alpha-Linolenic acid

α-Linolenic acid
Names
Preferred IUPAC name
(9Z,12Z,15Z)-Octadeca-9,12,15-trienoic acid[1]
Other names
ALA; LNA; Linolenic acid; cis,cis,cis-9,12,15-Octadecatrienoic acid; (9Z,12Z,15Z)-9,12,15-Octadecatrienoic acid; Industrene 120
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.006.669
UNII
  • InChI=1S/C18H30O2/c1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16-17-18(19)20/h3-4,6-7,9-10H,2,5,8,11-17H2,1H3,(H,19,20)/b4-3-,7-6-,10-9- Y
    Key: DTOSIQBPPRVQHS-PDBXOOCHSA-N Y
  • InChI=1/C18H30O2/c1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16-17-18(19)20/h3-4,6-7,9-10H,2,5,8,11-17H2,1H3,(H,19,20)/b4-3-,7-6-,10-9-
    Key: DTOSIQBPPRVQHS-PDBXOOCHBH
  • O=C(O)CCCCCCC\C=C/C\C=C/C\C=C/CC
  • CC/C=C\C/C=C\C/C=C\CCCCCCCC(=O)O
Properties
C18H30O2
Molar mass 278.436 g·mol−1
Density 0.9164 g/cm3
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y verify (what is YN ?)
Infobox references
This article is about alpha-linolenic acid. For other uses, see Linolenic acid.

α-Linolenic acid (ALA), (from Greek 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.[2] In physiological literature, it is listed by its lipid number, 18:3, and (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 an isomer 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 linoleic acid's omega-6 double bond produces oleic 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. 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 linolenic acid, that shows up as sharp resonances in high-resolution carbon-13 NMR spectra, invariably.[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.

Properties of vegetable oils[7][8]
TypeProcessing
treatment[9]		Saturated
fatty acids		Monounsaturated
fatty acids		Polyunsaturated
fatty acids		Smoke point
Total[7]Oleic
acid
(ω-9)		Total[7]α-Linolenic
acid
(ω-3)		Linoleic
acid
(ω-6)		ω-6:3
ratio
Avocado[10]11.670.652–66[11]13.5112.512.5:1250 °C (482 °F)[12]
Brazil nut[13]24.832.731.342.00.141.9419:1208 °C (406 °F)[14]
Canola[15]7.463.361.828.19.118.62:1238 °C (460 °F)[14]
Coconut[16]82.56.361.7175 °C (347 °F)[14]
Corn[17]12.927.627.354.715858:1232 °C (450 °F)[18]
Cottonseed[19]25.917.81951.915454:1216 °C (420 °F)[18]
Flaxseed/linseed[20]9.018.41867.853130.2:1107 °C (225 °F)
Grape seed 10.514.314.3  74.7–74.7very high216 °C (421 °F)[21]
Hemp seed[22]7.09.09.082.022.054.02.5:1166 °C (330 °F)[23]
Olive[24]13.873.071.310.50.79.814:1193 °C (380 °F)[14]
Palm[25]49.337.0409.30.29.145.5:1235 °C (455 °F)
Peanut[26]16.257.155.419.90.31819.6very high232 °C (450 °F)[18]
Rice bran oil2538.42.234.4[27]15.6232 °C (450 °F)[28]
High-oleic safflower oil[29]7.575.275.212.8012.8very high212 °C (414 °F)[14]
Sesame[30]?14.239.739.341.70.341.3138:1
Soybean[31]partially hydrogenated14.943.042.537.62.634.913.4:1
Soybean[32]15.622.822.657.77517.3:1238 °C (460 °F)[18]
Walnut oil[33]unrefined9.122.822.263.310.452.95:1160 °C (320 °F)[34]
Sunflower[35]8.9963.462.920.70.1620.5very high227 °C (440 °F)[18]
Cottonseed[36]hydrogenated93.61.50.60.20.31.5:1
Palm[37]hydrogenated88.25.70
The nutritional values are expressed as percent (%) by mass of total fat.

Potential role in nutrition and health
Flax is a rich source of α-linolenic acid.

Although the best source of ALA is seeds, most seeds and seed oils are much richer in an n−6 fatty acid, linoleic acid. Exceptions include flaxseed (must be ground for proper nutrient absorption) and chia seeds. Linoleic acid is the other essential fatty acid, but it, and the other n−6 fatty acids, compete with n−3s for positions in cell membranes and have very different effects on human health. There is a complex set of essential fatty acid interactions.

α-Linolenic acid can only be obtained by humans 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 can also be synthesized by humans from dietary α-linolenic acid: ALA → stearidonic acid → eicosatetraeonic acid → eicosapentaenoic acid → docosapentaenoic acid → 9,12,15,18,21-tetracosapentaenoic acid → 6,9,12,15,18,21-yetracosahexaenoic acid → docosahexaenoic acid, but with an efficiency of only a few percent.[38] 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.[39][40] Conversion of ALA to DHA is higher in women than in men.[41]

Stability and hydrogenation

α-Linolenic acid is relatively more susceptible to oxidation and will become rancid more quickly than many other oils. Oxidative instability of α-linolenic acid is one reason why producers choose to partially hydrogenate oils containing α-linolenic acid, such as soybean oil.[42] 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.[43]

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.[44]

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.[45] 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.

