Alpha-Linolenic acid

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.^[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 n end. Thus, ALA is a polyunsaturated n−3 (omega-3) fatty acid. It is an isomer of γ-linolenic acid, a polyunsaturated n−6 (omega-6) fatty acid.

Dietary sources

Seed oils are the richest sources of ALA, notably those of rapeseed (canola), soybeans, walnuts, flaxseed (Linseed), perilla, chia and hemp. ALA 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

ALA, an n−3 fatty acid, is a member of the group of essential fatty acids, so called because they are an essential dietary requirement for all mammals. 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.^[5]

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.^{citation needed}

Studies have found evidence that ALA is related to a lower risk of cardiovascular disease.^[6]^[7] 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 cardiac arrhythmia is exerted by ALA itself, or by these metabolic products. Some studies have linked ALA with rapidly progressing prostate cancer^[8] and macular degeneration,^[9] 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.^[10]

A large 2006 study found no association between total ALA intake and overall risk of prostate cancer.^[11]

ALA

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.^[12]^[13] 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-ALA soybeans. These yield a more stable oil requiring hydrogenation less often, and therefore providing trans-free alternatives into many applications such as frying oil.^[14] Several consortia are bringing low-ALA 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 ALA and LA.^[15]

Cardiovascular

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."^[16]

Drying oils

Contents

Dietary sources

Role in nutrition and health

ALA

Cardiovascular

Drying oils

See also

References