Quantification of carbohydrate oxidation by respiratory gas exchange and isotopic tracers

Abstract

Estimates of glucose oxidation measured by indirect respiratory calorimetry and by [U-13C]glucose tracer were compared as a function of respiratory exchange ratio (RER) in 14 studies performed on 9 healthy adult subjects. RER was varied between 0.7 and 1.04, either by fasting or by infusing glucose. 13C enrichment of plasma glucose and expired CO2 were measured by mass spectrometry. The two methods gave similar results 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 was 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 discrepancy may be the consequence of lipogenesis. We conclude that respiratory calorimetry and [13C]glucose tracer give comparable results only in a narrow range of RER. These data are important when the disposal of glucose is compared using these techniques in different metabolic states with varying respiratory quotients.

Pushing the limits of glucose kinetics: how rainbow trout cope with a carbohydrate overload

“…Therefore, it is a measure of the highest possible rate of glucose oxidation when non-oxidative disposal is nil. In the resting state, mammals only oxidize about 50% of R d glucose as reported for rats (43%) (Brooks and Donovan, 1983), dogs (30-50%) (Paul and Bella Issekutz, 1967;Wasserman et al, 1992) and humans (40-60%) (Glamour et al, 1995;Katz et al, 1992). The fraction of R d glucose actually oxidized in resting trout has never been measured, but is most likely less than 100%, as in mammals.…”

Carbohydrate as nutrient in the infant and child: range of acceptable intake

“…When the contribution of glucose to CO 2 was measured by tracer C, a constant fraction 50 ± 60% of glucose C appeared in CO 2 . The biological signi®cance of this observation is unclear and may represent the relation between pyruvate cycling and the cycling of carbon in the TCA cycle (Glamour et al, 1995). Nonetheless these data are important as they point to the problems of tracer-determined oxidation of glucose.…”

“…Such a rate of oxidation will meet only $ 50% of the energy need at any age. We chose to calculate oxidation of glucose from respiratory quotient data rather than tracer isotope data because respiratory calorimetry data include all sources of glucose oxidized including intracellular glycogen (Glamour et al, 1995).…”

“…The relation between exogenously administered glucose and the contribution of carbohydrate to total energy was systematically evaluated in normal healthy adults by Glamour et al (1995). Fourteen healthy non-diabetic adults were studied either during fasting or during intravenous glucose infusion at constant rate.…”

A peripheral perspective of weight regain

“…Therefore, for the same amount of consumed energy, more weight will be gained if the ingested fat is stored and the carbohydrate is used for energy. One additional consideration in this discussion, however, is that de novo lipogenesis is likely occurring during weight regain, an effect that will elevate RQ and make it less reflective of substrate oxidation (13). As Evans et al (9) point out in their study, the RQ above 1.0 in refeeding rats is a fairly good indication that a significant amount of lipogenesis is occurring (1).…”