Research has highlighted that docosahexanoic acid (DHA, C22:6 (n-3)) is important for brain development and function. The brain preferentially accumulates plasma DHA because it is necessary for efficient synaptic firing, membrane fluidity and possibly, neurogenesis. Deficiency of DHA causes an increase in the desturation of n-6 oils to form docosapentanoic acid (DPA, C22:5 (n-6)), which enters the brain in place of the DHA but does not allow proper brain function because of structural differences to DHA. Endogenous formation of DHA can occur via the plant derived alpha linolenic acid (ALA, C18:3 (n-3)) or via the fish derived oil eicosapentanoic acid (EPA, C20:5 (n-3)). However, evidence suggests that the conversion of ALA to DHA is not high in humans because of genetic deficiencies in one the necessary desaturase enzyme. This is problematic for vegetarians that use ALA as a source of n-3 fatty acids.
Researchers1 have investigated the conversion of EPA and ALA to DHA in by using C13 labelled EPA and deuterium labelled ALA. One week old infants were given doses of 2H5-ALA and 13C-U-EPA and blood was sampled so to assess the amount of ALA and EPA that was converted to DHA. The results showed that 94 % of the labelled fatty acids were absorbed. Of the 2H5-ALA that appeared in plasma, 4 % was converted to 2H5-EPA, which supports results from other studies that show around 5 % conversion of ALA to EPA. A relatively small amount of the 13C-U-EPA was converted to 13C-U-CDA (clupanodonic acid; (CDA, C22:5 (n-3)) an intermediate of the conversion of EPA to DHA), compared with the conversion of 2H5-EPA to 2H5-CDA. This suggests that EPA derived from ALA is more readily converted to DHA than dietary DHA.
From this study, the authors concluded that preformed (dietary) EPA was 3.6 times more effective at DHA synthesis than ALA. Although endogenous EPA is more readily converted to DHA than dietary EPA, the conversion of ALA to EPA is only around 4 %. From this the authors calculated that newborn infants require ~5 mg of dietary DHA per kg of body weight per day. The same low conversion of ALA to EPA has been reported in adults, which suggests a general human genetic trait of low desaturase activity. This is more evidence that dietary ALA from sources such as flax oil is not the most effective method to raise n-3 plasma levels of EPA or DHA. Fish oils or DHA derived from algae, appear to be superior methods of attaining n-3 requirements in both adults and children.
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