Influence of the bicarbonate pool and on the occurrence of 13CO2 in exhaled air

Exogenous carbohydrate oxidation during ultraendurance exercise

The purposes of this study were: 1) to obtain a measure of exogenous carbohydrate (CHOExo) oxidation and plasma glucose kinetics during 5 h of exercise; and 2) to compare CHOExo following the ingestion of a glucose solution (Glu) or a glucose + fructose solution (2:1 ratio, Glu+Fru) during ultraendurance exercise. Eight well-trained subjects exercised three times for 5 h at 58% maximum O2 consumption while ingesting either Glu or Glu+Fru (both delivering 1.5 g/min CHO) or water. The CHO used had a naturally high 13C enrichment, and five subjects received a primed continuous intravenous [6,6-2H2]glucose infusion. CHOExo rates following the ingestion of Glu leveled off after 120 min and peaked at 1.24 ± 0.04 g/min. The ingestion of Glu+Fru resulted in a significantly higher peak rate of CHOExo (1.40 ± 0.08 g/min), a faster rate of increase in CHOExo, and an increase in the percentage of CHOExo oxidized (65–77%). However, the rate of appearance and disappearance of Glu continued to increase during exercise, with no differences between trials. These data suggest an important role for gluconeogenesis during the later stages of exercise. Following the ingestion of Glu+Fru, cadence (rpm) was maintained, and the perception of stomach fullness was reduced relative to Glu. The ingestion of Glu+Fru increases CHOExo compared with the ingestion of Glu alone, potentially through the oxidation of CHOExo in the liver or through the conversion to, and oxidation of, lactate.

Metabolic response to carbohydrate ingestion during exercise in males and females

The present study investigated potential sex-related differences in the metabolic response to carbohydrate (CHO) ingestion during exercise. Moderately endurance-trained men and women (n = 8 for each sex) performed 2 h of cycling at approximately 67% Vo(2 max) with water (WAT) or CHO ingestion (1.5 g of glucose/min). Substrate oxidation and kinetics were quantified during exercise using indirect calorimetry and stable isotope techniques ([(13)C]glucose ingestion, [6,6-(2)H(2)]glucose, and [(2)H(5)]glycerol infusion). In both sexes, CHO ingestion significantly increased the rates of appearance (R(a)) and disappearance (R(d)) of glucose during exercise compared with WAT ingestion [males: WAT, approximately 28-29 micromol x kg lean body mass (LBM)(-1) x min(-1); CHO, approximately 53 micromol x kg LBM(-1) x min(-1); females: WAT, approximately 28-29 micromol x kg LBM(-1) x min(-1); CHO, approximately 61 micromol x kg LBM(-1) x min(-1); main effect of trial, P < 0.05]. The contribution of plasma glucose oxidation to the energy yield was significantly increased with CHO ingestion in both sexes (from approximately 10% to approximately 20% of energy expenditure; main effect of trial, P < 0.05). Liver-derived glucose oxidation was reduced, although the rate of muscle glycogen oxidation was unaffected with CHO ingestion (males: WAT, 108 +/- 12 micromol x kg LBM(-1) x min(-1); CHO, 108 +/- 11 micromol x kg LBM(-1) x min(-1); females: WAT, 89 +/- 10 micromol x kg LBM(-1) x min(-1); CHO, 93 +/- 11 micromol x kg LBM(-1) x min(-1)). CHO ingestion reduced fat oxidation and lipolytic rate (R(a) glycerol) to a similar extent in both sexes. Finally, ingested CHO was oxidized at similar rates in men and women during exercise (peak rates of 0.70 +/- 0.08 and 0.65 +/- 0.06 g/min, respectively). The present investigation suggests that the metabolic response to CHO ingestion during exercise is largely similar in men and women.

