Recovery of [13C]-bicarbonate as respiratory 13CO2 in parenterally fed infants

Ten infants on continuous total parenteral nutrition (TPN) were infused with NaH13CO3 for 6 h in order to assess the amount of 13C recovered as breath 13CO2. Protein intake was 2.8 +/- 0.3 g/kg/d and non-protein energy intake 107 +/- 4 kcal/kg/d (447 +/- 18 kJ/kg/d), provided either as glucose alone or as an isoenergetic glucose-lipid mixture. In the five infants receiving glucose as the sole non-protein energy source, total CO2 production (559 +/- 50 mumol/kg/min), natural 13C abundance of breath CO2 (-11.8 +/- 0.6 delta % versus PDB) and basal 13CO2 production (6.1 +/- 0.6 mumol/kg/min) were higher than in the five infants infused the glucose-lipid mixture (465 +/- 30 mumol/kg/min, P less than 0.02; -16.1 +/- 0.5 delta %, P less than 0.01 and 5.0 +/- 0.3 mumol/kg min, P less than 0.02, respectively). There was a good agreement, in the glucose-infused infants, between the net glucose oxidation rate measured by indirect calorimetry (25.6 +/- 2 g/kg/d) and the glucose oxidation rate estimated from the 13C natural abundances of breath CO2 and infused substrates (23.5 +/- 3 g/kg/d). Steady state 13C enrichment of breath CO2 was reached in all infants after 120 min infusion and ranged from 11.0 to 21.5 delta % over baseline. Steady state 13C enrichment was negatively related to total CO2 production (r = -0.72; P less than 0.02). In contrast, steady state 13CO2 production in excess of baseline was only correlated to bicarbonate infusion rate (r = 0.95; P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)