Steady-state Respiratory Exchange Ratio Research Articles

In Vivo Muscle Bioenergetics Predict Whole-body Metabolic Responses During A 30-minute Treadmill Walk

While the bioenergetics of working skeletal muscle have been well-studied in humans, the impact of muscle energetic characteristics on the performance of everyday mobility tasks is less clear. PURPOSE: To evaluate the relationships between muscle oxidative and non-oxidative bioenergetic variables, an indicator of muscle substrate use (acetylcarnitine), and whole-body energy metabolism during a standardized mobility activity. METHODS: After obtaining written consent, intramyocellular energetics were measured in vivo in the vastus lateralis of 7 males (mean 28.4 yr, range 25-45) using proton and phosphorus magnetic resonance spectroscopy in response to 2 knee extension protocols: 1) 24 s of maximal isokinetic contractions (0.5 Hz), and 2) 8 min of incremental isotonic contractions (0.5 Hz, 2-min stages at 6, 9, 12, and 15% maximal torque). Continuous measures of ATP production via oxidative phosphorylation, glycolysis and the creatine kinase reaction (CK), and changes in [acetylcarnitine] from pre- to post-contractions, were made. Whole-body metabolic responses to a 30-min treadmill walk (30MTW) at 1.3 m∙s-1 were measured using indirect calorimetry by metabolic cart, from which the rate constant for oxygen (O2) deficit and excess post-exercise oxygen consumption (EPOC), and mean steady state respiratory exchange ratio (RER) were calculated. Spearman rank correlation coefficients (rs) were calculated to evaluate associations between muscle and whole-body metrics. RESULTS: The O2 deficit from the 30MTW was correlated with muscle non-oxidative ATP production by CK (rs = 0.82, p = 0.03) and glycolysis (rs = 0.79, p = 0.05) during the first 2 min of the incremental protocol, as well as with the decrease in [ATP] at the end of the protocol (rs = 0.90, p = 0.01). Muscle [acetylcarnitine] following the incremental protocol was correlated with RER during the 30MTW (rs = 0.81, p = 0.03), and EPOC was related to both non-oxidative ATP production during the incremental protocol (rs = 0.86, p = 0.02), and the rate of phosphocreatine recovery (reflecting oxidative flux) following this protocol (rs = -0.96, p = 0.003). CONCLUSIONS: The results provide novel evidence linking in vivo muscle oxidative and non-oxidative energetics to whole-body metabolic responses during a mobility task relevant to everyday activities.

Breath holding as an example of extreme hypoventilation: experimental testing of a new model describing alveolar gas pathways.

What is the central question of this study? We modelled the alveolar pathway during breath holding on the hypothesis that it follows a hypoventilation loop on the O2 -CO2 diagram. What is the main finding and its importance? Validation of the model was possible within the range of alveolar gas compositions compatible with consciousness. Within this range, the experimental data were compatible with the proposed model. The model and its characteristics might allow predictions of alveolar gas composition whenever the alveolar ventilation goes to zero; for example, static and dynamic breath holding at the surface or during ventilation/intubation failure in anaesthesia. According to the hypothesis that alveolar partial pressures of O2 and CO2 during breath holding (BH) should vary following a hypoventilation loop, we modelled the alveolar gas pathways during BH on the O2 -CO2 diagram and tested it experimentally during ambient air and pure oxygen breathing. In air, the model was constructed using the inspired and alveolar partial pressures of O2 ( and , respectively) and CO2 ( and , respectively) and the steady-state values of the pre-BH respiratory exchange ratio (RER). In pure oxygen, the model respected the constraint of . To test this, 12 subjects performed several BHs of increasing duration and one maximal BH at rest and during exercise (30W cycling supine), while breathing air or pure oxygen. We measured gas flows, and before and at the end of all BHs. Measured data were fitted through the model. In air, =150±1mmHg and =0.3±0.0mmHg, both at rest and at 30W. Before BH, steady-state RER was 0.83±0.16 at rest and 0.77±0.14 at 30W; =107±7mmHg at rest and 102±8mmHg at 30W; and =36±4mmHg at rest and 38±3mmHg at 30W. By model fitting, we computed the RER during the early phase of BH: 0.10 [95% confidence interval (95% CI) =0.08-0.12] at rest and 0.13 (95% CI=0.11-0.15) at 30W. In oxygen, model fitting provided : 692 (95% CI=688-696) mmHg at rest and 693 (95% CI=689-698) mmHg at 30W. The experimental data are compatible with the proposed model, within its physiological range.

Independent and combined effects of carbohydrate and caffeine ingestion on aerobic cycling performance in the fed state

The purpose of this study was to examine the independent and combined effects of carbohydrate and caffeine ingestion on performance and various physiological parameters during aerobic cycling (∼1 h). Ten male cyclists (28 ± 9 years, 73 ± 6 kg, 66 ± 9 mL·kg(-1)·min(-1) maximal oxygen consumption) performed 20 min of steady-state (SS) cycling (60% peak power (W(max))) followed by a simulated 20-km time trial (TT) under placebo (PLA), carbohydrate (CHO), caffeine (CAF), and combined CAF-CHO conditions, all of which were performed in the fed state. CAF-CHO improved TT performance by 3.4% ± 2% (84 ± 57 s) compared with PLA (p < 0.05), whereas no differences were detected among CHO, CAF, and PLA. The SS respiratory exchange ratio was elevated in CHO (0.92 ± 0.03), CAF (0.96 ± 0.07), and CAF-CHO (0.95 ± 0.02) compared with PLA (0.89 ± 0.03) (p < 0.05). Post-SS and post-TT blood glucose levels were also elevated in CAF-CHO (88.3 ± 16.7 mg·dL(-1) and 111.2 ± 33.5 mg·dL(-1), respectively) compared with PLA (74.5 ± 9.8 mg·dL(-1) and 85.4 ± 17.6 mg·dL(-1), respectively) (p < 0.05). Treatment conditions did not differentially impact SS pulmonary ventilation, oxygen consumption, heart rate, peak quadriceps muscle strength, rating of perceived exertion, or blood lactate. CAF and CHO improved TT performance when taken together but not independently. Although the present work did not yield any definitive physiological mechanisms for the performance findings, these data suggest that cyclists in the fed state should ingest carbohydrate and caffeine together to improve time trial performance.