Measurement of gastric emptying by the13C-octanoate breath test—rationalization with scintigraphy

The (13)C-octanoate breath test has not achieved universal acceptance for the measurement of solid phase gastric emptying, largely because the results are not comparable with those from direct methods such as scintigraphy. To convert breath-test data to their scintigraphic equivalent scale corrections are applied which have been obtained from population studies, but there is no guarantee that these are applicable in all cases. We propose an alternative method applicable on an individual basis based upon a simple physiological model which does not require any change to the breath-test protocol. It is demonstrated by comparison with scintigraphy and the octanoate saliva test. Results from an existing dataset of simultaneous breath test, saliva test and scintigraphic determinations of solid phase gastric emptying from nine healthy subjects were re-analysed. The corrected breath tests gave gastric emptying curves which were shown to be not significantly different to those obtained from scintigraphy. The method provides a simple but effective way of generating gastric emptying curves from breath-test data that are directly comparable with direct measurement methods, which is advantageous since it allows the whole of the gastric emptying profile to be generated, not just values for the lag phase and half-emptying times.

Metabolic fate of a large amount of 13C-glycerol ingested during prolonged exercise

We have shown that the oxidation rate of exogenous glycerol and glucose during prolonged exercise were similar when ingested in small amounts (0.36 g/kg) (J Appl Physiol 90:1685,2001). The oxidation rate of exogenous carbohydrate increases with the amount ingested. We, thus, hypothesized that the oxidation rate of exogenous glycerol would also be larger when ingested in large amount. The study was conducted on six male subjects exercising for 120 min at 64 (2)% VO(2)max while ingesting 1 g/kg of (13)C-glycerol. Substrate oxidation was measured using indirect respiratory calorimetry corrected for protein oxidation, and from V(13)CO(2) at the mouth. The (13)C enrichment of plasma glucose was also measured in order to follow the possible conversion of (13)C-glycerol into glucose. In spite of the large amount of glycerol ingested and absorbed (plasma glycerol concentration = 8.0 (0.3) mmol/l at min 100), exogenous glycerol oxidation over the last 80 min of exercise [8.8 (1.6) g providing 4.1 (0.7)% of the energy yield] was similar to that observed when 0.36 g/kg was ingested. The comparison between the (13)C enrichment of plasma glucose and the oxidation rate of (13)C-glycerol showed that a portion of exogenous glycerol was converted into glucose before being oxidized, but also suggested that another portion could have been directly oxidized in peripheral tissues.

Exogenous carbohydrate oxidation rates are elevated after combined ingestion of glucose and fructose during exercise in the heat

The first purpose of this study was to investigate whether a glucose (GLU)+fructose (FRUC) beverage would result in a higher exogenous carbohydrate (CHO) oxidation rate and a higher fluid availability during exercise in the heat compared with an isoenergetic GLU beverage. A second aim of the study was to examine whether ingestion of GLU at a rate of 1.5 g/min during exercise in the heat would lead to a reduced muscle glycogen oxidation rate compared with ingestion of water (WAT). Eight trained male cyclists (maximal oxygen uptake: 64+/-1 ml.kg-1.min-1) cycled on three different occasions for 120 min at 50% maximum power output at an ambient temperature of 31.9+/-0.1 degrees C. Subjects received, in random order, a solution providing either 1.5 g/min of GLU, 1.0 g/min of GLU+0.5 g/min of FRUC, or WAT. Exogenous CHO oxidation during the last hour of exercise was approximately 36% higher (P<0.05) in GLU+FRUC compared with GLU, and peak oxidation rates were 1.14+/-0.05 and 0.77+/-0.08 g/min, respectively. Endogenous CHO oxidation was significantly lower (P<0.05) in GLU+FRUC compared with WAT. Muscle glycogen oxidation was not different after ingestion of GLU or WAT. Plasma deuterium enrichments were significantly higher (P<0.05) in WAT and GLU+FRUC compared with GLU. Furthermore, at 60 and 75 min of exercise, plasma deuterium enrichments were higher (P<0.05) in WAT compared with GLU+FRUC. Ingestion of GLU+FRUC during exercise in the heat resulted in higher exogenous CHO oxidation rates and fluid availability compared with ingestion of GLU and reduced endogenous CHO oxidation compared with ingestion of WAT.

Glucose Polymer Molecular Weight Does Not Affect Exogenous Carbohydrate Oxidation

PURPOSE

To compare the effects of high (HMW) versus low molecular weight (LMW) glucose polymer solutions on the pattern of substrate oxidation during exercise.

