Food and Behaviour Research

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

Burdge GC, Jones AE and Wootton SA (2002) Br J Nutr  88 355-363 

Web URL: Licensed users of Br J Nutr online can view this paper here

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 13CO2 on breath over the first 24 h. [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 14 d, 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 21 d. 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.

FAB RESEARCH COMMENT:

This study found that when young men were fed the short-chain omega-3 fatty acid (alpha-linolenic acid, or ALA, found in some plant and seed oils), their ability to convert this into the long-chain omega-3 most important for health (EPA and DHA, found naturally in fish and seafood) was very limited - and in the case of DHA, effectively absent. 

Adequate supplies of DHA are essential for normal brain structure and function - so these findings underscore the importance of a direct dietary supply of DHA for men.

Sex hormones are thought to influence the conversion of short- to long-chain omega-3 - and in keeping with this, a parallel study from the same group showed that although conversion of ALA to EPA and DHA was also limited in young women, they were at least able to make some DHA. See: