The essential fatty acid alpha linolenic acid (ALA, C18:3 (n-3)) is metabolised to the highly unsaturated fatty acids eicosapentanoic acid (EPA, C20:5 (n-3)) and docosahexanoic acid (DHA, C22:6 (n-3)). The brain preferentially accumulates DHA from plasma where it is incorporated into the cell membranes of neurones. It has been shown that the fluidity of the cell membrane is influenced by the type of fatty acids they contain. Saturated fatty acids and cholesterol molecules tend to reduce membrane fluidity whereas highly unsaturated fatty acids such as DHA and EPA tend to increase membrane fluidity. In addition, the ratio of n-3 (EPA, DHA) to n-6 (arachidonic acid; AA, C20:4 (n-6)) fatty acids in membranes influences the inflammatory conditions of the cell because n-3 and n-6 fatty acids are metabolised into eicosanoids that inhibit or stimulate inflammation, respectively.
High levels of DHA in the membranes of brain cells are therefore associated with reduced inflammation and possibly improved cognitive efficiency. It is thought that there is a general reduction in the concentration of DHA in the brain during ageing and that this might be associated with cognitive decline. For example, research1 published in the American Journal of Clinical Nutrition investigated the association between the erythrocyte fatty acid composition and cognitive decline in free-living individuals. The study measured the erythrocyte membrane fatty acid composition of 246 elderly men and women (age 63 to 74y) and assessed their cognitive ability 4 years later. The results showed that those individuals who had higher concentrations of stearic acid (STA, C18:0) and n-6 polyunsaturated fatty acids had an increased risk of cognitive decline when compared to those with higher concentrations of n-3 fatty acids.
These results support other research that shows that higher levels of n-3 fatty acids in plasma membranes are protective of the cognitive decline associated with ageing. It is known that higher DHA levels in plasma membranes are associated with higher levels of DHA in the membranes of neuronal cells in the brain and this in turn may have a number of beneficial effects. Theses include increases membrane fluidity, increased synaptic transmission and possibly promotion neurogenesis. Endogenously, DHA can be produced by eleongation and desaturation of ALA, but the conversion in humans is poor. Exogenous sources of DHA have been shown to increase erythrocyte plasma levels more efficiently and fatty fish is a good source of DHA. Fish oil capsules have also shown to be effective at increasing plasma levels of DHA and EPA.
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