INFLUENCE OF THE BICARBONATE POOL AND ON THE OCCURRENCE OF CO-13(2) IN EXHALED AIR

In (CO2)-C-13 breath tests, based on C-13:C-12 ratio measurements, the appearance of C-13 in exhaled CO2 was monitored after the administration of a C-13-labelled compound. Independently of the substrate used, the existence of a bicarbonate pool into which the CO2 produced enters before being exhaled, imposes a delay on the appearance of changes in the C-13:C-12 ratio. To estimate the nature and magnitude of this delay, we applied a two-compartment model to describe the kinetics of the body bicarbonate pool and we evaluated the C-13:C-12 ratio of CO2 entering that pool from the measured C-13:C-12 ratio in the exhaled CO2 after an oral intake of "naturally labelled" C-13-glucose. Our results demonstrated that discrepancies between total and exogenous glucose oxidation in relation to the peak occurrence time, as well as the absolute quantities, could be adequately explained by the interference of the bicarbonate stores.