METHODS

Eight cyclists (VO(2max): 63 +/- 8 mL.kg(-1).min(-1)) performed three 150-min cycling trials at 64 +/- 5% VO(2max) while ingesting 11.25% HMW (500-750 kg.mol(-1), 21 mOsm.kg(-1)) or LMW (8 kg.mol(-1), 110 mOsm.kg(-1)) solutions providing 1.8 g of carbohydrate per minute, or plain water. Substrate oxidation was determined using stable-isotope methods and indirect calorimetry.

RESULTS

Exogenous carbohydrate oxidation rate was not affected by carbohydrate molecular weight (P = 0.89, peak rate: 0.93 x// 1.37 g.min(-1)). There was no effect of carbohydrate molecular weight on endogenous carbohydrate or fat oxidation rates (P = 0.30), plasma free fatty acid (P = 0.14), lactate (P = 0.38), or glucose concentrations (P = 0.98), nor were there any serious gastrointestinal complaints reported for either of the two solutions during exercise.

CONCLUSIONS

Despite previous reports of faster gastric emptying and glycogen resynthesis suggesting enhanced glucose delivery, a markedly hypotonic HMW glucose polymer solution had no effect on exogenous and endogenous substrate oxidation rates during exercise, relative to a LMW glucose polymer solution. These data are consistent with there being no effect of carbohydrate structure or solution osmolality or viscosity on exogenous glucose oxidation and that ingested glucose polymers can only be oxidized on average up to 1.0 g.min during exercise.

Measurement of exogenous carbohydrate oxidation: a comparison of [U-14C]glucose and [U-13C]glucose tracers

The purpose of this study was to assess the level of agreement between two techniques commonly used to measure exogenous carbohydrate oxidation (CHO(EXO)). To accomplish this, seven healthy male subjects (24 +/- 3 yr, 74.8 +/- 2.1 kg, V(O2(max)) 62 +/- 4 ml x kg(-1) x min(-1)) exercised at 50% of their peak power for 120 min on two occasions. During these exercise bouts, subjects ingested a solution containing either 144 g glucose (8.7% wt/vol glucose) or water. The glucose solution contained trace amounts of both [U-13C]glucose and [U-14C]glucose to allow CHO(EXO) to be quantified simultaneously. The water trial was used to correct for background 13C enrichment. 13C appearance in the expired air was measured using isotope ratio mass spectrometry, whereas 14C appearance was quantified by trapping expired CO(2) in solution (using hyamine hydroxide) and adding a scintillator before counting radioactivity. CHO(EXO) measured with [13C]glucose ([13C]CHO(EXO)) was significantly greater than CHO(EXO) measured with [14C]glucose ([14C]CHO(EXO)) from 30 to 120 min. There was a 15 +/- 4% difference between [13C]CHO(EXO) and [14C]CHO(EXO) such that the absolute difference increased with the magnitude of CHO(EXO). Further investigations suggest that the difference is not because of losses of CO2 from the trapping solution before counting or an underestimation of the "strength" of the trapping solution. Previous research suggests that the degree of isotopic fractionation is small (S. C. Kalhan, S. M. Savin, and P. A. Adam. J Lab Clin Med89: 285-294, 1977). Therefore, the explanation for the discrepancy in calculated CHO(EXO) remains to be fully understood.

Oxidation of combined ingestion of maltodextrins and fructose during exercise

PURPOSE

To determine whether combined ingestion of maltodextrin and fructose during 150 min of cycling exercise would lead to exogenous carbohydrate oxidation rates higher than 1.1 g.min.

METHODS

Eight trained cyclists VO2max: 64.1 +/- 3.1 mL.kg.min) performed three exercise trials in a random order. Each trial consisted of 150 min cycling at 55% maximum power output (64.2+/-3.5% VO2max) while subjects received a solution providing either 1.8 g.min of maltodextrin (MD), 1.2 g.min of maltodextrin + 0.6 g.min of fructose (MD+F), or plain water. To quantify exogenous carbohydrate oxidation, corn-derived MD and F were used, which have a high natural abundance of C.

