Α-Linolenic acid

Etymology

The word linolenic is an irregular derivation from linoleic, which itself is derived from the Greek word linon (flax). Oleic means "of or relating to oleic acid" because saturating an omega-6 double bond of linoleic acid produces oleic acid. Similarly saturating one of linolenic acid's double bonds produces linoleic acid.

Dietary sources

Seed oils are the richest sources of α-linolenic acid, notably those of hempseed, chia, perilla, flaxseed (linseed oil), rapeseed (canola), and soybeans. Additionally, small amounts of α-linolenic acid can be obtained through the consumption of meats such as chicken. α-Linolenic acid is also obtained from the thylakoid membranes in the leaves of Pisum sativum (pea leaves).{{cite journal

Metabolism

α-Linolenic acid can be obtained by humans only through their diets. Humans lack the Δ-12 and Δ-15 desaturase enzymes required for processing stearic acid into A-linoleic acid or other unsaturated fatty acids.

Dietary α-linolenic acid is metabolized to stearidonic acid, a precursor to a collection of polyunsaturated 20-, 22-, 24-, etc fatty acids (eicosatetraenoic acid, eicosapentaenoic acid, docosapentaenoic acid, tetracosapentaenoic acid, 6,9,12,15,18,21-tetracosahexaenoic acid, docosahexaenoic acid).{{cite journal |author1=Breanne M Anderson |author2=David WL Ma | year = 2009 Because the efficacy of n−3 long-chain polyunsaturated fatty acid (LC-PUFA) synthesis decreases down the cascade of α-linolenic acid conversion, DHA synthesis from α-linolenic acid is even more restricted than that of EPA.{{cite journal

Biosynthesis

α-Linolenic acid is synthesized starting from stearic acid, which itself is synthesized through a general fatty acid synthesis pathway. Stearic acid is subsequently converted into oleic acid by Δ-9-desaturase. Then, oleic acid is converted into linoleic acid by Δ-12-desaturase. Finally, linoleic acid is converted into α-Linolenic acid by Δ-15-desaturase.

Stability and hydrogenation

Compared to many other oils, α-linolenic acid is more susceptible to oxidation. It becomes rancid more quickly in air. Oxidative instability of α-linolenic acid is one reason why producers choose to partially hydrogenate oils containing α-linolenic acid, such as soybean oil.{{cite web |access-date=2007-01-11 |archive-url=https://web.archive.org/web/20060929134832/http://www.metabolicengineering.gov/me2005/Kinney.pdf |archive-date=29 September 2006

The source of susceptibility to oxidation in α-linolenic acid is presence of four bis-allylic hydrogen atoms. The Gibbs free energy of abstracting these hydrogens is uniquely low due to the potential for radical delocalization in the lipid radical product. This potential for delocalization confers a stabilizing effect that makes the overall oxidation pathway more favorable. Additionally, α-linolenic acid has four bis-allylic hydrogens compared to just 2 in linoleic acid, making abstraction more likely.

Hydrogenation of ALA-containing fats can introduce trans fats. Consumers are increasingly avoiding products that contain trans fats, and governments have begun to ban trans fats in food products, including the US government as of May 2018. These regulations and market pressures have spurred the development of soybeans low in α-linolenic acid. These new soybean varieties yield a more stable oil that often do not require hydrogenation for many applications.{{cite web |access-date = 2007-01-06 |archive-url = https://web.archive.org/web/20061211071206/http://www.monsanto.com/monsanto/layout/media/06/01-12-06.asp |archive-date = 11 December 2006

Health

ALA is an essential fatty acid, meaning consumption of ALA in the diet is required for human life, along with all other mammals. ALA consumption is associated with a lower risk of cardiovascular disease and a reduced risk of fatal coronary heart disease. Dietary ALA intake can improve lipid profiles by decreasing triglycerides, total cholesterol, high-density lipoprotein, and low-density lipoprotein. A 2021 review found that ALA intake is associated with a reduced risk of mortality from all causes, cardiovascular disease, and coronary heart disease but a slightly higher risk of cancer mortality.

History

In 1887, linolenic acid was discovered and named by the Austrian chemist Karl Hazura of the Imperial Technical Institute at Vienna (although he did not separate its isomers). α-Linolenic acid was first isolated in pure form in 1909 by Ernst Erdmann and F. Bedford of the University of Halle an der Saale, Germany,See:

• On p. 1329 they distinguish one of the isomers of linolenic acid: "Wir bezeichnen diese in Leinöl vorhandene Linolensäure, welche das feste Hexabromid liefert, zum Unterschied von einer später zu erwähnenden Isomeren als α-Linolensäure." (We designate this linolenic acid, which the solid hexabromide [of linolenic acid] provides, as α-linolenic acid in order to distinguish [it] from an isomer [that will be] mentioned later.)
• The structure of α-linolenic acid appears on p. 1343. and by Adolf Rollett of the Universität Berlin, Germany, working independently, as cited in J. W. McCutcheon's synthesis in 1942, and referred to in Green and Hilditch's 1930s survey. It was first artificially synthesized in 1995 from C6 homologating agents. A Wittig reaction of the phosphonium salt of [(Z-Z)-nona-3,6-dien-1-yl]triphenylphosphonium bromide with methyl 9-oxononanoate, followed by saponification, completed the synthesis.

References

References

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