Alpha linolenic acid (ALA, 18:3 (n-3)) is a long chain polyunsaturated n-3 fatty acid that is essential to health. The typical Western diet is deficient in ALA, which must be consumed (or one of its metabolites) in order to prevent deficiency diseases. The current recommendations are to eat more fatty fish because the n-3 metabolites it contains feed into the same metabolic pathways as ALA, albeit at a later reaction step. Eicosapentanoic acid (EPA, 20:5 (n-3)) and docosaxexanoic acid (DHA, 22:6 (n-3)) in fish oil has been researched with regard to their ability to prevent cardiovascular disease, cancer, arthritis, diabetes and dementias. However, vegetarians are not able to consume EPA or DHA from fish oil and so must attain their n-3 fatty acids from other sources, most commonly flax oil, which is a plant oil rich in ALA.
Some research suggests that flax is not as beneficial as fish oil at increasing membrane concentrations of EPA and DHA, the precursors to the beneficial series 3 prostanoids. To investigate the effectiveness of flax or fish oil supplementation, researchers1 fed low doses of flax and fish oil to subjects and measured the concentration of various n-3 fatty acids in cell membranes. Subjects received flax seed oil containing 35 mg of ALA or fish oil containing 35 mg of EPA and DHA per kg of body weight per day for 3 months. After 3 months subjects were tested for lipoprotein concentrations of n-3 fatty acid as well as plasma lipid levels, before switching to the other oil and having a second blood test. The results showed that total, LDL and HDL cholesterol levels were not affected by either (n-3) fatty acid.
However, more interestingly fish oil was able to increase the lipoprotein concentrations of EPA, which confirms the results from other similar studies. However, in contrast the flax oil only caused very small mostly non-significant rises in the lipoprotein concentrations of EPA. Flax in contrast was able to increase lipoprotein concentrations of ALA to a greater extent than fish oil consumption. These results suggest that flax oil at the doses in this study are not enough to cause conversion to EPA, or that (more likely) conversion of ALA via the elongase and desaturase enzymes is inhibited for some reason. Perhaps because the amount of ALA was too low to compete for n-6 oil for these enzymes. Because it is lipoprotein concentrations EPA that determine the synthesis of series 3 prostanoids, it appears that only fish oil supplementation (at these doses) would be able to substantially influence eicosanoid production.
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