Alpha-linolenic acid

Alpha-linolenic acid (ALA) is a an organic compound found in many common vegetable oils. Systematically, it is named all-cis-9,12,15-octadecatrienoic acid.^[1] In physiological literature, it is given the name 18:3(n-3).

ALA 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 omega end. Thus, ALA is both a polyunsaturated fatty acid, and an omega-3 fatty acid. It is an isomer of γ-linolenic acid, an omega-6 fatty acid.

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Dietary sources

  Seed oils are the richest sources of alpha linolenic acid, notably those of rapeseed (canola), soybeans, walnuts, flaxseed (Linseed), perilla, chia and hemp. Alpha linolenic acid is also obtained from the thylakoid membranes of the green leaves of broadleaf plants (the membranes responsible for photosynthesis).^[2] Greens, therefore, and animals that eat greens, are often a good source of ALA.

Role in nutrition and health

Alpha-linolenic acid is a member of the group of essential fatty acids called omega-3 fatty acids, so called because they are an essential dietary requirement for all mammals. Most seeds and seed oils are much richer in the omega-6 fat linoleic acid. Linoleic acid is also an essential fat, but it, and the other omega-6 fats, compete with omega-3s for positions in cell membranes and have very different effects on human health. (See Essential fatty acid interactions.)

Eicosapentaenoic acid (EPA; 20:5n–3) and docosahexaenoic acid (DHA; 22:6n–3) play a vital role in many metabolic processes. Although these 2 fatty acids are readily available from fish, these marine-derived fatty acids can also be synthesized by humans from alpha-linolenic acid (ALA; 18:3n–3). 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.^{[citation needed]} It is generally assumed that linoleic acid (LA; 18:2n–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.^[5][6] However, the mechanism is still unclear: The body converts ALA into the longer chain fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), and it is unknown whether the protective effect against cardiac arrhythmia is exerted by ALA itself, or by these metabolic products. Some studies have linked ALA with rapidly progressing prostate cancer^[7] and macular degeneration,^[8] 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.^[9]

A large 2006 study found no association between total alpha-linolenic acid intake and overall risk of prostate cancer.^[10]

Linolenic acid and trans fats

When partially hydrogenated, all unsaturated fatty acids form trans fats. Soybeans are the largest source of edible oils in the U.S., and 40% of soy oil production is partially hydrogenated.^[11][12] The low oxidative stability of ALA is one reason that soybean oil undergoes partial hydrogenation. Regulations forcing the listing or banning of trans fats have spurred the development of low-linolenic soybeans. These yield a more stable oil requiring hydrogenation less often, and therefore providing trans-free alternatives into many applications such as frying oil.^[13] Several consortia are bringing low-linolenic 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 and linoleic acids. ^[14]

In heart disease

Dietary ALA has been assessed for its role in cardiovascular health. Clinical benefits have been seen in some but not all studies. Still, a review in 2005 concluded "The weight of the evidence favors recommendations for modest dietary consumption of ALA (2 to 3 g per day) for the primary and secondary prevention of CHD."^[15]

Other uses—drying oils

See main article: drying oil

ALA is the most abundant unsaturated component of several drying oils (e.g. perilla, walnut and linseed oils.)

See also

• Essential fatty acid
• List of omega-3 fatty acids
• Essential nutrient

References