Conversion of octanoic acid into long-chain saturated fatty acids in premature infants fed a formula containing medium-chain triglycerides

Abstract

A large number of very-low-birth weight infants are fed formulas containing medium-chain triglycerides (MCT) to enhance fat and calcium absorption. Studies are available on the intestinal absorption of MCT, which is nearly complete, but uncertainties exist on the metabolic fate of octanoic acid, the major component of MCT. Oxidation accounts for approximately 50% of the dietary intake, and losses as dicar☐ylic acids in the urine are neglibible. Since storage in adipose tissue is limited, conversion into long-chain fatty acids (LCFA) is likely to be an important route. To study the nonoxidative metabolism of MCT, six preterm infants fed a standard premature formula containing 38 weight% (wt%) MCT (54 mol% medium-chain fatty acids (MCFA), of which 35 mol% is octanoic acid) were studied at 4 weeks of age, when on full oral intake and receiving on average 130 kcal/kg/d. The study consisted of an oral primed constant-rate infusion of [ 13C]-octanoate and the measurement of the 13C enrichment of individual fatty acids in plasma triglycerides (TG) by a highly sensitive on-line combustion method using gas chromatography-isotope ratio mass spectrometry (GC-IRMS). We observed a significant incorporation of the dietary [ 13C]-octanoic acid in plasma TG (10.0% ± 4.5% of the enrichment of the diet). A noticeable incorporation of the label was detected in myristic and palmitic acids (4.6% ± 2.5% and 7.8% ± 4.1% of the octanoic enrichment of the diet). The absolute amount of the fatty acids was studied with conventional GC, and the plasma TG fatty acid profile differed markedly from the diet. Octanoic and decanoic acids in plasma were only 7.3% and 32%, respectively, of their mol% content of the diet, and myristic and palmitic acids were increased by 225% and 343%. Our findings demonstrate for the first time in vivo the conversion of octanoic acid into long-chain saturated fatty acids. We speculate that this process, apart from being metabolically inefficient, could be responsible for the high levels of long-chain saturated and monounsaturated fatty acids found in subjects fed MCT-containing diets, and it may interfere with the metabolism of other fatty acids.