RESULTS

Peak exogenous carbohydrate oxidation (last 30 min of exercise) rates were approximately 40% higher with combined MD+F ingestion compared with MD only ingestion (1.50+/-0.07 and 1.06+/-0.08 g.min, respectively, P<0.05). Furthermore, the average exogenous carbohydrate oxidation rate during the last 90 min of exercise was higher with combined MD+F ingestion compared with MD alone (1.38+/-0.06 and 0.96+/-0.07 g.min, respectively, P<0.05).

CONCLUSIONS

The present study demonstrates that with ingestion of large amounts of maltodextrin and fructose during cycling exercise, exogenous carbohydrate oxidation can reach peak values of approximately 1.5 g.min, and this is markedly higher than oxidation rates from ingesting maltodextrin alone.

Oxidation of combined ingestion of glucose and sucrose during exercise

The first purpose of the study was to examine whether combined ingestion of glucose and sucrose at an intake rate of 1.2 g/min would lead to higher oxidation rates compared with the ingestion of an isocaloric amount of glucose or sucrose alone. The second aim of the study was to investigate whether a mixture of glucose and sucrose when ingested at a high rate (2.4 g/min) would result in exogenous CHO oxidation rates higher than 1.2 to 1.3 g/min. Eight trained cyclists (maximal oxygen consumption: 64 +/- 2 mL . kg -1 . min -1 , mean +/- SE) performed 5 exercise trials in random order. Each trial consisted of 120 minutes of cycling at 50% maximum power output (63% +/- 2% maximal oxygen consumption), whereas subjects received a solution providing either 1.2 g/min of glucose (GLU), 1.2 g/min of sucrose (SUC), 0.6 g/min of glucose + 0.6 g/min of sucrose (M-GLU+SUC), 1.2 g/min of glucose + 1.2 g/min of sucrose (H-GLU+SUC), or water (WAT). Peak exogenous CHO oxidation rates in the H-GLU+SUC trial (1.20 +/- 0.07 g/min) were significantly higher ( P < .01) compared with the GLU, M-GLU+SUC, and SUC trials (0.77 +/- 0.04, 0.90 +/- 0.07, 0.98 +/- 0.04 g/min, respectively). Furthermore, peak exogenous CHO rates in M-GLU+SUC and SUC trials were significantly higher ( P < .05) compared with the GLU trial. In conclusion, combined ingestion of moderate amounts of glucose and sucrose (144 g) during cycling exercise resulted in approximately 21% higher exogenous CHO oxidation rates compared with the ingestion of an isocaloric amount of glucose. Furthermore, when a mixture of glucose and sucrose was ingested at high rates (2.4 g/min), exogenous CHO oxidation rates reached peak values of approximately 1.20 g/min.

Caffeine increases exogenous carbohydrate oxidation during exercise

Both carbohydrate (CHO) and caffeine have been used as ergogenic aids during exercise. It has been suggested that caffeine increases intestinal glucose absorption, but there are also suggestions that it may decrease muscle glucose uptake. The purpose of the study was to investigate the effect of caffeine on exogenous CHO oxidation. In a randomized crossover design, eight male cyclists (age 27 +/- 2 yr, body mass 71.2 +/- 2.3 kg, maximal oxygen uptake 65.7 +/- 2.2 ml x kg(-1) x min(-1)) exercised at 64 +/- 3% of maximal oxygen uptake for 120 min on three occasions. During exercise subjects ingested either a 5.8% glucose solution (Glu; 48 g/h), glucose with caffeine (Glu+Caf, 48 g/h + 5 mg x kg(-1) x h(-1)), or plain water (Wat). The glucose solution contained trace amounts of [U-13C]glucose so that exogenous CHO oxidation could be calculated. CHO and fat oxidation were measured by indirect calorimetry, and 13C appearance in the expired gases was measured by continuous-flow IRMS. Average exogenous CHO oxidation over the 90- to 120-min period was 26% higher (P < 0.05) in Glu+Caf (0.72 +/- 0.04 g/min) compared with Glu (0.57 +/- 0.04 g/min). Total CHO oxidation rates were higher (P < 0.05) in the CHO ingestion trials compared with Wat, but they were highest when Glu+Caf was ingested (1.21 +/- 0.37, 1.84 +/- 0.14, and 2.47 +/- 0.23 g/min for Wat, Glu, and Glu+Caf, respectively; P < 0.05). There was also a trend (P = 0.082) toward an increased endogenous CHO oxidation with Glu+Caf (1.81 +/- 0.22 g/min vs. 1.27 +/- 0.13 g/min for Glu and 1.12 +/- 0.37 g/min for Wat). In conclusion, compared with glucose alone, 5 mg x kg(-1) x h(-1) of caffeine coingested with glucose increases exogenous CHO oxidation, possibly as a result of an enhanced intestinal absorption.

