α-Linolenic acid Names Preferred IUPAC name Other names Identifiers ChEBI ChEMBL ChemSpider DrugBank ECHA InfoCard 100.006.669 UNII Properties C18H30O2 Molar mass 278.436 g·mol−1 Density 0.9164 g/cm3 Melting point −11 °C (12 °F; 262 K)[2] Boiling point 232 °C (450 °F; 505 K) at 17.0 mmHg[2] Solubility in acetonitrile 309.057 g/L (-34.5 °C)[3]
α-Linolenic acid, also known as alpha-linolenic acid (ALA) (from Greek alpha denoting “first” and linon meaning flax), is an n−3, or omega-3, essential fatty acid. ALA is found in many seeds and oils, including flaxseed, walnuts, chia, hemp, and many common vegetable oils.
In terms of its structure, it is named all–cis-9,12,15-octadecatrienoic acid.[4] In physiological literature, it is listed by its lipid number, 18:3 (n−3). It is a carboxylic acid with an 18-carbon chain and three cis double bonds. The first double bond is located at the third carbon from the methyl end of the fatty acid chain, known as the n end. Thus, α-linolenic acid is a polyunsaturated n−3 (omega-3) fatty acid. It is a regioisomer of gamma-linolenic acid (GLA), an 18:3 (n−6) fatty acid (i.e., a polyunsaturated omega-6 fatty acid with three double bonds).
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.
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.[5]
Some studies state that ALA remains stable during processing and cooking.[6] However, other studies state that ALA might not be suitable for baking as it will polymerize with itself, a feature exploited in paint with transition metal catalysts. Some ALA may also oxidize at baking temperatures.[7] ALA percentages in the table below refer to the oils extracted from each item.
Common name Alternate name Linnaean name % ALA†(of oil) ref. Chia chia sage Salvia hispanica 64% [8] Kiwifruit seeds Chinese gooseberry Actinidia chinensis 62% [8] Perilla shiso Perilla frutescens 58% [8] Flax linseed Linum usitatissimum 55% [8] Lingonberry cowberry Vaccinium vitis-idaea 49% [8] Camelina camelina Camelina sativa 37% [9] Purslane portulaca Portulaca oleracea 35% [8] Cuckoo flower mayflower Cardamine pratensis 35% [10] Cranberry American cranberry Vaccinium macrocarpon 35% [10] Sea buckthorn seaberry Hippophae rhamnoides L. 32% [11] Raspberry raspberry Rubus idaeus 31% [10] Blueberry bilberry Vaccinium myrtillus L. 29% [10] Hemp cannabis Cannabis sativa 20% [8] Walnut English walnut / Persian walnut Juglans regia 10.4% [12] Rapeseed canola Brassica napus 10% [4] Soybean soya Glycine max 8% [4] †average value
α-Linolenic acid is also obtained from the thylakoid membranes in the leaves of Pisum sativum (pea leaves).[13] Plant chloroplasts consisting of more than 95 percent of photosynthetic thylakoid membranes are highly fluid due to the large abundance of ALA, evident as sharp resonances in high-resolution carbon-13 NMR spectra.[14]
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.[15]
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).[16] 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.[17] Conversion of ALA to DHA is higher in women than in men. This is likely due to the increased need for DHA in women in order to support a developing fetus and in producing breastmilk containing DHA.[18]
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.[19]
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.[20] Soybeans are the largest source of edible oils in the U.S., and, as of a 2007 study, 40% of soy oil production was partially hydrogenated.[21]
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.[22]
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.[23] 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.[22]
ALA is an essential fatty acid, meaning consumption of ALA in the diet is required for human life,[19] along with all other mammals.[18] ALA consumption is associated with a lower risk of cardiovascular disease and a reduced risk of fatal coronary heart disease.[15][5] Dietary ALA intake can improve lipid profiles by decreasing triglycerides, total cholesterol, high-density lipoprotein, and low-density lipoprotein.[24] 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.[25]
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).[26] 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,[27] and by Adolf Rollett of the Universität Berlin, Germany,[28] working independently, as cited in J. W. McCutcheon’s synthesis in 1942,[29] and referred to in Green and Hilditch’s 1930s survey.[30] 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.[31]
- Canola oil
- Flax seed oil
- γ-Linolenic acid
- Drying oil
- Essential fatty acid
- List of n−3 fatty acids
- Essential nutrient
- Wheat germ oil
