{{Infobox Chemikalie
 | Strukturformel      = 330px
 | Andere Namen        = 
* (all-''cis'')-Octadeca-9,12,15-triensäure, 
* 9c,12c,15c-Octadecatriensäure, 
* Linolensäure
* ALA
 | Summenformel        = C<sub>18</sub>H<sub>30</sub>O<sub>2</sub>
 | CAS                 = 463-40-1
 | Beschreibung        = farblose Flüssigkeit
 | Molare Masse        = 278,43 g·mol<sup>–1</sup>
 | Aggregat            = flüssig
 | Dichte              = 0,92 g·cm<sup>–3</sup><ref name="GESTIS"></ref>
 | Schmelzpunkt        = –11 °C<ref name="GESTIS"/>
 | Siedepunkt          = 232 °C (23 hPa)<ref name="GESTIS"/>
 | Dampfdruck          = 
 | Löslichkeit         = unlöslich in Wasser<ref name="GESTIS"/>, gut löslich in organischen Lösungsmitteln
 | Quelle GefStKz      =<ref name="GESTIS"/>
 | Gefahrensymbole     = 
 | R                   = 
 | S                   = 
 | MAK                 = nicht festgelegt
}}
'''alpha-Linolensäure''' (&alpha;-''Linolensäure'', oft auch nur ''Linolensäure'' genannt) ist eine dreifach ungesättigte Fettsäure mit 18 Kohlenstoffatomen. Sie ist eine (all-cis)-Octadeca-9,12,15-triensäure, Kurzbezeichnung C18:3(9c,12c,15c) und gehört zur Gruppe der Omega-3-Fettsäuren. 

Mehrere andere Fettsäuren tragen den Begriff ''Linolensäure'' im Namen, unterscheiden sich jedoch erheblich. Die  Dihomogamma''linolensäure'' ((all-cis)-Eicosa-8,11,14-triensäure) besteht aus 20 Kohlenstoffatomen. Linolensäure ist leicht mit Linolsäure zu verwechseln.

== Geschichte ==
Der Name ''Linolensäure'' leitet sich vom griechischen Wort ''linos'' für Lein (Flachs) ab.

==Vorkommen==
Linolensäure ist ein chemischer Bestandteil vieler Triglyceride, welche den Hauptanteil der natürlichen Fette und Öle ausmachen. Eine ganze Reihe natürlich gewonnener pflanzlicher Öle ist reich an Linolensäure. Hierzu gehören unter anderem:
*Leinöl (ca. 50%), 
*Hanföl (ca. 17% alpha-Linolensäure und ca. 4% gamma-Linolensäure)
*Walnussöl (ca. 15%)
*Rapsöl (ca. 9%)
*Sojaöl (ca. 8%)
*Chiaöl (möglicherweise höchster Gehalt überhaupt mit 50-65%).

== Gewinnung und Darstellung ==
Die Fettsäure lässt sich durch alkalische Verseifung aus den entsprechenden Triglyceriden gewinnen, indem die entsprechenden Fette oder Öle mit Alkalien gekocht werden. Da die natürlichen Fette und Öle stets viele unterschiedliche Fettsäuren enthalten, schließt sich in der Regel eine Trennung des entstandenen Gemisches an.

== Eigenschaften ==
=== Physikalische Eigenschaften ===
Sorgfältig gereinigte und unter Luftausschluss aufbewahrte Linolensäure ist eine farblose, ölige und fast geruchlose Flüssigkeit.

=== Chemische Eigenschaften ===
Linolensäure ist sehr oxidationsempfindlich und altert an der Luft rasch unter Gelbfärbung, die auf die Bildung von Hydroperoxiden zurückzuführen ist. Im weiteren Verlauf der Oxidation kommt es zur Verharzung unter Bildung von Firnis, dieser Vorgang wird auch als ''Trocknung'' bezeichnet.

