Research suggests that consuming certain cold water fish is beneficial to the health because of the fatty acids they contain. Mackerel, herring, sardines, trout, salmon and tuna contain high concentrations of omega 3 (n-3) oils in their fat, mainly eicosapentanoic acid (EPA, C20:5 (n-3)) and docosahexanoic acid (DHA, C22:6 (n-3)). The fatty acid EPA is a precursor to the series 3 prostanoids, high levels of which are able to reduce inflammation through inhibition of the pro-inflammatory series 2 prostanoids derived from arachidonic acid. Conversion of DHA to EPA occurs via a series of elongase and desaturase enzymes. In addition, DHA is also concentrated by the brain, where it increases fluidity in neuronal membranes, improves neuronal efficiency and modulates cellular signalling. However, vegetarians are unable to consume fish oils and so they must acquire their n-3 fatty acids from other sources.
Flax oil is a commercially available alternative source of n-3 fatty acids for vegetarians, and is often consumed by non-vegetarians. Flax oil contains the essential fatty acid alpha linolenic acid (ALA, C18:3 (n-3)), which is metabolised in the body to EPA and then DHA via a series of elongase and desaturase enzymes. However, the conversion of ALA to EPA is not good in humans, and research indicates that dietary EPA is 2.5 to 5 times as effective as ALA in raising blood levels of EPA. In addition, dietary ALA does not raise blood levels of DHA in pregnant women or new born infants. The metabolism of ALA to DHA is estimated to be less than 1 %, with the conversion of EPA to DHA being the slowest step. This poor conversion is because of genetically low desaturase activity in humans.
A better alternative for vegetarians is DHA derived from salt water algae such the dinoflagellate Crypthecodinium cohnii. Algae such as this and others, are now becoming commercially available and can supply good quality DHA. The algae are single celled organisms and if fed in tanks can accumulate lipids to around 20 to 50 % of their dry mass, of which 30 to 40 % can be DHA. The high concentration of DHA makes them commercially viable organism to grow and harvest. Another advantage of algal derived DHA is that it does not exposed to pollutants such as polychlorinated biphenyls (PCB), mercury and dioxins, which are known to accumulate in fish, and be present in some commercially available oils. Because manufacturing of DHA from algae is easier than fish, the oil is less likely to become oxidised in the process.
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