Ischaemic heart disease is characterised by a reduced blood flow to the heart caused by blockage of the coronary artery as seen in coronary artery disease. The long chain n-3 polyunsaturated fatty acids (PUFA) in fish oils, eicosapentanoic acid (EPA, C20:5 (n-3)) and docosahexanoic acid (DHA, C22:6 (n-3)), have been shown to have cardioprotective effects that include antithrombotic, antihypertensive, antiarrhythmic and antiinflammatory effects. Both EPA and DHA can be synthesised endogenously from their precursor essential fatty acid α-linolenic acid (ALA, C18:3 (n-3)). However the conversion in humans is inefficient due to genetic deficiency of the necessary desturase enzyme. Because EPA and DHA are absent from some diets, researchers are interested in comparing their physiological efficacy with the precursor fatty acid (ALA) which tends to be present at higher intakes in most diets.
For example, researchers1 have investigated the association between intake of ALA, EPA and DHA and risk of ischaemic heart disease. Because linoleic acid (LA, C18:2 (n-6)) competes for the same desturase enzymes, the modifying effect of LA was also investigated. Using a prospective design study, researchers observed a Danish cohort of 3277 healthy subjects, and a follow-up period of 23.3 years resulted in 471 cases of ischaemic heart disease. The results showed that ALA intake was not significantly associated with a decrease in the risk of ischaemic heart disease, but there was a stepwise trend between tertiles. Intakes of long chain n-3 PUFAs were associated inversely with ischaemic heart disease, but only in women. However, there was no association between LA and ischaemic heart disease suggesting that the lack of association between ALA and Ischaemic heart disease was not due to LA interference.
These results support previous studies that have shown cardioprotective effects for long chain PUFAs from fish. In this study ALA did not show cardioprotective effects, but in some studies positive results have been reported. For example, 1g/d ALA has been reported to be associated with a 47% reduction in the risk of ischaemic heart disease. This suggest that confounding variables are creating uncertain results, and this may relate to the other components of the diet. Some dietary components are known to affect desturase enzymes involved in ALA metabolism and this may therefore have an influence on results. For example, the long chain PUFAs from fish can cause inhibition of Δ6-desaturase, and thus the conversion of ALA to EPA and DHA. However, in this study there was no evidence of modification of the effects of ALA by EPA or DHA on ischaemic heart disease risk.
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