Linoleic acid

Linoleic acid (LA) is an organic compound with the formula Template:Chem2. Both alkene groups (Template:Chem2) are cis. It is a fatty acid sometimes denoted 18:2 (n−6) or 18:2 cis-9,12. A linoleate is a salt or ester of this acid.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Linoleic acid is a polyunsaturated, omega−6 fatty acid. It is a colorless liquid that is virtually insoluble in water but soluble in many organic solvents.<ref name="crc97"/> It typically occurs in nature as a triglyceride (ester of glycerin) rather than as a free fatty acid.<ref name="Is there a fatty acid taste">Template:Cite journal</ref> It is one of two essential fatty acids for humans, who must obtain it through their diet,<ref>Template:Cite journal</ref> and the most essential, because the body uses it as a base to make the others.

The word "linoleic" derives Template:Ety, reflecting the fact that it was first isolated from linseed oil.

History

In 1844, F. Sacc, working at the laboratory of Justus von Liebig, isolated linoleic acid from linseed oil.<ref name=sacc1844/><ref name=sacc1845/> In 1886, K. Peters determined the existence of two double bonds.<ref name= Peters >Template:Cite journal</ref> Its essential role in human diet was discovered by G. O. Burr and others in 1930.<ref name=burr1930/> Its chemical structure was determined by T. P. Hilditch and others in 1939, and it was synthesized by R. A. Raphael and F. Sondheimer in 1950.<ref name=raph1950/>

In physiology

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} The consumption of linoleic acid is vital to proper health, as it is an essential fatty acid.<ref name="Whelan & Fritsche">Template:Cite journal</ref>

Metabolism and eicosanoids

File:Linoleic Acid Metabolism.gifTemplate:Clear left Linoleic acid (LA: Template:Chem; 18:2,n−6) is a precursor to arachidonic acid (AA: Template:Chem; 20:4,n−6) with elongation and unsaturation.Template:R AA is the precursor to some prostaglandins,<ref>Template:Cite journal</ref> leukotrienes (LTA, LTB, LTC), thromboxane (TXA)<ref>Template:Cite journal</ref> and the N-acylethanolamine (NAE) arachidonoylethanolamine (AEA: Template:Chem; 20:4,n−6),<ref>Template:Cite journal</ref> and other endocannabinoids and eicosanoids.<ref>Template:Cite journal</ref>

The metabolism of LA to AA begins with the conversion of LA into gamma-linolenic acid (GLA), effected by Δ6 desaturase.<ref name="David F. Horrobin 1993">Evidence suggests that infants must acquire Δ6-desaturase breast milk. Breast-milk fed babies have higher concentrations of GLA than formula-fed babies, while formula-fed babies have elevated concentrations of LA. Template:Cite journal</ref> GLA is converted to dihomo-γ-linolenic acid (DGLA), the immediate precursor to AA.

LA is also converted by various lipoxygenases, cyclooxygenases, cytochrome P450 enzymes (the CYP monooxygenases), and non-enzymatic autoxidation mechanisms to mono-hydroxyl products viz., 13-Hydroxyoctadecadienoic acid, and 9-Hydroxyoctadecadienoic acid; these two hydroxy metabolites are enzymatically oxidized to their keto metabolites, 13-oxo-octadecadienoic acid and 9-oxo-octadecdienoic acid. Certain cytochrome P450 enzymes, the CYP epoxygenases, catalyze oxidation of LA to epoxide products viz., its 12,13-epoxide, vernolic acid, and its 9,10-epoxide, coronaric acid. These linoleic acid products are implicated in human physiology and pathology.<ref>Template:Cite journal</ref>

Hydroperoxides derived from the metabolism of anandamide (AEA: Template:Chem; 20:4,n−6), or its linoleoyl analogues, are by a lipoxygenase action found to be competitive inhibitors of brain and immune cell FAAH, the enzyme that breaks down AEA and other endocannabinoids, and the compound linoleoyl-ethanol-amide (Template:Chem; 18:2,n−6), an N-acylethanolamine,Template:Clarify - the ethanolamide of linoleic acid (LA: Template:Chem; 18:2,n−6) and its metabolized incorporated ethanolamine (MEA: Template:Chem),<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> is the first natural inhibitor of FAAH, discovered.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Uses and reactions

Linoleic acid is a component of quick-drying oils, which are useful in oil paints and varnishes. These applications exploit the lability of the doubly allylic Template:Chem2 groups (Template:Chem2) toward oxygen in air (autoxidation). Addition of oxygen leads to crosslinking and formation of a stable film.<ref>Template:Ullmann</ref>

Reduction of the carboxylic acid group of linoleic acid yields linoleyl alcohol.<ref>Template:Cite journal</ref>

Linoleic acid is a surfactant with a critical micelle concentration of 1.5 x 10⁻⁴ M @ pH 7.5.Template:Citation needed

Linoleic acid has become increasingly popular in the beauty products industry because of its beneficial properties on the skin. Research points to linoleic acid's anti-inflammatory, acne reductive, skin-lightening and moisture retentive properties when applied topically on the skin.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Linoleic acid is also used in some bar of soap products.

