Bioavailability and Potential Uses of Vegetarian Sources of Omega-3 Fatty Acids: A Review of the Literature

Presently alpha linolenic acid (ALA) is the most widely used vegetarian LC3PUFA, but only marginal amounts are converted into eicosapentaenoic (EPA) and docosahexaenoic acid (DHA); both of which are strongly related to human health.
Currently, fish oils represent the most prominent dietary sources of EPA and DHA; however, these are unsuitable for vegetarians. Alternative sources include flaxseed, echium, walnut, and algal oil but their conversion to EPA and DHA must be considered. The present systematic review sets out to collate information from intervention studies examining the bioavailability of alternative vegetarian long chain omega 3 (n 3) polyunsaturated fatty acids (LC3PUFA) sources. Ten key papers published over the last 10 years were identified with seven intervention studies reporting that ALA from nut and seed oils was not converted to DHA at all. Three studies showed that ingestion of micro-algae oil led to significant increases in blood erythrocyte and plasma DHA. Further work is now needed to identify optimal doses of alternative vegetarian LC3PUFAs and how these can be integrated within daily diets. The potential role of algal oils appears to be particularly promising and an area in which further research is warranted. (Authors abstract)

This review paper focuses on long chain omega 3 fatty acids. Research has identified that vegetarians and vegans, nonfish eaters and pregnant mothers may not consume adequate quantities of LC3PUFA’s. A vegan diet is completely devoid of DHA and vegetarian diets contain smaller amounts of DHA than that of meat and particularly fish eaters. Clinical studies suggest that tissue levels of LC3PUFA’s are at low levels in vegetarians and particularly low in vegans. Currently, the most significant vegetarian dietary form of n 3 is ALA. Abundant ALA commodities include flaxseed, walnut, and echium seed oils.  Most vegetarian diets are rich in LA, a dietary source of n 6 which can be converted to the longer chain AA in the n 6 metabolic pathway. The increasing popularity of vegetable oils such as corn, sunflower, and safflower has lead to a rise in n-6 fatty acid intakes in US and Western diets, whilst intakes of n 3 have declined. The authors do not review this important point in this paper. In the metabolic pathway n 3 and n 6 fatty acids compete for the enzyme that is able to convert them. Diets with a high ratio of LA:ALA can suppress DHA synthesis in favor of docosapentenoic acid  which takes the place of DHA in the retinal and neural tissues. Delta 6 desaturase is the enzyme responsible for synthesizing LCPUFA’s from ALA and LA. The activity of this enzyme can be reduced by aging, stress, diabetes, eczema, and some types of infection. Various dietary and lifestyle factors can impair LCPUFA synthesis including high intakes of saturated, hydrogenated or “trans” fatty acids, a lack of vitamin and mineral cofactors and lifestyle choices such as smoking and the use of alcohol and caffeine. Therefore, usually, very little ALA is converted to EPA and even less, if any to DHA.  Consequently, non fish eaters could represent a portion of the population who may be at risk from the health consequences of a decreased LC3PUFA status.
A number of alternative sources of LC3PUFA are available.   ALA rich oils would be appropriate supplements for vegetarians and vegans. Currently, there are no official n 3 recommendations for vegetarians and vegans although some experts suggest vegetarians and those receiving no direct dietary sources of EPA and DHA should at least double the recommended intake of ALA. A considerable amount of research has examined the LC3PUFA status of humans after supplementation with ALA rich oils. All of the studies included in this review have shown that LC3PUFA’s can successfully be used to increase blood levels of ALA but conversion of ALA to its longer chain relatives EPA and DHA is limited within the n 3 metabolic pathway.  Overall, findings indicate that vegetarian sources of n 3 oils such as flaxseed, echium, and walnut may elicit similar results to marine sources of LC3PUFA, although further research is necessary to indicate optimal levels of supplementation. The conversion process can be affected by several factors such as current LC3PUFA status and consumption of n 6 fatty acids.
Flaxseed oil is a rich source of n 3 and has been proven to have good bioavailability. (Editors comments)