-A derivatives of the fatty acids, RC(O)S–CoA:

:{{chem2| HOCH2\sCH(OH)\sCH2\sOPO3H-  +  RC(O)S\sCoA  +  R'C(O)S\sCoA  ->  RC(O)O\sCH2\sCH(\sOC(O)R')\sCH2\sOPO3H-  +  2HS\sCoA}}

The phosphate ester linkage is then hydrolysed to make way for the introduction of a third fatty acid ester:

:{{chem2| RC(O)O\sCH2\sCH(\sOC(O)R')\sCH2\sOPO3H-  +  H2O  ->  RC(O)O\sCH2\sCH(\sOC(O)R')\sCH2OH  +  H2PO4-}}

:{{chem2| RC(O)O\sCH2\sCH(\sOC(O)R')\sCH2OH  +  R"C(O)S\sCoA  ->  RC(O)O\sCH2\sCH(\sOC(O)R')\sCH2\sOC(O)R"  +  HS\sCoA}}

==Nomenclature==
===Common fat names===
Fats are often named after their source, e.g., [[olive oil]], [[cod liver oil]], [[shea butter]], [[tail fat]]. Some have traditional names of their own, e.g.,  butter, lard, [[ghee]], and [[margarine]]. The composition of these natural fats are somewhat variable.  The [[oleic acid]] component in [[olive oil]] can vary from 64% to 86%.

===Chemical fatty acid names===
Triglycerides are then commonly named as esters of those acids, as in glyceryl 1,2-dioleate 3-palmitate, the name for a brood pheromone of the honey bee.<ref name=koen1983>N. Koeniger and H. J. Veith (1983): "Glyceryl-1,2-dioleate-3-palmitate, a brood pheromone of the honey bee (''Apis mellifera'' L.)". ''Experientia'', volume 39, pages 1051–1052 {{doi | 10.1007/BF01989801}}</ref>  Where the fatty acid residues in a triglyceride are all the same, names like [[olein]] (for glyceryl trioleate) and [[palmitin]] (for glyceryl tripalmitate) are common.{{cn|date=November 2024}}

===IUPAC===
In the [[International Union of Pure and Applied Chemistry]]'s (IUPAC's) general [[IUPAC nomenclature of organic chemistry|chemical nomenclature for organic compounds]],<ref>{{cite book|title = Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names, 2013|year = 2014|publisher = [[Royal Society of Chemistry]]|author1 = Henri A. Favre |author2 = Warren H. Powell |collaboration = International Union of Pure and Applied Chemistry|isbn = 978-1-84973-306-9|location = Cambridge, England|url = https://iupac.qmul.ac.uk/BlueBook/PDF/BlueBookV2.pdf|oclc = 865143943}}</ref> any organic structure can be named by starting from its corresponding [[hydrocarbon]] and then specifying differences so as to describe its structure completely. For fatty acids, for example, the position and orientation of carbon-carbon double bonds is specified counting from the [[carboxyl functional group]]. Thus, oleic acid is formally named (9''Z'')-octadec-9-enoic acid, which describes that the compound has:
* an 18 carbon chain ("octadec-") with the carbon of the carboxyl ("-oic acid") given the number 1
* all carbon-carbon bonds are single except for the double bond then joins carbon 9 ("9-en") to carbon 10
* the chain connects to each of the carbons of the double bond on the same side (hence, ''cis'', or "(9''Z'')" - the "''Z''" being an abbreviation for the German word [[zusammen]], meaning together).
IUPAC nomenclature can also handle branched chains and derivatives where hydrogen atoms are replaced by other chemical groups.  Triglycerides take formal IUPAC names according to the rule governing naming of esters.  For example, the formal name propane-1,2,3-tryl 1,2-bis((9''Z'')-octadec-9-enoate) 3-(hexadecanoate) applies to the pheromone informally named as glyceryl 1,2-dioleate-3-palmitate,<ref name=koen1983 /> and also known by other common names including 1,2-dioleoyl-3-palmitoylglycerol, glycerol dioleate palmitate, and 3-palmito-1,2-diolein.

