Metabolism in preterm infants on the first days of life: The effect of corticosteroids

During pregnancy, the fetus receives from its mother a continuous intravenous supply of energy substrates via the placenta. In most species, plasma lipids do not readily cross the placenta. In the human, where the placental transfer oflipids is limited, these substrates do not contribute to fetal oxidative metabolism since fetal tissues have a very low capacity for free fatty acids (FFA) oxidation. The FFA that cross the placenta are stored in adipose tissue and liver. 'When the mother is well fed, the supply of glucose and amino acids to the fetus is sufficient to cover its needs for oxidative metabolism and growth, and the fetus has no need to produce endogenous glucose. Excess of glucose is stored as glycogen in many tissues, particularly in the liver and as lipids in adipose tissue. After birth, the continuous intravenous high-carbohydrate low-fat diet ofthe fetus is replaced by a discontinuous enteral high-fat lowcarbohydrate diet

Cyst(e)ine requirements in enterally fed very low birth weight preterm infants

OBJECTIVE. Optimal nutrition is of utmost importance for the preterm infant's later health and developmental outcome. Amino acid requirements for preterm infants differ from those for term and older Infants, because growth rates differ. Some nonessential amino acids, however, cannot be sufficiently synthesized endog-enously. Cyst(e)ine is supposed to be such a conditionally essential amino acid in preterm infants. The objective of this study was to determine, at 32 and 35 weeks' postmenstrual age, cyst(e)ine requirements in fully enterally fed very low birth weight preterm infants with gestational ages of <29 weeks. METHODS. Infants were randomly assigned to 1 of the 5 graded cystine test diets that contained generous amounts of methionine. Cyst(e)ine requirement was determined with the indicator amino acid oxidation technique ([I-13C]phenylaIanlne) after 24-hour adaptation. RESULTS.Fractional [I13-CJphenylalanine oxidation was established in 47 very low birth weight preterm infants (mean gestational age: 28 weeks ± 1 week SD; birth weight: 1.07 kg ± 0.21 kg SD). Increase in dietary cyst(e)ine intake did not result in a decrease in fractional [l-13,CJphenylalanine oxidation. CONCLUSIONS.These data do not support the hypothesis that endogenous cyst(e)ine synthesis is limited in very low birth weight preterm infants with gestational ages of <29 weeks, both at 32 and 35 weeks postmenstrual age. It is safe to conclude that cyst(e)ine requirement is <I8 mg/kg per day in enterally fed very low birth weight preterm infants who are older than 32 weeks' postmenstrual age and whose methionine intake is adequate. Therefore, cyst(e)ine is probably not a conditionally essential amino acid in these infants. Copyrigh

Endogenous surfactant turnover in preterm infants measured with stable isotopes

We studied surfactant synthesis and turnover in vivo in preterm infants using the stable isotope [U-13C]glucose, as a precursor for the synthesis of palmitic acid in surfactant phosphatidylcholine (PC). Six preterm infants (birth weight, 916 +/- 244 g; gestational age, 27.7 +/- 1.7 wk) received a 24-h [U-13C]glucose infusion on the first day of life. The 13C-enrichment of palmitic acid in surfactant PC, obtained from tracheal aspirates, was measured by gas chromatography-combustion interface-isotope ratio mass spectrometry. We observed a significant incorporation of carbon-13 from glucose into surfactant PC palmitate. PC palmitate became enriched after 19.4 +/- 2.3 (16.5 to 22.3) h and reached maximum enrichment at 70 +/- 18 (48 to 96) h after the start of the label infusion. The fractional synthesis rate (FSR) of surfactant PC palmitate from glucose was 2.7 +/- 1.3%/d. We calculated the absolute production rate of surfactant PC to be 4.2 mg/kg/d, and the half-life to be 113 +/- 25 (87 to 144) h. Data on endogenous surfactant production and turnover were obtained for the first time in human infants with the use of stable isotopes. This novel and safe method could be applied to address many important issues concerning surfactant metabolism in preterm infants, children, and adults