|Jmol-3D images||Image 1
|Molar mass||278.43 g mol−1|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
α-Linolenic acid (ALA) is an essential omega-3 fatty acid found in seeds (chia, flaxseed, see also table below), nuts (notably walnuts), and many common vegetable oils. In terms of its structure, it is named all-cis-9,12,15-octadecatrienoic acid. In physiological literature, it is given the name 18:3 (n−3).
α-Linolenic acid 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 methyl end of the fatty acid chain, known as the n end. Thus, α-linolenic acid is a polyunsaturated n−3 (omega-3) fatty acid. It is an isomer of gamma-linolenic acid, a polyunsaturated n−6 (omega-6) fatty acid.
α-Linolenic acid was first isolated by Rollett as cited in J. W. McCutcheon's synthesis in 1942, and referred to in Green and Hilditch's 1930s survey. It was first artificially synthesized in 1995 from C6 homologating agents. A Wittig reaction of the phosphonium salt of [(Z-Z)-nona-3,6-dien-1-yl]triphenylphosphonium bromide with methyl 9-oxononanoate, followed by saponification, completed the synthesis.
Seed oils are the richest sources of α-linolenic acid, notably those of chia, perilla, flaxseed (linseed oil), rapeseed (canola), and soybeans. α-Linolenic acid is also obtained from the thylakoid membranes in the leaves of Pisum sativum (pea leaves). Plant chloroplasts consisting of more than 95 percent of photosynthetic thylakoid membranes are highly fluid due to large abundance of linolenic acid, that shows up as sharp resonances in high resolution carbon-13 NMR spectra, invariably. Some studies state that ALA remains stable during processing and cooking. However, other studies state that ALA might not be suitable for baking, as it will polymerize with itself, a feature exploited in paint with transition metal catalysts. Some ALA may also oxidize at baking temperatures. %ALA in the table below is for the oil extracted from each item.
|Common name||Alternate name||Linnaean name||% ALA†||ref.|
|Chia||chia sage||Salvia hispanica||64%|||
|Kiwifruit seeds||Chinese gooseberry||Actinidia chinensis||62%|||
|Sea buckthorn||seaberry||Hippophae rhamnoides L.||32%|||
Potential role in nutrition and health
α-Linolenic acid, an n−3 fatty acid, is a member of the group of essential fatty acids (EFAs), so called because they cannot be produced within the body and must be acquired through diet. Most seeds and seed oils are much richer in an n−6 fatty acid, linoleic acid. Exceptions include flaxseed (must be ground for proper nutrient absorption) and walnuts. Linoleic acid is also an EFA, but it, and the other n−6 fatty acids, compete with n−3s for positions in cell membranes and have very different effects on human health. (See Essential fatty acid interactions.)
α-Linolenic acid can only be obtained by humans through their diets because the absence of the required 12- and 15-desaturase enzymes makes de novo synthesis from stearic acid impossible. Eicosapentaenoic acid (EPA; 20:5, n−3) and docosahexaenoic acid (DHA; 22:6, n−3) are readily available from fish and algae oil and play a vital role in many metabolic processes. These can also be synthesized by humans from dietary α-linolenic acid, but with an efficiency of only a few percent. Because the efficacy of n−3 long-chain polyunsaturated fatty acid (LC-PUFA) synthesis decreases down the cascade of α-linolenic acid conversion, DHA synthesis from α-linolenic acid is even more restricted than that of EPA.
A 2005 study found that daily administration of α-linolenic acid significantly reduced both self-reported anxiety, stress levels, and objective measured cortisol levels in college age students.
A large 2006 study found no association between total α-linolenic acid intake and overall risk of prostate cancer. Multiple studies have shown a relationship between α-linolenic acid and an increased risk of prostate cancer. This risk was found to be irrespective of source of origin (e.g., meat, vegetable oil). A recent (2009) meta-analysis, however, found evidence of publication bias in earlier studies, and concluded that if ALA contributes to increased prostate cancer risk, the increase in risk is quite small. In contrast, alpha-linoleic acid was recently shown to negatively regulate the growth of cancer cells, but not healthy cells, in vitro.
A 2011 longitudinal study of over 50,000 women, conducted at Harvard University, over a period of ten years, found that a higher intake of α-linolenic acid (combined with a lower intake of linoleic acid) was positively associated with a significant reduction in depression in the same group (the same study also found that by contrast an intake of EPA and DHA found in fish oils did not reduce depression).
Stability and hydrogenation
α-Linolenic acid is relatively more susceptible to oxidation and will become rancid more quickly than many other oils. Oxidative instability of α-linolenic acid is one reason why producers choose to partially hydrogenate oils containing α-linolenic acid, such as soybean oil. Soybeans are the largest source of edible oils in the U.S., and 40% of soy oil production is partially hydrogenated.
However, when partially hydrogenated, part of the unsaturated fatty acids become unhealthy trans fats. Consumers are increasingly avoiding products that contain trans fats, and governments have begun to ban trans fats in food products. These regulations and market pressures have spurred the development of low-α-linolenic acid soybeans. These new soybean varieties yield a more stable oil that doesn't require hydrogenation for many applications, thus providing trans fat-free products, such as frying oil.
Several consortia are bringing low-α-linolenic acid 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 acid and linoleic acid. Monsanto Company has introduced to the market Vistive, their brand of low α-linolenic acid soybeans, which is less controversial than GMO offerings, as it was created via conventional breeding techniques.
Dietary α-linolenic acid 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 α-linolenic acid (2 to 3 g per day) for the primary and secondary prevention of coronary heart disease."
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