Fats - n3 Polyunsaturated (Pre-formed Long Chain)

Background

Smaller amounts of polyunsaturated fatty acids with double bonds in the n-3 position also occur in the diet. These are sometimes referred to as omega fatty acids. Humans are unable to insert a double bond at the n-3 position of a fatty acid and thus require a dietary source. The parent fatty acid of the n-3 series is α-linolenic (ALA, 18:3). ALA is found in legumes, canola oils and margarines, linseed oils and products, certain nuts such as walnuts, and in small amounts in leafy vegetables. Canola oils and margarines and linseed oils are rich sources and legumes contribute some. A second group of n-3 fatty acids are the long chain (LC) acids eicosapentaenoic acid (EPA, 20:5), docosahexaenoic acid (DHA, 22:6) and docosapentaenoic acid (DPA, 22:5) that are found predominantly in oily fish such as mackerel, herrings, sardines, salmon and tuna and other seafood. Whilst α-linolenic acid predominates in western diets, the fish oils, DHA, EPA and DPA predominate in other communities consuming their traditional diet, such as the Inuit (Holman et al 1982).

ALA primarily functions as a precursor for the synthesis of EPA which in turn forms DHA but may also have an independent role in protection against coronary heart disease via different mechanisms (Crawford et al 2000). Conversion of ALA to EPA and DHA is limited and varies according to the intakes of other fatty acids (Burdge et al 2003, Emken 2003, Pawlosky et al 2001). Thus, a typical intake of ALA may be less able to satisfy the physiological requirements for LC n-3 fatty acids than the smaller and often more variable intakes of pre-formed LC n-3 fatty acids.

DHA plays an important role as a structural membrane lipids, particularly in nerve tissue and the retina, and can also act as a precursor to certain eicosanoids. EPA is the precursor of the 3 series of prostaglandins and the 5 series of leukotrienes. In recent years, research has shown both cardiovascular and anti-inflammatory benefits of LC n-3 fatty acids (Albert et al 1998, 2002, Burr et al 1989, Dallongeville et al 2003, Djousse et al 2001, Dolecek 1992, GISSI-Prevenzione Investigators 1999, Hu et al 1999, Pischon et al 2003, WHO 2003). Early on, because of the nature of the fish oils used in studies, these benefits were attributed to EPA and its impact on eicosanoid production (Simopoulos 1991) but recent studies suggest that DHA is the primary mediator of cardiovascular benefits, influencing gene expression of key metabolic regulators, particularly in endothelial cells (Mori et al 1999). The potential role of DPA, as a very minor component of fish oil, has been largely ignored, despite the fact that recent research shows DPA contributes almost 30% of total LC n-3 in our diet (Howe et al 2003, 2005).

Until dose-response relationships have been established, the relative efficacy of EPA, DPA and DHA remains uncertain. Moreover, the extent of their interconversion is also uncertain. Hence it is not possible to differentiate between intake requirements for EPA, DPA and DHA at this stage.