Measurement of 13C-glucose oxidation rate using mass spectrometric determination of the CO2: Ar ratio and spirometry.

A new method was developed and validated for measuring the CO2 concentration in the breath by mass spectrometric analysis. Argon, an inert gas that is present in air in a constant concentration of 0.923%, was used as an internal standard. By determining the ratio of CO2 (mass 44) to Ar (mass 40) in a breath sample, it was possible to read the CO2 concentration from a standard curve, relating CO2 concentration to CO2: Ar ratio. By combining mass spectrometric determination of CO2 concentration in breath with spirometric measurement of expired volumes, the CO2 production was determined in 67 subjects at rest. The mean value was 8.86 mmol kg-1 h-1, but there was considerable interindividual variation. This new method was applied to glucose oxidation studies in 10 normal subjects, 10 post-gastrectomy patients and 7 obese type II diabetic subjects. Measurement of the 13CO2 exhalation with quantitative determination of CO2 production allowed more accurate determination of the CO2 excretion rate in relation to blood levels of glucose, insulin and free fatty acids than assuming the constant CO2 production of 300 mmol unit body surface -1 h-1 or 9 mmol kg-1 h-1. It also resulted in a better discrimination between normal subjects and diabetics.