Docosahexanoic acid (DHA, C22:6 (n-3)) and eicosapentanoic acid (EPA, C20:5 (n-3)) are two long chain fatty acids that are metabolites of the essential fatty acid alpha linolenic acid (ALA, C18:3 (n-3)). Dietary ALA undergoes a series of elongation and desaturation reactions that ultimately produced EPA and DHA. However, the conversion rate is low and in both adult and infant humans it has been estimated to be around 4%, probably due to genetic deficiency of the necessary desaturase enzymes. For most people this problem can be overcome by increasing dietary sources of DHA and EPA, which are present in high amounts in fatty fish. However, generally vegetarian individuals must rely on vegetable sources of ALA to provide their DHA and EPA requirements. Interestingly, women appear to have higher plasma levels of DHA than men, and this is thought to be due to hormonal differences.
For example, research1 has measured the plasma DHA levels of men and women in order to investigate how sex differences influence essential fatty acid metabolism. Researchers compared the DHA plasma levels of 73 male and 103 female healthy individuals that were fed a controlled diet with constant amounts of ALA. The results showed that DHA levels were 14.6 % higher in women than men, but there was no difference in EPA levels between the sexes and men had slightly higher levels of ALA. When the women were split into groups based on their contraceptive use, those not taking oral contraceptives had DHA levels only 10.2% higher than the men. Transsexual male to female subjects who were treated with the synthetic oestrogen drug ethinyl estradiol had a 42% increase in DHA levels. Testosterone administration to female to male transsexual subjects decrease DHA by 22%.
Research has demonstrated that women are protected from cardiovascular disease, possibly because they have higher levels of oestrogen. Many protective mechanisms have been proposed for oestrogen against cardiovascular disease, but these results suggest that higher levels of DHA may be the reason for this protection. Both EPA and DHA have been shown to be protective of cardiovascular disease in numerous studies. The higher levels of ALA in men are probably because the conversion to DHA is lower than in women, and as a result the levels of the precursor ALA builds up to a greater extent. It appears therefore that oestrogen is able to upregulate the synthesis of DHA from ALA contained in vegetable oils, possibly via hepatic pathways. This is probably a protective mechanism to ensure adequate foetal DHA and accounts for raises in DHA seen during pregnancy.
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