Comparison of indirect calorimetry and a new breath 13C/12C ratio method during strenuous exercise

Abstract

A new stable isotope method for the determination of substrate oxidation rates in vivo is described and compared with indirect calorimetry at rest and during high-intensity exercise (30 min at 80-85% maximal O2 uptake capacity) in six well-trained cyclists. This method uses the absolute ratios of 13C/12C in expired air, endogenous glucose, fat, and protein in addition to O2 consumption and is independent of CO2 production (VCO2). Carbohydrate and fat oxidation rates at rest, calculated by both methods, were not significantly different. During exercise the breath 13C/12C ratio increased and reached a steady state after 15-20 min. Carbohydrate oxidation rates during exercise were 39.4 +/- 5.2 and 41.7 +/- 5.7 mg.kg-1.min-1 [not significant (NS)], and fat oxidation rates were 7.3 +/- 1.3 and 6.9 +/- 1.2 mg.kg-1.min-1 (NS), using indirect calorimetry, and the breath ratio method, respectively. We conclude that the breath 13C/12C ratio method can be used to calculate substrate oxidation under different conditions, such as the basal state and exercise. In addition, the results obtained by this new method support the validity of the underlying assumption that indirect calorimetry regards VCO2 as a reflection of tissue CO2 production, during exercise in trained subjects, even up to 80-85% maximal O2 uptake.

Measurement and Physiological Relevance of the Maximal Lipid Oxidation Rate During Exercise (LIPOXmax)

“…Thus, the increase in RER in these exercise conditions is almost completely explained by the balance between oxidized carbohydrates and lipids, independent of blood lactate. The validity of this calorimetric approach is further confirmed by a classical work of Romijn (Romijn et al, 1992) who showed in highly trained sportsmen that up to 80-85% VO 2 max calorimetric calculations based on respiratory exchanges during exercise closely fit with much more sophisticated measurements using stable isotopes (MacRae et al, 1995). Concerning proteins, if one compares exercise bouts at 33 and 66% of VO 2 max it can be demonstrated that their use for oxidation remains stable at the various levels of exercise, supporting the basal assumption that the balance of substrates may be interpreted in terms of respective percentage of oxidized fat and carbohydrates.…”

“…Our protocol based on 6-minutes workloads seems preferable (Bordenave et al, 2007). As already developed above, Romijn (Romijn et al, 1992) compared, in highly trained endurance cyclists, calorimetric results and isotopic measurement during exercise tests up to 85% VO 2 max and showed that at this level calorimetry is fully reliable. However, a look at the figures of this paper shows that the steady state of RER occurs after 4 min and is not obtained after 2 minutes.…”

“…The theoretical concern was that, when exercise is performed above the lactate threshold, there is an extra CO 2 production which can be assumed to interfere with the calculations (MacRae et al, 1995). In fact, below 75% of the VO 2max , this increase in CO 2 has no measurable effect on calorimetric calculations (Romijn et al, 1992), so that these calculations predict closely oxidation rates measured by stable isotope labeling (Christmass et al, 1999). Clearly, even at high intensity exercise, respiratory gases are mostly the reflect of the balance of substrate oxidation.…”

Constrained optimization in human walking: cost minimization and gait plasticity

“…Oxygen and CO 2 exchange rates were converted to caloric equivalents (Lusk, 1924), according to the assumption of low- to-moderate intensity work load and a respiratory exchange ratio (RER) below the blood CO 2 buffer level (Romijn et al, 1992). Metabolic cost measurements were considered acceptable if the average RER value over the final 2·min of each trial was 0.92 or less.…”

Substrate oxidation during exercise: impact of time interval from the last meal in obese women

“…19 In fact, it has been demonstrated that the contribution of bicarbonatederived CO 2 to VCO 2 is rather negligible and that VCO 2 can be considered to be a reliable reflection of muscle tissue CO 2 production during exercise, including at high intensity, as long as RER is lower than 1. 20,21 Furthermore, as previously shown, this type of exercise testing can be used to characterize alterations in substrate utilization in the obese during exercise 6 and has clear beneficial effects. [8][9][10] In this study, the values of W max , expressed in absolute value, in terms of percentage of predetermined W max and in terms of heart rate corresponding to COP ox , were very close to those reported previously in overweight females.…”