The omega-3 fatty acid alpha linolenic acid (ALA, C18:3 (n-3)) is essential to the health. In humans, ALA is metabolised to eicosapentaenoic acid (EPA, C20:5 (n-3)) and subsequently accumulates in cell membranes. Here EPA is converted into a series of eicosanoid hormones including prostaglandins, thromboxanes and leukotrienes that can inhibit the production of proinflammatory eicosanoids synthesised from the omega-6 fatty acid arachidonic acid (AA, C20:4 (n-6)). As well as being synthesised from the ALA, EPA can be derived from the diet directly, with fish oils being a particularly rich source. In fact the conversion of ALA to EPA in many humans of Western European descent is poor, due to a genetic deficiency of one of the necessary enzymes, delta 6-desaturase, the rate limiting step in the conversion pathway. Consuming fatty fish is therefore a good way to increase cell membrane levels of EPA because it supplies the EPA directly, thus bypassing the rate limiting step (here).
Because EPA is stored in cell membranes, a daily intake might not be necessary. Current recommendations are to consume fatty fish 2 to 3 times per week, and this recommendation is based on the ability of the EPA to accumulate in cell membranes. Further, docosahexaenoic acid (DHA, C22:6 (n-3)), also present in fish, accumulates in cell membranes and is able to be converted to EPA through a series of elongation and desaturation enzymes. Therefore cell membrane DHA acts as a reservoir for EPA. While consumption of fish is usually sporadic, some choose to take fish oil capsules containing fish oils, and these tend to be consumed daily. Studies two have used both infrequent fish consumption and daily fish oil consumption to investigate the health effects of fish oil. To investigate the possible differences in accumulation of omega-3 fatty acids from different delivery methods, researchers assessed the effects of fish oil capsules providing 6.54 grams per week EPA either in two portions or a daily intake1.
The subjects consumed the fish oil following one of the two protocols for 12 months over which period they had their blood levels of EPA and DHA monitored. Following 12 months the group that consumed the fish oil capsules daily had higher EPA, DHA and EPA plus DHA in platelets, and higher EPA and EPA plus DHA in mononuclear cells, compared to the group taking fish oil capsules only twice per week. Therefore the pattern of intake does affect the incorporation of EPA and DHA into blood cells and this may have relevance to previous studies that have used differing methods of fish oil delivery. However, it should be noted that not biological parameters relating to fish oil consumption were measured, so it is not possible to say that the levels of EPA achieved following daily consumption were beneficial, although it could be inferred that they may be. Consuming smaller portions of fish more frequently, or fish oil capsules daily, may therefore be a more optimal strategy than infrequent consumption of larger amounts of fish.
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