V˙o2kinetics of mild exercise are altered by RER

We propose that variations in fat and carbohydrate (CHO) oxidation by working muscle alter O(2) uptake (VO(2)) kinetics. This hypothesis provides two predictions: 1) the kinetics should comprise two exponential components, one fast and the other slow, and 2) their contribution should change with variations in fat and CHO oxidation, as predicted by steady-state respiratory exchange ratio (RER). The purpose of this study was to test these predictions by evaluating the VO(2) kinetic model: VO(2)(t) = alpha(R) + alpha(F)(1 - exp[(t - TD)/-tau(F)]) + alpha(C)(1 - exp[(t - TD)/-tau(C)]) for short-term, mild leg cycling in 38 women and 44 men, where VO(2)(t) describes the time course, alpha(R) is resting VO(2), t is time after onset of exercise, TD is time delay, alpha(F) and tau(F) are asymptote and time constant, respectively, for the fast (fat) oxidative term, and alpha(C) and tau(C) are the corresponding parameters for the slow (CHO) oxidative term. We found that 1) this biexponential model accurately described the VO(2) kinetics over a wide range of RERs, 2) the contribution of the fast (alpha(F), fat) component was inversely related to RER, whereas the slow (alpha(C), CHO) component was positively related to RER, and 3) this assignment of the fast and slow terms accurately predicted steady-state respiratory quotient and CO(2) output. Therefore, the kinetic model can quantify the dynamics of fat and CHO oxidation over the first 5-10 min of mild exercise in young adult men and women.

Carbohydrate supercompensation and muscle glycogen utilization during exhaustive running in highly trained athletes

Three female and three male highly trained endurance runners with mean maximal oxygen uptake (VO2max) values of 60.5 and 71.5 ml.kg-1.min-1, respectively, ran to exhaustion at 75%-80% of VO2max on two occasions after an overnight fast. One experiment was performed after a normal diet and training regimen (Norm), the other after a diet and training programme intended to increase muscle glycogen levels (Carb). Muscle glycogen concentration in the gastrocnemius muscle increased by 25% (P less than 0.05) from 581 mmol.kg-1 dry weight, SEM 50 to 722 mmol.kg-1 dry weight, SEM 34 after Carb. Running time to exhaustion, however, was not significantly different in Carb and Norm, 77 min, SEM 13 vs 70 min, SEM 8, respectively. The average glycogen concentration following exhaustive running was 553 mmol.kg-1 dry weight, SEM 70 in Carb and 434 mmol.kg-1 dry weight, SEM 57 in Norm, indicating that in both tests muscle glycogen stores were decreased by about 25%. Periodic acid-Schiff staining for semi-quantitative glycogen determination in individual fibres confirmed that none of the fibres appeared to be glycogen-empty after exhaustive running. The steady-state respiratory exchange ratio was higher in Carb than in Norm (0.92, SEM 0.01 vs 0.89, SEM 0.01; P less than 0.05). Since muscle glycogen utilization was identical in the two tests, the indication of higher utilization of total carbohydrate appears to be related to a higher utilization of liver glycogen. We have concluded that glycogen depletion of the gastrocnemius muscle is unlikely to be the cause of fatigue during exhaustive running at 75%-80% of VO2max in highly trained endurance runners. Furthermore, diet- and training-induced carbohydrate super-compensation does not appear to improve endurance capacity in such individuals.

1463 THE RESPIRATORY EXCHANGE RATIO IN GROWING LOW BIRTH WEIGHT INFANTS

As part of an ongoing study of energy balance, 22 growing low birth weight infants were studied for three four hour periods during which minute by minute measurements of oxygen consumption and carbon dioxide production were made using a Kippdiaferometer calibrated with human expired gas. The three study periods were set one week apart so that each baby was studied over a two week interval. 16 babies were fed with formula (SIMILAC 70 kcals/100 gm) and 6 were fed with their own mother's expressed breast milk. Mean gestational age was 30 weeks (range 27 to 34 weeks) and mean birth weight 1.29 kg (range 0.98 kg to 1.7 kg). The age on entry to the study ranged from 5 to 29 days and average weight gain was 17 g/kg.day (range 12-22 g/kg.day). In 65 study periods mean oxygen consumption was 7.0 (S.D. 1.5) mls/kg.min, mean carbon dioxide production was 8.1 (S.D. 1.1) mls/kg.min and mean respiratory exchange ratio was 1.20 (S.D. 0.27). The respiratory exchange ratio increased with postnatal age, due both to a decrease in oxygen consumption and an increase in carbon dioxide production. A steady-state respiratory exchange ratio above unity indicates that processes other than oxidative catabolism are contributing to gaseous exchange, such as the conversion of glucose to fat. Such processes must be taken into account when calculating a thermal equivalent for oxygen consumption.