Alpha linolenic acid (ALA, C18:3 (n-3)) is an essential fatty acid. Therefore alpha linolenic acid is required for heath but cannot be synthesised endogenously, and so must be present in the diet. Most humans possess a deficiency of the gene required for the production of the delta 6-desaturase enzyme, the rate limiting step in the conversion of alpha linolenic acid to longer chain more unsaturated fatty acids. As a result, dietary sources of alpha linolenic acid may not provide adequate plasma levels of the important metabolites eicosapentanoic acid (EPA, C20:5 (n-3)) or docosahexanoic acid (DHA, C22:6 (n-3)). This is not a problem for omnivores because both EPA and DHA can be derived from fish oils, thus negating the need for dietary sources of alpha linolenic acid. However, vegetarians must derive their n-3 fatty acids from the plant based alpha linolenic acid, putting them at risk of essential fatty acid deficiency diseases.
Because alpha linolenic acid is poorly converted, consumption of plant based n-3 fatty acids may not provide the same benefits as those derived from fish oils. For example, fish oils have been extensively researched and found to be protective of cardiovascular disease. This is likely because the increased dietary intake of EPA and DHA raises plasma levels of n-3 fatty acids, and these have beneficial anti-inflammatory and anti-thrombotic effects. That alpha linolenic acid is not as cardioprotective when compared to EPA or DHA has been demonstrated in a number of papers. For example, in one study1, researchers investigated the effects of dietary alpha linolenic acid on the incidence of coronary heart disease in 2957 adults aged over 65 years of age. The results showed that there was no association between alpha linolenic acid intake and the incidence of coronary heart disease in the highest compared to the lowest quartile for alpha linolenic acid consumption.
However, previous studies have found inverse associations between seafood long chain fatty acids (EPA and DHA) and the incidence of coronary heart disease. This leaves vegetarians with a problem because they cannot consume the beneficial fish oils. However, there are two solutions to this problem. Firstly, marine algae synthesise DHA, and capsules containing farmed algae grown in tanks have recently become available. Supplementation with such a product could therefore provide the required amounts of DHA. Secondly, alcohol increases the flux through the n-3 fatty acid pathway, by upregulating the activity of the delta 6-desaturase enzyme. Regular consumption of alcohol has been shown to increase the EPA and DHA content of plasma and this therefore could allow beneficial effect to be obtained from plant based n-3 fatty acids. Interestingly, this may be the mechanism by which alcohol is beneficial to the health.