{{Chembox new
|   Name = α-Linolenic acid
|   ImageFile = Linolenic-acid-3D-vdW.png
|   ImageFile1 = ALAnumbering.png
|   ImageSize1 = 350px
|   OtherNames = ALA; Linolenic acid; cis, cis,cis-9,12,15-Octadecatrienoic acid; (Z,Z,Z)-9,12,15-Octadecatrienoic acid; Industrene 120
| Section1 = {{Chembox Identifiers
|   CASNo = 463-40-1
  }}
| Section2 = {{Chembox Properties
|   Formula = C<sub>18</sub>H<sub>30</sub>O<sub>2</sub>
|   MolarMass = 278.43 g//mol
|   Density =
|   MeltingPt = 
|   BoilingPt = 
  }}
}}

'''α-Linolenic acid''' ('''ALA''') is an organic compound found in many common vegetable oils.  Systematically, it is named ''all''-''cis''-9,12,15-octadecatrienoic acid.<ref name=Beare>{{cite web
|author=Beare-Rogers
|title=IUPAC Lexicon of Lipid Nutrition
|year=2001
|url=http://www.iupac.org/publications/pac/2001/pdf/7304x0685.pdf
|accessmonthday=22 February | accessyear=2006
|format=pdf}}</ref> In physiological literature, it is given the name 18:3 (''n''−3).

ALA is a polyunsaturated ''n''−3 (omega-3) fatty acid. It is an isomer of  ''n''−6 (omega-6) fatty acid.

==Dietary sources==
thumb|120px|right|Flax is a rich source of ALA.
Seed oils are the richest sources of ALA, notably those of perilla, thylakoid membranes of the green leaves of broadleaf plants (the membranes responsible for photosynthesis).<ref>{{cite journal
  |author= Chapman, David J.; De-Felice, John and Barber, James 
  |journal=Plant Physiol |year=1983 |month=May|volume=72 | issue=1|pages=225–228
  |title=Growth Temperature Effects on Thylakoid Membrane Lipid and Protein Content of Pea Chloroplasts 1
  |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1066200
  |accessdate=2007-01-15
  |pmid=16662966}}</ref>  Greens, therefore, and animals that eat greens, are often a good source of ALA. 

{| class="wikitable" border=1 cellspacing=0 cellpadding=1 style="border-collapse:collapse;"
!|Common name||Alternate name||Linnaean name||% ALA<sup>†</sup>||ref.
|-
|Chia ||chia sage||''Salvia hispanica''||64% ||<ref name=sofadb> [http://www.bagkf.de/sofa/ Seed Oil Fatty Acids - SOFA Database Retrieval]</ref>
|-
|Kiwifruit||Chinese gooseberry||''Actinidia chinensis''||62%||<ref name=sofadb />
|-
|Perilla||shiso||''Perilla frutescens''||58%||<ref name=sofadb />
|-
|Flax||linseed||''Linum usitatissimum''||55%||<ref name=sofadb />
|-
|Lingonberry||cowberry||''Vaccinium vitis-idaea''||49%||<ref name=sofadb />
|-
|Purslane||portulaca||''Portulaca oleracea''||35%||<ref name=sofadb />
|-
| Sea Buckthorn ||seaberry ||''Hippophae rhamnoides L.''||32%||<ref name=Li>{{cite conference
  | first = Thomas S. C.
  | last = Li
  | authorlink = 
  | coauthors = 
  | title = Sea buckthorn: New crop opportunity
  | booktitle = Perspectives on new crops and new uses
  | pages = 335–337
  | publisher = ASHS Press 
  | date = 1999
  | location = Alexandria, VA
  | url = http://www.hort.purdue.edu/newcrop/proceedings1999/v4-335.html
  | doi = 
  | id = 
  | accessdate =  2006-10-28}}</ref>
|-
|Hemp||cannabis||''Cannabis sativa''||20%||<ref name=sofadb />
|-
| Rapeseed||canola||''Brassica napus''||10%||<ref name=Beare />
|-
| Soybean ||soya||''Glycine max''||8%||<ref name=Beare />
|-
| colspan=3|&nbsp;
| colspan=2|<small><sup>†</sup>average val</small>
|}

==Role in nutrition and health==
ALA, an ''n''−3 fatty acid, is a member of the group of essential fatty acids, so called because they cannot be produced within the body and must be acquired through diet.  Most seeds and seed oils are much richer in an ''n''−6 fatty acid, linoleic acid. Linoleic acid is also an essential fat, but it, and the other ''n''−6 fats, compete with ''n''−3s for positions in cell membranes and have very different effects on human health. ''(See Essential fatty acid interactions.'')  