Dietary sources

Template:See also It is abundant in safflower, and corn oil, and comprises over half their composition by weight. It is present in medium quantities in soybean oils, sesame, and almonds.<ref name="USDA">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite journal</ref>

Name	% LA†	ref.
Salicornia oil	75%	<ref name= Alfheeaid >Template:Cite journal</ref>
Poppyseed oil	74%	<ref name="Bozan">Template:Cite journal</ref>
Safflower oil	72–78%	<ref name="Hall2015">Template:Cite book</ref>
Grape seed oil	70%	<ref name="Martin">Template:Cite journal</ref>
Evening Primrose oil	65–80%	citation	CitationClass=web }}</ref>
Cardoon oil	60%	<ref name="Varvouni">Template:Cite journal</ref><ref name="Barbosa">Template:Cite journal</ref>
Wheat germ oil	56%	<ref name="Rahim">Template:Cite journal</ref><ref name="Siraj">Template:Cite journal</ref>
Hemp oil	54.3%	<ref>Template:Cite journal</ref>
Cottonseed oil	54%	<ref name="Lukonge">Template:Cite journal To download the article, click the title (not doi) and choose option [PDF] academia.edu.</ref><ref name="Yang">Template:Cite journal</ref>
Corn oil	51.9%	citation	CitationClass=web }}</ref>
Prickly Pear seed oil	50–78%	<ref name="Al-Naqeb">Template:Cite journal</ref>
Walnut oil	50–72%	<ref name="Masoodi">Template:Cite journal</ref><ref name="Walnut">{{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
Melon seed oil	50–70%	<ref name="Cucumis melo">Template:Cite journal</ref>
Soybean oil	50.9%	citation	CitationClass=web }}</ref>
Sesame oil	45%	citation	CitationClass=web }}</ref><ref name= Oboulbiga >Template:Cite journal</ref>
Pumpkin seed oil	42–59%	<ref>Template:Cite journal</ref>
Rice bran oil	39%
Argan oil	37%
Pistachio oil	32.7%
Peach oil	29%	<ref name=sdpk>Template:Cite journal</ref>
Almonds	24%
Sunflower oil	20.5%	citation	CitationClass=web }}</ref>
Peanut oil	19.6%	citation	CitationClass=web }}</ref>
Chicken fat	18–23%	<ref>Template:Cite journal</ref>
Canola oil	17.8%	citation	CitationClass=web }}</ref>
Egg yolk	16%
Linseed oil (flax), cold pressed	14.2%	citation	CitationClass=web }}</ref>
Lard	10%
Palm oil	10%
Olive oil	8.4%	citation	CitationClass=web }}</ref>
Tallow	3%
Cocoa butter	3%
Macadamia oil	2%
Butter	2%
Coconut oil	2%
	†average value, except the items where a range is given

Other occurrences

Cockroaches release oleic and linoleic acid upon death, which discourages other roaches from entering the area. This is similar to the mechanism found in ants and bees, which release oleic acid upon death.<ref name="BBC">Template:Cite news</ref>

Related compounds

While polyunsaturated fatty acids are unusual in plant cuticles, a diunsaturated dicarboxylic acid has been reported as a component of the surface waxes or polyesters of some plant species. Thus, Template:Chem name, a derivative of linoleic acid, is present in Arabidopsis and Brassica napus cuticle.<ref>Template:Cite journal</ref> Taxoleic acid is isomeric to linoleic acid.

Health effects

Consumption of linoleic acid has been associated with lowering the risk of cardiovascular disease, diabetes and premature death.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> There is high-quality evidence that increased intake of linoleic acid decreases total blood cholesterol and low-density lipoprotein.<ref>"Systematic review of the evidence for relationships between saturated, cis monounsaturated, cis polyunsaturated fatty acids and selected individual fatty acids, and blood cholesterol concentration". foodstandards.gov.au. Retrieved 10 January 2023.</ref> Higher in vivo circulating and tissue levels of linoleic acid are associated with a lower risk of major cardiovascular events.<ref>Template:Cite journal</ref> Clinical trials have shown that increased linoleic acid intake does not increase markers of inflammation or oxidative stress.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

The American Heart Association advises people to replace saturated fat with linoleic acid to reduce CVD risk.<ref>Template:Cite journal</ref>

See also

• Conjugated linoleic acid
• Essential fatty acid interactions
• Essential nutrients
• Laballenic acid
• Linolein

References

<references>

<ref name=sacc1844>F. Sacc (1844). "Ueber das Leinöl, seine physicalischen und chemischen Eigenschaften und seine Oxydationsproducte". Liebigs Annalen, volume 51, issue 2, pages 213–230. {{#invoke:doi|main}}.</ref>

<ref name=sacc1845>F. Sacc (1845). "Expériences sur les propriétés physiques et chimiques de l'huile de Lin". Neue Denkschriften der Allg. Schweizerischen Gesellschaft für die Gesammten Naturwissenschaften, volume 7, pages 191–208 in pdf.</ref>

<ref name=burr1930>Template:Cite journal</ref>

<ref name=raph1950>Template:Cite journal</ref>

</references>

Further reading

• {{#invoke:citation/CS1|citation

|CitationClass=web }}

External links

• Linoleic acid MS Spectrum
• Fatty Acids: Methylene-Interrupted Double Bonds, AOCS Lipid Library