In terms of its
structure, it is named all-cis-9,12,15-octadecatrienoic acid.[2] 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 cisdouble 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
polyunsaturatedn−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).
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.
Seed oils are the richest sources of α-linolenic acid, notably those of hempseed,
chia,
perilla,
flaxseed (
linseed oil),
rapeseed (
canola), and
soybeans. α-Linolenic acid is also obtained from the
thylakoid membranes in the leaves of Pisum sativum (pea leaves).[3] 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.[4] Some studies state that ALA remains stable during processing and cooking.[5] 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.[6] ALA percentages in the table below refer to the oils extracted from each item.
α-Linolenic acid can be obtained by humans only through their diets, because the absence of the required 12- and 15-desaturase enzymes makes de novo synthesis from
stearic acid impossible.
Eicosapentaenoic acid (EPA; 20:5, n−3) and
docosahexaenoic acid (DHA; 22:6, n−3) are readily available from fish and algae oil, and play a vital role in many metabolic processes. These also can be synthesized by humans from dietary α-linolenic acid: ALA →
stearidonic acid →
eicosatetraenoic acid →
eicosapentaenoic acid →
docosapentaenoic acid → tetracosapentaenoic acid →
6,9,12,15,18,21-tetracosahexaenoic acid →
docosahexaenoic acid, but with an efficiency of only a few percent.[12]
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.[13] Conversion of ALA to DHA is higher in women than in men.[14]
Stability and hydrogenation
Compared to many other oils, α-linolenic acid is more susceptible to oxidation and will become rancid more quickly.
Oxidative instability of α-linolenic acid is one reason why producers choose to partially
hydrogenate oils containing α-linolenic acid, such as
soybean oil.[15]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.[16]
However, when partially hydrogenated, part of the unsaturated fatty acids become unhealthy
trans fats. Consumers are increasingly avoiding products that contain trans fats, and governments have begun to ban trans fats in food products. These regulations and market pressures have spurred the development of low-α-linolenic acid soybeans. These new soybean varieties yield a more stable oil that doesn't require hydrogenation for many applications, thus providing trans fat-free products, such as frying oil.[17]
Several consortia are bringing low-α-linolenic acid soy to market.
DuPont's effort involves
silencing the FAD2 gene that codes for
Δ6-desaturase, giving a soy oil with very low levels of both α-linolenic acid and linoleic acid.[15]Monsanto Company has introduced to the market
Vistive, their brand of low α-linolenic acid soybeans, which is less controversial than
GMO offerings, as it was created via conventional breeding techniques.[citation needed]
Health
ALA consumption is associated with a lower risk of
cardiovascular disease and a reduced risk of fatal coronary heart disease.[18][19] Dietary ALA intake can improve lipid profiles by decreasing
triglycerides,
total cholesterol,
high-density lipoprotein, and
low-density lipoprotein.[20] 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.[21]
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).[22] 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,[23] and by Adolf Rollett of the
Universität Berlin, Germany,[24] working independently, as cited in J. W. McCutcheon's synthesis in 1942,[25] and referred to in Green and Hilditch's 1930s survey.[26] 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.[27]
^Loreau O, Maret A, Poullain D, Chardigny JM, Sébédio JL, Beaufrère B, Noël JP (2000). "Large-scale preparation of (9Z,12E)-[1-13C]-octadeca-9,12-dienoic acid, (9Z,12Z,15E)-[1-13C]-octadeca-9,12,15-trienoic acid and their 1-13C all-cis isomers". Chemistry and Physics of Lipids. 106 (1): 65–78.
doi:
10.1016/S0009-3084(00)00137-7.
PMID10878236.
^Manthey FA, Lee RE, Hall CA (2002). "Processing and cooking effects on lipid content and stability of alpha-linolenic acid in spaghetti containing ground flaxseed". J. Agric. Food Chem. 50 (6): 1668–71.
doi:
10.1021/jf011147s.
PMID11879055.
^Hazura K (1887).
"Über trocknende Ölsäuren IV. Abhandlung" [On drying oily acids 4th paper]. Monatshefte für Chemie (in German). 8: 260–270.
doi:
10.1007/BF01510049.
S2CID197767239.
Archived from the original on 18 January 2021. Retrieved 1 November 2020. Linolenic acid is named on p. 265: "Für die Säure C18H32O2 schlage ich den Namen Linolsäure, für die Säure C18H30O2 den Namen Linolensäure vor." (For the acid C18H32O2 I suggest the name "linolic acid"; for the acid C18H30O2 [I suggest] the name "linolenic acid".) Linolenic acid is discussed on pp. 265-268.
Erdmann E, Bedford F (1909).
"Über die im Leinöl enthaltene Linolensäure" [On linolenic acid [that's] contained in flax oil]. Berichte der Deutschen Chemischen Gesellschaft (in German). 42: 1324–1333.
doi:
10.1002/cber.190904201217.
Archived from the original on 26 January 2021. Retrieved 31 October 2020. 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.)
Erdmann E, Bedford F, Raspe F (1909).
"Konstitution der Linolensäure" [Structure of linolenic acid]. Berichte der Deutschen Chemischen Gesellschaft (in German). 42: 1334–1346.
doi:
10.1002/cber.190904201218.
Archived from the original on 1 February 2021. Retrieved 31 October 2020. The structure of α-linolenic acid appears on p. 1343.
^Sandri, J., Viala, J. (1995). "Direct preparation of (Z,Z)-1,4-dienic units with a new C6 homologating agent: synthesis of α-linolenic acid". Synthesis. 1995 (3): 271–275.
doi:
10.1055/s-1995-3906.
S2CID196696819.