Gamma-Linolenic acid

γ-Linolenic acid (gamma-linolenic acid or GLA, sometimes called gamolenic acid) is an essential fatty acid found primarily in vegetable oils. It is sold as a dietary supplement for treating problems with inflammation and auto-immune diseases. The efficacy of such use is disputed.

Chemistry

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). Chemically, 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 the n−3 fatty acid found in flax seed.

History

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.

Dietary Sources

GLA is obtained from vegetable oils, such as evening primrose (Oenothera biennis) oil, blackcurrant seed oil, borage oil and hemp seed oil, and from spirulina, a cyanobacterium. Each contains varying amounts of the fatty acid, with borage oil usually being the most heavily concentrated form. All are widely available in pharmacies, health food stores, or online shops.

The human body produces GLA from linoleic acid (LA). This reaction is catalized by Δ⁶-desaturase (D6D), an enzyme which 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 where there is excessive consumption of GLA metabolites.

A 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. PGE₁ has a role in regulation of immune system function and is used as the medicine alprostadil. TXA₁ modulates the pro-inflammatory properties of the thromboxane TXA₂.

Unlike AA and EPA, DGLA cannot yield leukotrienes. However it can inhibit the formation of pro-inflammatory leukotrienes from AA.

Although GLA is an n−6 fatty acid, a type of acid which is generally pro-inflammatory, it has anti-inflammatory properties. (See discussion at Essential fatty acid interactions: The paradox of dietary GLA.)

Health and Medicine

GLA is sometimes prescribed in the belief that it has anti-inflammatory properties lacking some of the common side effects of other anti-inflammatory drugs. Herbal medicine advocates recommend GLA for autoimmune disorders, arthritis, eczema and PMS with noticeable results not expected for months. Research is ongoing, investigating GLA as a potential anticancer agent. GLA is unique among the omega-6 polyunsaturated fatty acids (linoleic acid, GLA and arachidonic acid) in its potential to suppress tumor growth and metastasis.

GLA can also form a lithium salt, increasing its solubility in water. The resulting compound is Li-GLA, also called lithium gammalinolenate. Li-GLA is currently in phase II clinical trials to determine whether it is useful in the treatment of HIV infections, since it has the ability to destroy HIV-infected T cells in vitro. It has a number of side-effects, including a reduction in hemoglobin, hematuria, gastrointestinal disturbance, fatigue and headache. CTN#083,LiGLA

Atopic Eczema

Conflicting data are found for GLA in the treatment of eczema. The UK's Medicines and Healthcare products Regulatory Agency has withdrawn GLA's product licence for atopic eczema. Still, the US National Institute of Health's MedlinePlus states that there is 'B' grade evidence ('good scientific evidence') for the efficacy of evening primrose oil in the treatment of eczema and skin irritation. But it cautions that large well-designed studies are still needed. A controlled study of borage oil for eczema found no benefit to GLA; it underperformed placebo.

History

The medical use of GLA has been controversial. David Horrobin published much research on the use of GLA (as evening primrose oil) as a dietary supplement for treating atopic eczema. He also founded Scotia Pharmaceuticals, which sold this oil as a pharmaceutical, which led to controversy even after his death.

References

See also

• Linolenic acid