QUANTIFICATION OF CARBOHYDRATE OXIDATION BY RESPIRATORY GAS-EXCHANGE AND ISOTOPIC TRACERS

Estimates of glucose oxidation measured by indirect respiratory calori metry and by [U-C-13]glucose tracer were compared as a function of res piratory exchange ratio (RER) in 14 studies performed on 9 healthy adu lt sub jects. RER was varied between 0.7 and 1.04, either by fasting o r by infusing glucose. C-13 enrichment of plasma glucose and expired C O2 were measured by mass spectrometry. The two methods gave similar re sults when the nonprotein respiratory quotient (NPRQ) was between 0.76 and 0.90. Glucose oxidation by the tracer method was quantified to be higher than that by respiratory calorimetry when NPRQ was <0.76; it w as lower than the respiratory calorimetry estimate when NPRQ was >0.90 . The discrepancy between the two methods at low RER may represent the contribution of gluconeogenesis, whereas, at high RER, the discrepanc y may be the consequence of lipogenesis. We conclude that respiratory calorimetry and [C-13]glucose tracer give comparable results only in a narrow range of RER. These data are important when the disposal of gl ucose is compared using these techniques in different metabolic states with varying respiratory quotients.