This study aimed to investigate energy substrate utilisation and nitrogen balance in low birthweight infants receiving total parenteral nutrition during the first days of life, and in particular, to determine the effect of two different glucose intakes on carbon dioxide production. Twenty infants (mean (SE) birthweight 1314 (65) g, mean (SE) gestation 30.9 (0.4) weeks) were recruited to the study. Immediately after birth they were randomised to a carbohydrate intake of 8 g/kg/day (5.5 mg/kg/minute) or 12 g/kg/day (8.3 mg/kg/minute). After 24 hours they were changed to the alternative regimen which was continued for a further 24 hours. Fat and protein intakes were kept constant throughout the study. Indirect calorimetry was performed during each of the regimens, urine was collected for urinary nitrogen, and substrate utilisation calculated for 12 infants. The carbohydrate utilisation rate was increased during the higher carbohydrate intake. Lipid utilisation rates were significantly different, with net lipid synthesis occurring during high carbohydrate intake. Protein utilisation rates were not influenced by the different carbohydrate intakes. The mean plasma glucose concentration was higher during the high carbohydrate intake but the mean highest and lowest values were not significantly different during the two study periods. A plasma glucose below 2.6 mmol/l was recorded more frequently during the low glucose intake (9/20 v 5/20). Capillary PCO2 values measured during high and low glucose intakes were similar (5.9 (0.2) v 6.2 (0.3) kPa. Carbon dioxide production rates were increased during the higher carbohydrate intake but the differences were not significant. Similarly, there was no significant difference in the respiratory quotients (RQ), oxygen consumption, or energy expenditure during the two study periods.
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