Gamma-linolenic acid (GLA) is an essential fatty acid (EFA) in the omega-6 family that is found primarily in plant-based oils. EFAs are essential to human health but cannot be made in the body. For this reason, they must be obtained from food. EFAs are needed for normal brain function, growth and development, bone health, stimulation of skin and hair growth, regulation of metabolism, and maintenance of reproductive processes.

Y-Linolenic acid (gamma-linolenic acid or GLA, sometimes called gamoleic acid) is a fatty acid found primarily in vegetable oils. It is sold as a dietary supplement for treating problems with inflammation and auto-immune diseases.

GLA is categorized as an n-6 (also called w-6 or omega-6) fatty acid, meaning that the first double bond on the methyl end 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 a-linolenic acid, which is the n-3 fatty acid found in flax 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 Luft extracted the oil from evening primrose seeds and described an unusual linolenic acid, which they name Y-, Later, the exact chemical structure was characterized by Riley.

Although there are a- and y-forms of linolenic acid, there is no ß-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: evening primrose (Oenothera biennis) oil, blackcurrant seed oil, borage oil, and hemp seed oil. GLA is also found in considerable quantities in eatable hemp seeds 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 catalyzed by 6-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.