High oxidation rates from combined carbohydrates ingested during exercise

Studies that have investigated oxidation of a single carbohydrate (CHO) during exercise have reported oxidation rates of up to 1 g x min(-1). Recent studies from our laboratory have shown that a mixture of glucose and sucrose or glucose and fructose ingested at a high rate (1.8 g x min(-1)) leads to peak oxidation rates of approximately 1.3 g x min(-1) and results in approximately 20 to 55% higher exogenous CHO oxidation rates compared with the ingestion of an isocaloric amount of glucose.

PURPOSE

The purpose of the present study was to examine whether a mixture of glucose, sucrose and fructose ingested at a high rate would result in even higher exogenous CHO oxidation rates (>1.3 g x min(-1)).

METHODS

Eight trained male cyclists (VO2max: 64 +/- 1 mL x kg(-1) BM x min(-1)) cycled on three different occasions for 150 min at 62 +/- 1% VO2max and consumed either water (WAT) or a CHO solution providing 2.4 g x min(-1) of glucose (GLU) or 1.2 g x min(-1) of glucose + 0.6 g x min(-1) of fructose + 0.6 g x min(-1) of sucrose (MIX).

RESULTS

High peak exogenous CHO oxidation rates were found in the MIX trial (1.70 +/- 0.07 g x min(-1)), which were approximately 44% higher (P < 0.01) compared with the GLU trial (1.18 +/- 0.04 g x min(-1)). Endogenous CHO oxidation was lower (P < 0.05) in MIX compared with GLU (0.76 +/- 0.12 and 1.05 +/- 0.06 g x min(-1), respectively).

CONCLUSION

When glucose, fructose and sucrose are ingested simultaneously at high rates (2.4 g x min(-1)) during cycling exercise, exogenous CHO oxidation rates can reach peak values of approximately 1.7 g x min(-1) and estimated endogenous CHO oxidation is reduced compared with the ingestion of an isocaloric amount of glucose.

Metabolic response to prolonged cycling with (13)C-glucose ingestion following downhill running

The purpose of this study was to describe the effect of muscle damage and delayed-onset muscle soreness (DOMS) on the metabolic response during a subsequent period of prolonged concentric exercise (120 min, approximately 61% V(.)O(2max), on a cycle ergometer), with ingestion of 3 g of (13)C-glucose/kg body mass. We hypothesized that the oxidation of plasma and exogenous glucose would be reduced, while the oxidation of glucose arising from muscle glycogen would be increased. Six male subjects were studied during exercise in a control situation and 2 days following downhill running, at a time when plasma creatine kinase (CK) activity was increased, and DOMS was present. Carbohydrate and lipid oxidation were computed from indirect respiratory calorimetry corrected for protein oxidation, while the oxidation of plasma glucose and muscle glycogen were computed from V(.)(13)CO(2) and the ratio of (13)C/(12)C in the plasma glucose. All data were presented as the mean and the standard error of the mean. The oxidation of protein (approximately 6% energy yield, in the control and the experimental trial), lipid (approximately 15 and approximately 18%), and carbohydrate (approximately 79 and approximately 76%), as well as that of plasma glucose (approximately 41 and approximately 46%), glucose from the liver (approximately 12 and approximately 14%), and glucose from muscle glycogen (approximately 38 and approximately 31%) were not significantly different between the control and experimental (DOMS) trials. The response of the plasma glucose, insulin, lactate, and free fatty acid concentrations was not modified by the previous eccentric exercise. These results indicate that the metabolic response to prolonged concentric exercise is not modified by muscle damage and DOMS resulting from a bout of eccentric exercise performed 2 days before.