===Fatty acid code===
A notation specific for fatty acids with unbranched chain, that is as precise as the IUPAC one but easier to parse, is a code of the form "{N}:{D} ''cis''-{CCC} ''trans''-{TTT}", where {N} is the number of carbons (including the carboxyl one), {D} is the number of double bonds, {CCC} is a list of the positions of the ''cis'' double bonds, and {TTT} is a list of the positions of the ''trans'' bonds.  Either or both ''cis'' and ''trans'' lists and their labels are omitted if there are no multiple bonds with that geometry.  For example, the codes for stearic, oleic, elaidic, and vaccenic acids are "18:0", "18:1 ''cis''-9", "18:1 ''trans''-9", and "18:1 ''trans''-11", respectively. [[Catalpic acid]], (9''E'',11''E'',13''Z'')-octadeca-9,11,13-trienoic acid according to IUPAC nomenclature, has the code "18:3 ''cis''-13 ''trans''-9,11".{{cn|date=November 2024}}

==Saturated and unsaturated fats==

For human nutrition, an important classification of fats is based on the number and position of [[double bond]]s in the constituent fatty acids.  '''Saturated fat''' has a predominance of [[saturated fatty acid]]s, without any double bonds, while '''unsaturated fat''' has predominantly [[unsaturated fatty acid|unsaturated acids]] with double bonds.  (The names refer to the fact that each double bond means two fewer hydrogen atoms in the chemical formula. Thus, a saturated fatty acid, having no double bonds, has the maximum number of hydrogen atoms for a given number of carbon atoms{{snd}}that is, it is "saturated" with hydrogen atoms.)<ref name=osu2014>{{cite web|title=Essential Fatty Acids|url=http://lpi.oregonstate.edu/mic/other-nutrients/essential-fatty-acids|publisher=Micronutrient Information Center, Oregon State University, Corvallis, OR|access-date=24 May 2017|date=May 2014}}</ref><ref name=mayo2017>{{cite web|title=Omega-3 fatty acids, fish oil, alpha-linolenic acid|url=http://www.mayoclinic.com/health/fish-oil/NS_patient-fishoil/|publisher=Mayo Clinic|date=2017|access-date=24 May 2017}}</ref>

Unsaturated fatty acids are further classified into '''[[monounsaturated fatty acid|monounsaturated]]''' (MUFAs), with a single double bond, and '''[[polyunsaturated fatty acid|polyunsaturated]]''' (PUFAs), with two or more.<ref name=osu2014/><ref name=mayo2017/>  Natural fats usually contain several different saturated and unsaturated acids, even on the same molecule. For example, in most vegetable oils, the saturated [[palmitic acid|palmitic]] (C16:0) and [[stearic acid|stearic]] (C18:0) [[acyl group|acid residues]] are usually attached to positions 1 and 3 (sn1 and sn3) of the glycerol hub, whereas the middle position (sn2) is usually occupied by an unsaturated one, such as [[oleic acid|oleic]] (C18:1, ω–9) or [[linoleic acid|linoleic]] (C18:2, ω–6).<ref name=ISEO2006>{{Cite web | url=http://www.iseo.org/foodfatsoils.pdf | title=Food Fats and oils | last=Institute of Shortenings and Edible oils | year=2006 | url-status=dead | archive-url=https://web.archive.org/web/20070326144952/http://www.iseo.org/foodfatsoils.pdf | archive-date=2007-03-26 | access-date=2009-02-19}}</ref>)

{| style="margin-left:1em;margin-right:auto;"
|-
| style="text-align:right;padding-right:1.3em;" | [[File:Stearic acid.svg|right|400x50px]]
| [[Stearic acid]] (saturated, C18:0)
|-
| style="text-align:right;padding-right:1.8em;" | [[File:Palmitoleic acid structure.png|right|400x50px]]
| [[Palmitoleic acid]] (mono-unsaturated, C16:1 ''cis''-9,  omega-7)
|-
| style="text-align:right;padding-right:1.8em;" | [[File:Oleic acid flat.svg|right|400x45px]]
| [[Oleic acid]] (mono-unsaturated, C18:1 ''cis''-9, omega-9)
|-
| style="text-align:right;padding-right:0.7em;" | [[File:Alpha-linolenic acid.svg|right|500x77px]]
| [[alpha-Linolenic acid|α-Linolenic acid]] (polyunsaturated, C18:3 ''cis''-9,12,15, omega-3)
|-
| style="text-align:right;padding-right:0.6em;" | [[File:Gamma-linolenic acid.svg|right|500x77px]]
| [[gamma-Linolenic acid|γ-Linolenic acid]] (polyunsaturated, C18:3 ''cis''-6,9,12, omega-6)
|}

Saturated fats generally have a higher melting point than unsaturated ones with the same molecular weight, and thus are more likely to be solid at room temperature.  For example, the animal fats [[tallow]] and [[lard]] are high in saturated fatty acid content and are solids.  Olive and linseed oils on the other hand are unsaturated and liquid.  Unsaturated fats are prone to [[oxidation]] by air, which causes them to become rancid and inedible.{{cn|date=November 2024}}

