Docosahexaenoic acid (DHA, C22:6 (n-3)) is a long chain polyunsaturated fatty acid found in cold water fatty fish such as trout, tuna, mackerel, salmon and sardines. These fish bioaccumulate DHA from marine algae that are able to synthesise the fatty acid. Unlike the land plant omega 3 fatty acid alpha linolenic acid (ALA, C18:3 (n-3)), DHA is efficiently converted to eicosapentaenoic acid (EPA, C20:5 (n-3)) in humans. This conversion is necessary to produce the series 3 eicosanoid hormones that modulate the inflammatory effects of the more pro-inflammatory series 1 eicosanoid hormones. In this way DHA is pivotal in regulating systemic inflammation. However, DHA also plays a structural role in the central nervous system in glial and neuronal cell membranes. Its structural role in the central nervous system makes it the predominant fatty acid in the brain. Although ALA can be converted to both EPA and DHA the rate of conversion is slow in humans. Therefore DHA is conditionally essential (here) and must be supplied performed in the diet.
Areas of the brain associated with memory and attention are able to concentration DHA, and this improves the efficiency of such areas. It is estimated that intakes of DHA are generally low in the typical Western diet because fish and other seafood sources of preformed DHA are absent from commonly eaten foods. Therefore it is possible that many Westerners living in developed nation have inhibited cognitive function. Indeed, this hypothesis was supported by a recent paper published in the American Journal of Clinical Nutrition1. In the study, researchers used a randomised double-blind placebo controlled design to feed healthy subjects either 1.16 grams of DHA per day or a placebo. The researchers then used a battery of tests to challenge cognitive performance. The reaction times of both episodic and working memory improved in the DHA group compared to the placebo group. Sex modulated the response to the DHA with reaction times of episodic memory and working memory improving more in women and men, respectively.
Analysis of the diets of the subjects revealed they ate a typical Western diet and that their intakes of DHA were low. Supplementation with the DHA increased erythrocyte concentrations by 2.6 % compared to the placebo, showing that the supplements were effective at increasing tissue concentrations of DHA. Supplementation also lowered the ratios of arachidonic acid (AA, C20:4 (n-6)) (the precursor to the pro-inflammatory series 2 eicosanoids) to DHA and EPA. The authors suggested that this shift in ratios may have contributed to the improved cognition. Some previous research has reported no cognitive benefits to DHA supplementation, which may relate to the short duration of the studies. In the above cited study improvements were seen after 6 months of supplementation, and this duration may be required for DHA to accumulate in the relevant areas of the brain. The sex differences shown may relate to the different problem solving strategies between men and women, although oestrogen is known to modulate DHA metabolism (here and here).
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