Eicosapentaenoic acid (EPA; 20:5, ''n''−3) and docosahexaenoic acid (DHA; 22:6, ''n''−3) play a vital role in many metabolic processes. Although these two fatty acids are readily available from fish, these marine-derived fatty acids can also be synthesized by humans from ALA. Humans, however, can obtain ALA only through their diets, because the absence of the required 12- and 15-desaturase enzymes makes de novo synthesis from stearic acid impossible. Furthermore, conversion of dietary ALA into EPA is limited. Because the efficacy of ''n''−3 long-chain polyunsaturated fatty acid (LC-PUFA) synthesis decreases down the cascade of ALA conversion, DHA synthesis from ALA is even more restricted than that of EPA.<ref>{{cite journal
| author=Shiels M. Innis
| year = 2007
| title = Fatty acids and early human development
| journal = Early Human Development
| volume = 83
| pages = 761–766
| doi = 10.1016/j.earlhumdev.2007.09.004
}}</ref>

It is generally assumed that linoleic acid (LA; 18:2, ''n''−6) reduces EPA synthesis because of the competition between ALA and LA for common desaturation and elongation enzymes.

Studies have found evidence that ALA is related to a lower risk of cardiovascular disease.<ref>{{cite journal
 | author = Penny M. Kris-Etherton, William S. Harris, [and] Lawrence J. Appel, for the Nutrition Committee
 | year = 2002
 | title = Fish Consumption, Fish Oil, Omega-3 Fatty Acids, and Cardiovascular Disease
 | journal = Circulation
 | volume = 106
 | issue = 21
 | pages = 2747–2757
 | url = http://circ.ahajournals.org/cgi/content/full/106/21/2747
 | format=pdf
 | accessdate = 2006-07-25
 | doi = 10.1161/01.CIR.0000038493.65177.94
 | pmid = 12438303 
 }} PMID 12438303</ref><ref>{{cite journal
 | author = William E. Connor
 | year = 2000
 | title = Importance of ''n''−3 fatty acids in health and disease
 | journal = American Journal of Clinical Nutrition
 | volume = 71
 | issue = 1 Suppl.
 | pages = 171S–175S
 | url = http://www.ajcn.org/cgi/reprint/71/1/171S
 | format=pdf
 | accessdate = 2006-07-25
 }} PMID 10617967</ref>
However, the mechanism is still unclear:  The body converts ALA into the longer chain fatty acids EPA and DHA, and it is unknown whether the protective effect against metabolic products. Some studies have linked ALA with rapidly progressing prostate cancer<ref>{{cite journal
 | author = Brouwer IA, Katan MB, Zock PL
 | year = 2004
 | title = Dietary α-linolenic acid is associated with reduced risk of fatal coronary heart disease, but increased prostate cancer risk: a meta-analysis.
 | journal = Journal of Nutrition
 | volume = 134
 | issue = 4
 | pages = 919–922
 | url = http://jn.nutrition.org/cgi/content/full/134/4/919
 | accessdate = 2006-11-13
 | pmid = 15051847
 }} PMID 15051847</ref> and macular degeneration,<ref>{{cite journal
 | author = Eunyoung Cho, Shirley Hung, Walter C Willett, Donna Spiegelman, Eric B Rimm, Johanna M Seddon, Graham A Colditz and Susan E Hankinson
 | year = 2001
 | title = Prospective study of dietary fat and the risk of age-related macular degeneration 
 | journal = American Journal of Clinical Nutrition
 | volume = 73
 | issue = 2
 | pages = 209–218
 | url = http://www.ajcn.org/cgi/content/full/73/2/209
 | accessdate = 2006-11-13
 | pmid = 11157315
 }} PMID 11157315</ref>
increasing the risk 70% over control subjects (over those that did not receive ALA). Research has also suggested a major neuroprotective effect of ALA in in-vivo models of both global ischemia and KA-induced epilepsy.<ref>{{cite journal
 | author = Inger Lauritzen, Nicolas Blondeau, Catherine Heurteaux, Catherine Widmann, Georges Romey and Michel Lazdunski
 | year = 2000
 | title = Polyunsaturated fatty acids are potent neuroprotectors
 | journal = The EMBO Journal
 | volume = 19
 | issue = 8
 | pages = 1784–1793
 | url = http://embojournal.npgjournals.com/cgi/content/full/19/8/1784
 | accessdate = 2005-10-06
 | pmid = 10775263
 | doi = 10.1093/emboj/19.8.1784}}</ref>