The double bonds in unsaturated fats can be converted into single bonds by reaction with hydrogen effected by a catalyst.  This process, called [[fat hydrogenation|hydrogenation]], is used to turn vegetable oils into solid or semisolid [[vegetable fat]]s like [[margarine]], which can substitute for tallow and butter and (unlike unsaturated fats) resist [[rancidification]]. Under some conditions, hydrogenation can creates some unwanted ''trans'' acids from ''cis'' acids.<ref>{{cite journal |last1=Marchand |first1=V |title=Trans fats: What physicians should know |journal=Canadian Paediatric Society |date=2010 |volume=6 |issue=15 |pages=373–375 |doi=10.1093/pch/15.6.373 |pmid=21731420 |pmc=2921725 }}</ref>

In cellular [[metabolism]], unsaturated fat molecules yield slightly less energy (i.e., fewer [[calories]]) than an equivalent amount of saturated fat. The heats of combustion of saturated, mono-, di-, and tri-unsaturated 18-carbon fatty acid esters have been measured as 2859, 2828, 2794, and 2750 kcal/mol, respectively; or, on a weight basis, 10.75, 10.71, 10.66, and 10.58 kcal/g{{snd}}a decrease of about 0.6% for each additional double bond.<ref name=kris1991>{{cite journal |doi=10.1007/BF02660311|title=Estimation of heat of combustion of triglycerides and fatty acid methyl esters|year=1991|last1=Krisnangkura|first1=Kanit|journal=Journal of the American Oil Chemists' Society|volume=68|pages=56–58|s2cid=84433984}}</ref>

The greater the degree of unsaturation in a fatty acid (i.e., the more double bonds in the fatty acid) the more vulnerable it is to [[lipid peroxidation]] ([[rancidification|rancidity]]). [[Antioxidant]]s can protect unsaturated fat from lipid peroxidation.

==Commercial applications==
While it is the ''nutritional'' aspects of polyunsaturated fatty acids that are generally of greatest interest, these materials also have non-food applications. 

Linseed oil and related oils are important components of useful products used in [[oil paint]]s and related coatings. Linseed oil is rich in di- and tri-unsaturated fatty acid components, which tend to harden in the presence of oxygen. This heat-producing hardening process is peculiar to these so-called ''drying oils''. It is caused by a [[polymerization]] process that begins with oxygen molecules attacking the carbon backbone.  Aside from llinseed oil, other oils exhibit drying properties and are used in more specialized applications.  These include [[tung oil|tung]], [[poppyseed oil|poppyseed]], [[perilla oil|perilla]], and [[walnut oil]].  All "[[polymerize]]" on exposure to [[oxygen]] to form solid films, useful in [[paint]]s and [[varnish]]es.<ref name=drying>{{cite book |doi=10.1002/14356007.a09_055 |chapter=Drying Oils and Related Products |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2001 |last1=Poth |first1=Ulrich |isbn=978-3-527-30385-4 }}</ref>

Triglycerides can also be split into methyl esters of the constituent fatty acids via [[transesterification]]: 
:{{chem2|RCO2CH(CH2O2CR')(CH2O2CR'')  +  3  CH3OH  ->  RCO2CH3 + R'CO2CH3 +  R''CO2CH3  +  HOCH(CH2OH)2}}
The resulting [[fatty acid methyl ester]]s can be used as fuel in [[diesel engine]]s, hence their name [[biodiesel]].

== Staining ==
[[Staining (biology)|Staining]] for fatty acids, triglycerides, lipoproteins, and other lipids is done through the use of [[lysochrome]]s (fat-soluble dyes).  These dyes can allow the qualification of a certain fat of interest by staining the material a specific color. Some examples: [[Sudan IV]], [[Oil Red O]], and [[Sudan Black B]].{{cn|date=November 2024}}

== Interactive pathway map ==
{{StatinPathway WP430|highlight=Triglyceride}}

== See also ==
* [[Diglyceride acyltransferase]], an enzyme that produces triglycerides
* [[Glycerol-3-phosphate acyltransferase]]s, enzymes involved in early step in biosynthesis of triglycerides
* [[Phosphatidic acid]]s, playing a role in biosynthesis of triglycerides
* [[Medium-chain triglycerides]]
* [[Lipid profile]]
* [[Lipid]]s
* [[Vertical auto profile]]
* [[Hypertriglyceridemia]], the presence of high amounts of triglycerides in the blood.

== References ==
{{Reflist|30em}}
== External links ==
{{Portal|Chemistry}}
* [https://www.emedicinehealth.com/how_can_i_lower_my_triglycerides_quickly/article_em.htm Lowering Triglycerides] (EMedicineHealth.com; October 2020)

{{Lipids}}
{{Lipoprotein metabolism}}
{{Lipid metabolism enzymes}}
{{Clinical biochemistry blood tests}}
{{Authority control}}

[[Category:Triglycerides| ]]
[[Category:Lipid disorders]]
[[Category:Esters]]