Eicosapentaenoic and docosapentaenoic acids are the principal products of a-linolenic acid metabolism in young men.

Key Findings:

The metabolism of [U-13C]ALNA in a typical UK diet was assessed. The results describe incorporation of 13C into n-3 LCPUFA in plasma, incorporation of 13C-labelled fatty acids into erythrocyte membrane phospholipids and the extent of partitioning of [13C]ALNA towards beta-oxidation. The principal products of desaturation and elongation of ALNA were EPA and DPA. There was no evidence for increased 13C enrichment in DHA. Increased plasma DHA concentration following increased ALNA intakes has been reported in some studies. In this study, the metabolic demands for DHA may have been met by existing pools of DHA within the body or by dietary supply of DHA thus making conversion from ALA not required.

ABSTRACT:

The capacity for conversion of alpha-linolenic acid (ALNA) to n-3 long-chain polyunsaturated fatty acids was investigated in young men. Emulsified [U- 13C]ALNA was administered orally with a mixed meal to six subjects consuming their habitual diet. Approximately 33% of administered [13C]ALNA was recovered as 13CO 2 on breath over the first 24h. [13C]ALNA was mobilised from enterocytes primarily as chylomicron triacylglycerol (TAG), while [13C]ALNA incorporation into plasma phosphatidylcholine (PC) occurred later, probably by the liver. The time scale of conversion of [13C]ALNA to eicosapentaenoic acid (EPA) and docosapentaenoic acid (DPA) suggested that the liver was the principal site of ALNA desaturation and elongation, although there was some indication of EPA and DPA synthesis by enterocytes. [13C]EPA and [13C]DPA concentrations were greater in plasma PC than TAG, and were present in the circulation for up to 7 and 14d, respectively. There was no apparent 13C enrichment of docosahexaenoic acid (DHA) in plasma PC, TAG or non-esterified fatty acids at any time point measured up to 21d. This pattern of 13C n-3 fatty acid labelling suggests inhibition or restriction of DHA synthesis downstream of DPA. [13C]ALNA, [13C]EPA and [ 13C]DPA were incorporated into erythrocyte PC, but not phosphatidylethanolamine, suggesting uptake of intact plasma PC molecules from lipoproteins into erythrocyte membranes. Since the capacity of adult males to convert ALNA to DHA was either very low or absent, uptake of pre-formed DHA from the diet may be critical for maintaining adequate membrane DHA concentrations in these individuals. (Author’s abstract)