Compartmental Analyses of Plasma 13C- and 2H-Labeled n-6 Fatty Acids Arising from Oral Administrations of 13C-U-18:2n-6 and 2H5-20:3n-6 in Newborn Infants

Efficacy of 13C-U-18:2n-6 and 2H5-20:3n-6 toward synthesis of labeled-20:4n-6 was studied in newborn infants utilizing compartmental models of plasma labeled n-6 fatty acids (FA). Ten infants received oral doses of 13C-U-18:2n-6 and 2H5-20:3n-6 ethyl esters (100 and 2 mg/kg, respectively). Rate constant coefficients and half-lives (t½) of n-6 FA were determined from the time-course concentrations of labeled-FA. Plasma n-6 FA values approximated steady state concentrations. Synthetic and utilization rates were calculated. Eight percent (range, 2–21%) of plasma 13C-U-18:2n-6 was used for synthesis of 13C-18:3n-6, -20:2n-6, and -20:3n-6. Seventy percent of 13C-20:3n-6 (mean, CV: 0.26) was available for synthesis of 13C-20:4n-6. The percentage of 2H5-20:3n-6 converted to 2H5-20:4n-6 was lower (mean: 26%, p < 0.02) than the 13C-labeled analogue. Turnover of 18:2n-6 in subjects and of 20:4n-6 in plasma was 4.2 g/kg/d (CV: 0.58) and 4.3 mg/kg/d (CV: 0.81), respectively. Intake of 18:2n-6 and 20:4n-6 were estimated to be 3.0 g/kg/d (±1.7) and 2.8 mg/kg/d (± 2.2), respectively. Infants required additional 18:2n-6 and 20:4n-6 (mean: 1.2 g and 1.5 mg/kg/d) above predicted intake amounts to maintain plasma concentrations of 18:2n-6 and 20:4n-6, in order to spare FA from fat stores.