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

The principal biological role of alpha-linolenic acid (alphaLNA; 18:3n-3) appears to be as a precursor for the synthesis of longer chain n-3 polyunsaturated fatty acids (PUFA). Increasing alphaLNA 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 alphaLNA intakes. Stable isotope tracer studies indicate that conversion of alphaLNA to EPA occurs but is limited in men and that further transformation to DHA is very low. The fractional conversion of alphaLNA 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 alphaLNA is used for beta-oxidation in women compared with men. Overall, alphaLNA 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 alphaLNA conversion in women may be important for meeting the demands of the fetus and neonate for DHA.