n-6 fatty acid metabolism in the newborn infant: is linoleic acid sufficient to meet the demand for arachidonic acid?

Laboratory of Metabolic Control, National Institutes on Alcohol Abuse & Alcoholism, NIH, Rm 1S-22 5625 Fishers Lane, Bethesda, MD 20892, Bethesda, Maryland, USA

^* bpawl@mail.nih.gov

Abstract

Two compartmental models were developed to assess the contributions of linoleic acid, 18:2n-6, and di-homo-g-linoelic acid, 20:3n-6, toward maintaining plasma homeostasis concentrations of arachidonic acid, 20:4n-6, in newborn infants. Ten infants received oral doses of ¹³C-U-18:2n-6 and ²H5-20:3n-6 ethyl esters (100 and 2 mg kg⁻¹, respectively). Rate constant coefficients of n-6 FAs were determined from the time-course concentrations of labeled-FAs and endogenous plasma n-6 FA values were used to approximate steady state concentrations. Eight percent (range: 2-21%) of plasma ¹³C-U-18:2n-6 was utilized for synthesis of ¹³C -18:3n-6, -20:2n-6 and -20:3n-6 and 70% 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 only 26%. Turnover of 18:2n-6 in subjects and of 20:4n-6 in plasma was 4.2 g kg⁻¹ d⁻¹ (CV: 0.58) and 4.3 mg kg⁻¹ d⁻¹ (CV: 0.81), respectively. Intake of 18:2n-6 and 20:4n-6 were estimated to be 3.0 g kg⁻¹ d⁻¹ (± 1.7) and 2.8 mg kg⁻¹ d⁻¹ (± 2.2), respectively. Infants required additional 18:2n-6 (1.2 g kg⁻¹ d⁻¹) above predicted intake amounts to maintain plasma concentrations of 18:2n-6. The percent conversion of 18:2n-6 to 20:4n-6 was incapable of sustaining plasma 20:4n-6 concentrations in nearly all subjects necessitating a supplemental intake of ~ 4 mg kg⁻¹ d⁻¹ of 20:4n-6.