Editing Triglyceride (section)

==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.