Metabolism of C-13-labeled linoleic acid in newborn infants during the first week of life

Linoleic acid metabolism was studied during the first week of life in 10 br east-fed, full-term infants. Uniformly C-13-labeled linoleic acid (1 mg/kg body weight) was given orally. The C-13 content was determined in expired C O2 over 6 h and in plasma phospholipid fatty acids over 3 d. Total CO2 prod uction determined by indirect calorimetry was 16.7 +/- 10.6 mL/min (mean +/ - SE). Over 6 h 7.4 +/- 0.6% of the ingested C-13-labeled linoleic acid was oxidized to CO2. Plasma phospholipid linoleic acid showed maximal C-13 enr ichment 24 h after tracer application (delta over baseline 178 +/- 24 parts per thousand). Enrichment of dihomo-gamma-linolenic acid increased from d 2 to d 5 of life (p < 0.002), with delta over baseline values of 2.1 +/- 0. 5 parts per thousand at 24 h, 3.7 +/- 10.9 parts per thousand at 48 h, and 4.4 +/- 1.0 parts per thousand at 72 h. C-13 content of arachidonic acid te nded to increase insignificantly. Areas under the curve of plasma tracer co ncentration over time were calculated for plasma n-6 phospholipid fatty aci ds. Percentages of total areas under the curve of the investigated n-6 fatt y acids were 97.3 +/- 0.8% for linoleic acid, 1.5 +/- 0.6% for dihomo-gamma -linolenic acid, and 1.2 +/- 0.6% for arachidonic acid. The proportion of l inoleic acid oxidized to CO2 did not correlate with the estimated conversio n to long-chain polyunsaturated metabolites. Breast-fed newborn infants syn thesize n-6 long-chain polyunsaturated fatty acids already during the first week of life, but the contribution of endogenous synthesis to the total pl asma long-chain polyunsaturated pool is small. A major portion of dihomo-ga mma-linolenic acid is converted to arachidonic acid.