Alpha linolenic acid (ALA, C18:3 (n-3)) is an omega 3 fatty acid and is essential to humans. The essentiality of ALA derives from its conversion to other longer chain fatty acids, which are in turn converted to eicosanoids. Eicosanoids are hormone like substances that regulate cellular activity and thereby influence inflammation, immune response, platelet aggregation and other important cellular functions. Unlike animals, plants can synthesise ALA, and so dietary sources of ALA are limited to plant foods. High concentrations of the fatty acid can be found in walnuts, hemp seeds and flax seeds. Green leafy vegetables are also sources of ALA but the concentrations are low. Recommendations are to eat more omega 3 fatty acids because the Western diet contains too much of the omega 6 fatty acid linoleic acid (LA, C18:2 (n-6)). This high ratio of omega 6 to omega 3 fatty acids in the diet is thought to initiate inflammation and the disease process. Consuming more omega 3 fatty acids and redressing the imbalance ratio is known to improve health outcome.
Theoretically, eating more plant sources of omega 3 fatty acids, and lowering intakes of omega 6 fatty acids, should restore the delicate omega 6 to omega 3 ratio. Current estimate suggest that the omega 6 to omega 3 ratio might be as high as 20 to 1 in some individuals, whereas recommendations are that the ratio be around 3 to 1. However, obtaining adequate omega 3 fatty acids in the form of ALA from plants is problematic for a number of reasons. Firstly, the enzyme system required for the conversion of the essential fatty acid ALA to the other omega 3 fatty acids required for eicosanoid formation is the same enzyme system used by LA and the omega 6 fatty acids. The high intakes of omega 6 fatty acids in the Western diet therefore inhibit the synthesis of omega 3 derived eicosanoids through competitive inhibition of enzymes. Secondly, most humans possess a slow rate of delta 5-desaturase activity, the rate limiting step in the essential fatty acid pathway, which causes a slow synthesis of the required products.
However, there is a solution to this problem because photosynthetic algae produce ALA. This algae is eaten by fish, who convert the ALA into longer chain more unsaturated fatty acids such as eicosapentaenoic acid (EPA, C20:5 (n-3)) and docosahexaenoic acid (DHA, C22:6 (n-3)). Unlike humans then, fish bioaccumulate these longer chain omega 3 fatty acids, which makes fish useful nutritionally. Because both EPA and DHA feed into the omega 3 fatty acid pathway past the rate limiting delta 5-desaturase enzyme, they are more readily synthesised into eicosanoids when compared to plant derived ALA. Consuming fish therefore makes more sense than consuming plant derived ALA, if the aim is to balance the excessive levels of omega 6 fatty acids in the diet. This theory is supported by clinical data that shows greater health improvements for EPA and DHA from fish compared to ALA from plants. Both EPA and DHA should therefore be re-classified as conditionally essential nutrients due to poor synthesis rates in humans.
RdB
