Effect of triacylglycerol structure on absorption and metabolism of isotope-labeled palmitic and linoleic acids by humans

The effect of dietary TAG structure and fatty acid acyl TAG position on palmitic and linoleic acid metabolism was investigated in four middle-aged male subjects. The study design consisted of feeding diets containing 61 g/d of native lard (NL) or randomized lard (RL) for 28 d. Subjects then received an oral dose of either 1,3-tetradeuteriopalmitoyl-2-dideuteriolinoleoyl-rac-glycerol or a mixture of 1,3-dideuteriolinoleoyl-2-tetradeuteriopalmitoyl-rac-glycerol and 1,3-hexadeuteriopalmitoyl-2-tetradeuteriolinoleoyl-rac-glycerol. Methyl esters of plasma lipids isolated from blood samples drawn over a 2-d period were analyzed by GC-MS. Results showed that absorption of the H-2-fatty acids (H-2-FA) was not influenced by TAG position. The H-2-FA at the 2-acyl TAG position were 85 +/- 4.6% retained after absorption. Substantial migration of H-2-1 6:0 (31.2 +/- 8.6%) from the sn-2 TAG position to the sn-1,3 position and H-2-18:2n-6 (52.8 +/- 6.4%) from the sn-1,3 position to the sn-2 position of chylomicron TAG occurred after initial absorption and indicates the presence of a previously unrecognized isomerization mechanism. Incorporation and turnover of the 2H-FA in chylomicron TAG, plasma TAG, and plasma cholesterol esters were not influenced by TAG acyl position. Accretion of (2)H16:0 from the sn-2 TAG position in 1-acylphosphatidylcholine was 1.7 times higher than 2H-1 6:0 from the sn-1,3 TAG positions. Acyl TAG position did not influence 2H-1 8:2n-6 incorporation in PC. The concentration of H-2-18:2n-6-derived H-2-20:4n-6 in plasma PC from subjects fed the RL diet was 1.5 times higher than for subjects fed the NL diet, and this result suggests that diets containing 16:0 located at the sn-2 TAG position may inhibit 20:4n-6 synthesis. The overall conclusion is that selective rearrangement of chylomicron TAG structures diminishes but does not totally eliminate the metabolic and physiological effects of dietary TAG structure.