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|Molar mass||278.43 g mol−1|
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
γ-Linolenic acid (gamma-linolenic acid or GLA, INN and USAN gamolenic acid) is a fatty acid found primarily in vegetable oils. It is sold as a dietary supplement for a variety of human health problems, although there is little or no evidence of its effectiveness.
GLA is categorized as an n−6 (also called ω−6 or omega-6) fatty acid, meaning that the first double bond on the methyl end (designated with n or ω) is the sixth bond. In physiological literature, GLA is designated as 18:3 (n−6). GLA is a carboxylic acid with an 18-carbon chain and three cis double bonds. It is an isomer of α-linolenic acid, which is a polyunsaturated n−3 (omega-3) fatty acid, found in rapeseed canola oil, soy beans, walnuts, flax seed (linseed oil), perilla, chia, and hemp seed.
GLA was first isolated from the seed oil of evening primrose. This herbal plant was grown by Native Americans to treat swelling in the body. In the 17th century, it was introduced to Europe and became a popular folk remedy, earning the name king's cure-all. in 1919, Heiduschka and Lüft extracted the oil from evening primrose seeds and described an unusual linolenic acid, which they name γ-. Later, the exact chemical structure was characterized by Riley.
Although there are α- and γ- forms of linolenic acid, there is no β- form. One was once identified, but it turned out to be an artifact of the original analytical process.
GLA is obtained from vegetable oils such as GLA safflower oil (Carthamus tinctorius), evening primrose (Oenothera biennis) oil, blackcurrant seed oil, borage oil, and hemp seed oil. GLA is also found in edible hemp seeds, oats, barley,[full citation needed] and spirulina. Each contains varying amounts of the fatty acid, with GLA safflower oil at 40% GLA being a novel concentrated form. This is a new genetically modified oil and has been available in commercial quantities since 2011. It should be noted that conventional safflower oils have zero GLA. Borage oil ranges from 15% to 20% and evening primrose oil ranges from 8% to 10% GLA.
The human body produces GLA from linoleic acid (LA). This reaction is catalyzed by Δ6-desaturase (D6D), an enzyme that allows the creation of a double bond on the sixth carbon counting from the carboxyl terminus. LA is consumed sufficiently in most diets, from such abundant sources as cooking oils and meats. However, a lack of GLA can occur when there is a reduction of the efficiency of the D6D conversion (for instance, as people grow older or when there are specific dietary deficiencies) or in disease states wherein there is excessive consumption of GLA metabolites.[dubious ]
Source of eicosanoids
From GLA, the body forms dihomo-γ-linolenic acid (DGLA). This is one of the body's three sources of eicosanoids (along with AA and EPA.) DGLA is the precursor of the prostaglandin PGH1, which in turn forms PGE1 and the thromboxane TXA1. Both PGE11 and TXA1 are anti-inflammatory; thromboxane TXA1, unlike its series-2 variant, induces vasodilation, and inhibits platelet consequently, TXA1 modulates (reduces) the pro-inflammatory properties of the thromboxane TXA2. PGE1 has a role in regulation of immune system function and is used as the medicine alprostadil.
Although GLA is an n−6 fatty acid, a type of acid that is, in general, pro-inflammatory, it has anti-inflammatory properties. (See discussion at Essential fatty acid interactions: The paradox of dietary GLA.)
Health and medicine
GLA has been promoted as medication for a variety of ailments including breast pain and eczema, most notoriously by David Horrobin (1939 – 2003), whose marketing of evening primrose oil was described by the British Medical Journal as ethically dubious – the substance was likely to be remembered as "a remedy for which there is no disease". In 2002, the UK's Medicines and Healthcare products Regulatory Agency withdrew marketing authorisations for evening primrose oil as an eczma remedy. The British Medical Journal commented that it had taken 20 years to demonstrate that the substance was of no use in atopic dermatitis, and called for more transparency in the research on which drug licensing decisions were taken.
GLA is also sometimes promoted as an anti-cancer agent. According to the American Cancer Society there is very little evidence for its effectiveness, and "neither GLA nor other GLA-rich supplements (such as evening primrose oil) have been convincingly shown to be useful in preventing or treating any other health conditions."
Notes and references
- Yung-Sheng Huang, Vincent A. Ziboh (2001). Gamma-Linolenic Acid: Recent Advances in Biotechnology and Clinical Applications. AOCS Press. p. 259. ISBN 1-893997-17-0. Retrieved 2007-12-07.
- Eckey, EW (1954). Vegetable Fats and Oils (volume 123 of American Chemical Society monograph series). Reinhold. p. 542.
- Qureshi AA et al. (1984). Fed. Proc. 43: 2626.
- Flider, Frank J (May 2005). "GLA: Uses and New Sources". INFORM 16 (5): 279–282.
- Nykiforuk, C.; Shewmaker, C. (19). "High level accumulation of gamma linolenic acid in transgenic safflower (Carthamus tinctorius) seeds". Transgenic Research 21 (2): 367–81. doi:10.1007/s11248-011-9543-5. PMID 21853296.
- Horrobin DF (1993). "Fatty acid metabolism in health and disease: the role of delta-6-desaturase" (pdf). Am. J. Clin. Nutr. 57 (5 Suppl): 732S–736S; discussion 736S–737S. PMID 8386433.
- King, Michael W. "Introduction to the Eicosanoids". The Medical Biochemistry Page. 1996–2013 themedicalbiochemistrypage.org, LLC. Retrieved 23 July 2013.
- Belch JJ, Hill A (2000). "Evening primrose oil and borage oil in rheumatologic conditions". Am. J. Clin. Nutr. 71 (1 Suppl): 352S–6S. PMID 10617996. Retrieved 2007-12-07. "DGLA itself cannot be converted to LTs but can form a 15-hydroxyl derivative that blocks the transformation of arachidonic acid to LTs. Increasing DGLA intake may allow DGLA to act as a competitive inhibitor of 2-series PGs and 4-series LTs and, thus, suppress inflammation."
- Richmond, C. (2003). "David Horrobin". BMJ 326 (7394): 885. doi:10.1136/bmj.326.7394.885.
- Williams, H. C (2003). "Evening primrose oil for atopic dermatitis: Time To Say Goodnight". BMJ 327 (7428): 1358–9. doi:10.1136/bmj.327.7428.1358. JSTOR 25457999. PMC 292973. PMID 14670851.
- Smith, R. (2003). "The drugs don't work". BMJ 327 (7428): 0–h. doi:10.1136/bmj.327.7428.0-h.
- "Gamma Linolenic Acid". American Cancer Society. 13 May 2010. Retrieved August 2013.