CONVERSION OF OCTANOIC-ACID INTO LONG-CHAIN SATURATED FATTY-ACIDS IN PREMATURE-INFANTS FED A FORMULA CONTAINING MEDIUM-CHAIN TRIGLYCERIDES

A large number of very-low-birth weight infants are fed formulas conta ining medium-chain triglycerides (MCT) to enhance fat and calcium abso rption. Studies are available on the intestinal absorption of MCT, whi ch is nearly complete, but uncertainties exist on the metabolic fate o f octanoic acid, the major component of MCT. Oxidation accounts for ap proximately 50% of the dietary intake, and losses as dicarboxylic acid s in the urine are negligible. Since storage in adipose tissue is limi ted, conversion into long-chain fatty acids (LCFA) is likely to be an important route. To study the nonoxidative metabolism of MCT, six pret erm infants fed a standard premature formula containing 38 weight% (wt %) MCT (54 mol% medium chain fatty acids (MCFA), of which 35 mol% is o ctanoic 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 ora l primed constant-rate infusion of [C-13]-octanoate and the measuremen t of the C-13 enrichment of individual fatty acids in plasma triglycer ides (TG) by a highly sensitive on-line combustion method using gas ch romatography-isotope ratio mass spectrometry (GC-IRMS). We observed a significant incorporation of the dietary [C-13]-octanoic acid in plasm a TG (10.0% +/- 4.5% of the enrichment of the diet). A noticeable inco rporation 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 di et. Octanoic and decanoic acids in plasma were only 7.3% and 32%, resp ectively, of their mol% content of the diet, and myristic and palmitic acids were increased by 225% and 343%. Our findings demonstrate for t he first time in vivo the conversion of octanoic acid into long-chain saturated fatty acids. We speculate that this process, apart from bein g metabolically inefficient, could be responsible for the high levels of long-chain saturated and monounsaturated fatty acids found in subje cts fed MCT-containing diets, and it may interfere with the metabolism of other fatty acids. Copyright (C) 1994 by W.B. Saunders Company