Conversion of $\alpha$-linolenic acid to longer-chain polyunsaturated fatty acids in human adults

The principal biological role of $\alpha$-linolenic acid ($\alpha$LNA; 18:3n-3) appears to be as a precursor for the synthesis of longer chain n-3 polyunsaturated fatty acids (PUFA). Increasing $\alpha$LNA intake for a period of weeks to months results in an increase in the proportion of eicosapentaenoic acid (EPA; 20:5n-3) in plasma lipids, in erythrocytes, leukocytes, platelets and in breast milk but there is no increase in docosahexaenoic acid (DHA; 22:6n-3), which may even decline in some pools at high $\alpha$LNA intakes. Stable isotope tracer studies indicate that conversion of $\alpha$LNA to EPA occurs but is limited in men and that further transformation to DHA is very low. The fractional conversion of $\alpha$LNA to the longer chain n-3 PUFA is greater in women which may be due to a regulatory effect of oestrogen. A lower proportion of $\alpha$LNA is used for $\beta$-oxidation in women compared with men. Overall, $\alpha$LNA appears to be a limited source of longer chain n-3 PUFA in humans. Thus, adequate intakes of preformed long chain n-3 PUFA, in particular DHA, may be important for maintaining optimal tissue function. Capacity to up-regulate $\alpha$LNA conversion in women may be important for meeting the demands of the fetus and neonate for DHA