Recovery of 13C in breath from infused NaH13CO3 increases during euglycaemic hyperinsulinaemia

1. The effect of euglycaemic hyperinsulinaemia on the recovery of 13C in expired CO2 has been assessed in six normal subjects. Each was studied on three occasions: once with a 6 h primed constant infusion of NaH13CO3 combined with a euglycaemic hyperinsulinaemic clamp for the last 3 h (study 1), once with a 6 h primed constant infusion of NaH13CO3 alone (study 2) and once with a 6 h infusion of normal saline combined with a hyperinsulinaemic clamp for the last 3 h (study 3). Measurements of 13C enrichment of expired CO2 were made in the third and sixth hour of each infusion. 2. There was no significant increase in enrichment during study 3 (3 h 0.00047 +/- 0.00016 versus 6 h 0.00069 +/- 0.00028 atom per cent excess) with potato-starch-derived D-glucose used to maintain euglycaemia. 13C recovery increased in the sixth hour of both study 1 and 2 (study 1: 3 h 74.4 +/- 2.0 versus 6 h 85.5 +/- 2.6%, P < 0.01; study 2: 3 h 72.1 +/- 2.4 versus 6 h 81.7 +/- 1.4%, P < 0.01). There was no significant difference in recovery between studies 1 and 2. 3. These results suggest that increased recovery during a sequential euglycaemic clamp is predominantly time-dependent. Studies which use this technique to examine the effect of insulin on substrate oxidation should take this into account.