Determinants of urea production and mineral retention in parenterally fed preterm infants.

OBJECTIVE To determine how weight for gestational age affects urea and mineral excretion by preterm infants receiving total parenteral nutrition (TPN). STUDY DESIGN Daily urine samples were collected from all preterm infants given high calcium TPN, providing 30 kcal/g amino acids, during its first 44 months of use, and from all those given standard TPN, providing 25 kcal/g amino acids, over the previous 24 months. Urine urea and mineral excretion were measured as follows: Urea excretion mmol/kg/day = Urine urea/urine creatinine X creatinine production Creatinine production μmol/kg/day = -2.07 + 2.34 X gestational age in weeks Results: High calcium TPN was evaluated in 52 infants. Urea excretion did not rise with increasing TPN intake. During the first week, urea excretion increased with weight for gestational age, with higher rates in above average than below average weight infants. It also increased with gestational age in above average but not below average weight infants. Below average weight infants had lower potassium and phosphate excretion than those above average. Standard TPN was evaluated in 20 infants. Urea excretion increased with TPN intake to higher levels than on high calcium, and also increased with weight for gestational age. CONCLUSION Urea excretion was simple to measure, with remarkably consistent daily results in individuals. Below 30 weeks gestation infants on TPN providing 30 kcal/g amino acids had urea excretion < 0.1 g urea N/kg/day, < 3.5 mmol/kg/day if below average weight, and < 0.12 g urea N/kg/day, < 4.3 mmol/kg/day if above average weight. Below average weight infants retained more potassium and phosphate during the first week than those above average, and their greater requirements were provided by the TPN.