Measurement of 13 C‐glucose oxidation rate using mass spectrometric determination of the CO 2 : Ar ratio and spirometry

A new method was developed and validated for measuring the CO 2 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 CO 2 (mass 44) to Ar (mass 40) in a breath sample, it was possible to read the CO 2 concentration from a standard curve, relating CO 2 concentration to CO 2 : Ar ratio. By combining mass spectrometric determination of CO 2 concentration in breath with spirometric measurement of expired volumes, the CO 2 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 13 CO 2 exhalation with quantitative determination of CO 2 production allowed more accurate determination of the CO 2 excretion rate in relation to blood levels of glucose, insulin and free fatty acids than assuming the constant CO 2 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.