Key Findings:
This was one of the first studies to determine the efficacy of ALA conversion to EPA and DHA. The effect of ALA deficiency on neurological function supports the role of ALA as a precursor to longer chain n-3 PUFA which are critical in the function of the central nervous system. Based on dated information, the author indicates that studies whereby increased ALA intake or using a single bolus of isotopically- labelled ALA indicate that conversion of ALA to longer-chain PUFA, particularly DHA, in humans appears to be limited. Women have an increased capacity for synthesis of EPA and DHA from ALA. If demands for EPA and DHA are modest and primarily serve to support membrane turnover and renewal in adults, then it is possible that in healthy individuals consuming a balanced diet limited capacity for synthesis of EPA and DHA may be sufficient to maintain tissue function. There may be physiological mechanisms by which ALA conversion is up-regulated.
ABSTRACT:
The principal biological role of α-linolenic acid (ALA; 18:3n-3) appears to be as a precursor for the synthesis of longer chain n-3 polyunsaturated fatty acids (PUFA). Increasing ALA 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 ALA intakes. Stable isotope tracer studies indicate that conversion of ALA to EPA occurs but is limited in men and that further transformation to DHA is very low. The fractional conversion of ALA 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 ALA is used for β-oxidation in women compared with men. Overall, ALA 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 ALA conversion in women may be important for meeting the demands of the fetus and neonate for DHA. (Authors abstract)