Health

According to a 2012 review, higher ALA consumption is associated with a moderately lower risk of cardiovascular disease, but wide variation in results across multiple studies highlights the need for additional research before drawing firm conclusions.[46]

Dietary ALA intake can improve lipid profiles by decreasing triglycerides, total cholesterol, high-density lipoprotein and low-density lipoprotein cholesterol.[47] 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.[48]

History

In 1887, linolenic acid was discovered and named by the Austrian chemist Karl Hazura of the Imperial Technical Institute at Vienna (although he didn't separate its optical isomers).[49] 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,[50] and by Adolf Rollett of the Universität Berlin, Germany,[51] working independently, as cited in J. W. McCutcheon's synthesis in 1942,[52] and referred to in Green and Hilditch's 1930s survey.[53] 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.[54]

See also

References
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  3. Chapman, David J.; De-Felice, John; Barber, James (May 1983). "Growth temperature effects on thylakoid membrane lipid and protein content of pea chloroplasts 1". Plant Physiol. 72 (1): 225–228. doi:10.1104/pp.72.1.225. PMC 1066200. PMID 16662966.
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  12. Marie Wong; Cecilia Requejo-Jackman; Allan Woolf (April 2010). "What is unrefined, extra virgin cold-pressed avocado oil?". Aocs.org. The American Oil Chemists’ Society. Retrieved 26 December 2019.
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  14. Katragadda, H. R.; Fullana, A. S.; Sidhu, S.; Carbonell-Barrachina, Á. A. (2010). "Emissions of volatile aldehydes from heated cooking oils". Food Chemistry. 120: 59–65. doi:10.1016/j.foodchem.2009.09.070.
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  17. "Corn oil, industrial and retail, all purpose salad or cooking, fat composition, 100 g". US National Nutrient Database, Release 28, United States Department of Agriculture. May 2016. Retrieved 6 September 2017.
  18. Wolke, Robert L. (16 May 2007). "Where There's Smoke, There's a Fryer". The Washington Post. Retrieved 5 March 2011.
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  32. "Soybean oil, salad or cooking, fat composition, 100 g". US National Nutrient Database, Release 28, United States Department of Agriculture. May 2016. Retrieved 6 September 2017.
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  44. Monsanto. "ADM to process Monsanto's Vistive low linolenic soybeans at Indiana facility". Archived from the original on 11 December 2006. Retrieved 6 January 2007.
  45. Kinney, Tony. "Metabolism in plants to produce healthier food oils" (PDF). Archived from the original (PDF) on 29 September 2006. Retrieved 11 January 2007.
  46. Pan A, Chen M, Chowdhury R, et al. (December 2012). "α-Linolenic acid and risk of cardiovascular disease: a systematic review and meta-analysis". Am. J. Clin. Nutr. (Systematic review). 96 (6): 1262–73. doi:10.3945/ajcn.112.044040. PMC 3497923. PMID 23076616.
  47. Hao Yue, Bin Qiu, Min Jia, Wei Liu, Xiao-fei Guo, Na Li, Zhi-xiang Xu, Fang-ling Du, Tongcheng Xu, Duo Li (2020). "Effects of α-linolenic acid intake on blood lipid profiles：a systematic review and meta-analysis of randomized controlled trials". Critical Reviews in Food Science and Nutrition. 61 (17): 2894–2910. doi:10.1080/10408398.2020.1790496.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  48. Naghshi S, Aune D, Beyene J, Mobarak S, Asadi M, Sadeghi O (2021). "Research Dietary intake and biomarkers of alpha linolenic acid and risk of all cause, cardiovascular, and cancer mortality: systematic review and dose-response meta-analysis of cohort studies". The BMJ. 375: n2213. doi:10.1136/bmj.n2213. PMID 34645650.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  49. 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. S2CID 197767239. 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.
  50. See:
    • 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. 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. The structure of α-linolenic acid appears on p. 1343.
  51. Rollett, A. (1909). "Zur Kenntnis der Linolensäure und des Leinöls" [[Contribution to our] knowledge of linolenic acid and flax oil]. Zeitschrift für physiologische Chemie. 62 (5–6): 422–431. doi:10.1515/bchm2.1909.62.5-6.422.
  52. J. W. McCutcheon (1955). "Linolenic acid". Organic Syntheses.; Collective Volume, vol. 3, p. 351
  53. Green, TG; Hilditch, TP (1935). "The identification of linoleic and linolenic acids". Biochem. J. 29 (7): 1552–63. doi:10.1042/bj0291552. PMC 1266662. PMID 16745822.
  54. Sandri, J.; Viala, J. (1995). "Direct preparation of (Z,Z)-1,4-dienic units with a new C6 homologating agent: synthesis of alpha-linolenic acid". Synthesis. 1995 (3): 271–275. doi:10.1055/s-1995-3906.
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