Dietary linoleic acid influences desaturation and acylation of deuterium-labeled linoleic and linolenic acids in young adult males

The purpose of this study was to investigate the effect of dietary linoleic acid (18:2(n − 6)) on the conversion of 18:2(n − 6) and 18:3(n − 3) to their respective n − 6 and n − 3 metabolites; to compare the incorporation of these fatty acids into human plasma lipids; to evaluate the importance of dietary 18:3(n − 3) as a precursor for the biosynthesis of long-chain length n − 3 fatty acids. The approach used was to feed young adult male subjects (n = 7) diets containing 2 levels of linoleic acid (SAT diet, 15 g/day; PUFA diet, 30 g/day) for 12 days. A mixture of triacylglycerols containing deuterated linolenic (18:3(n − 3)) and linoleic (18:2(n − 6)) acids was fed and blood samples were drawn over a 48 h period. Concentrations of deuterated 18:3(n − 3) in plasma total lipid ranged from 309.2 to 606.4 μg/ml and concentrations of 18:2(n − 6) ranged from 949.2 to 1743.3 μg/ml. The sum of the deuterated n − 3 long-chain length fatty acid metabolites in plasma total lipid were 116 ± 4.3 μg/ml (SAT diet) and 41.6 ± 12.4 jug/ml (PUFA diet). The total deuterated n − 6 fatty acid metabolites were 34.6 ± 12.2 μg/ml (SAT diet) and 9.8 ± 5.9 μg/ml (PUFA diet). The total percent conversion of deuterated 18:3(n − 3) to n − 3 fatty acid metabolites and deuterated 18:2(n − 6) to n − 6 fatty acid metabolites were 11–18.5% and 1.0–2.2%, respectively. The percentages for deuterated 20:5(n − 3), 22:5(n − 3) and 22:6(n − 3) (6.0%, 3.5%, and 3.8%) were much higher than for 20:3(n − 6) and 20:4(n − 6) (0.9% and 0.5%). Overall, conversion of deuterated 18:3(n − 3) and 18:2(n − 6) was reduced by 40–54% when dietary intake of 18:2(n − 6) was increased from 15 to 30 g/day. Comparison of the deuterated 18:3(n − 3) and 18:2(n − 6) data for plasma triacylglycerol and phosphatidylcholine (PC) indicated that 18:2(n − 6) was preferentially incorporated into PC. Dietary 18:2(n − 6) intake did not alter acyltransferase selectivity but activity was reduced when 18:2(n − 6) intake was increased. Based on these results, conversion of the 18:3(n − 3) in the US diet (2 g) is estimated to provide 75–85% of the long-chain length n − 3 fatty acids needed to meet daily requirements for some (but not all) adults.