The Effect of Glucose Infusion on Glucose Kinetics during a 1-h Time Trial

PURPOSE AND METHODS

To investigate the effect of glucose infusion on glucose kinetics and performance, six endurance cyclists (VO2max = 61.7 +/- 2.0 (mean +/- SE) mL x kg(-1) x min(-1)) completed two performance trials in which they had to accomplish a set amount of work as quickly as possible (991 +/- 41 kJ). Subjects were infused with either glucose (20% in saline; carbohydrate (CHO)) at a rate of 1 g x min(-1) or saline (0.9% saline; placebo (PLA)). It was hypothesized that time trial performance would be unaffected by the infusion of glucose, as endogenous stores of CHO would not be limiting in the PLA trial.

RESULTS

Plasma glucose concentration increased from 4.8 +/- 0.1 mmol x L(-1) to 5.9 +/- 0.3 mmol x L(-1) during the PLA trial and from 4.9 +/- 0.1 mmol x L(-1) at rest to 12.4 +/- 1.1 mmol x L(-1) during the CHO trial. These values were significantly higher at all time points during the CHO trial compared with PLA (P < 0.001). In the final stages of the time trial, Rd in the PLA trial was 49 +/- 5 micromol x kg(-1) x min(-1) compared with 88 +/- 7 micromol x kg(-1) x min(-1) in the CHO trial (P < 0.05). Despite these differences, there was no difference in performance time between PLA and CHO (60.04 +/- 1.47 min, PLA, vs 59.90 +/- 1.49 min, CHO, respectively). Infused carbohydrate oxidation in the last 25% of the CHO trial was at least 675 +/- 120 micromol x kg(-1) and contributed 17 +/- 4% to total carbohydrate oxidation.

CONCLUSION

The results demonstrate that glucose infusion had no effect on 1-h cycle time-trial performance, despite an increased availability of plasma glucose for oxidation and evidence of increased glucose uptake into the tissues.

Gender difference in the metabolic response to prolonged exercise with [13C]glucose ingestion

The metabolic response to a 120-min cycling exercise with ingestion of [(13)C]glucose (3 g kg(-1)) was compared in women in the follicular phase of the cycle [ n=6; maximum rate of oxygen uptake (VO(2max)) 44.7 (2.6) ml kg(-1) min(-1)] and in men [ n=6; VO(2max) 54.2 (4.3) ml kg(-1) min(-1)] working at the same relative workload (approximately 65% VO(2max): 107 and 179 W in women and men, respectively). We hypothesized that the contribution of endogenous substrate oxidations (indirect respiratory calorimetry corrected for protein oxidation) to the energy yield will be similar in men and women, but that women will rely more than men on exogenous glucose oxidation. Over the exercise period, the respective contributions of protein, lipid and carbohydrate oxidation to the energy yield, were similar in men [3.7 (0.9), 21.7 (2.9) and 74.6 (3.5)%] and women [3.4 (0.8), 21.5 (2.2), 75.1 (2.5)%]. The rate of exogenous glucose oxidation was approximately 45% lower in women than men (0.5 and 0.6 g min(-1) vs 0.7 and 0.9 g min(-1), between min 40 and 80, and min 80 and 120, respectively). However, when the approximately 39% difference in absolute workload and energy expenditure was taken into account, the contribution of exogenous glucose oxidation to the energy yield was similar in men and women: 22.5 vs 24.2% between min 40 and 80, and 25.7 and 28.5% between min 80 and 120, respectively. These data indicate that when fed glucose, the respective contributions of the oxidation of the various substrates to the energy yield during prolonged exercise at the same % VO(2max) are similar in men and in women in the follicular phase of the cycle.

Effects of carbohydrate availability on sustained shivering I. Oxidation of plasma glucose, muscle glycogen, and proteins

Carbohydrates (CHO) can play an important thermogenic role during shivering, but the effect of their availability on the use of other oxidative fuels is unclear. Using indirect calorimetry and tracer methods ([U-13C]glucose ingestion), we have determined the specific contributions of plasma glucose, muscle glycogen, proteins, and lipids to total heat production (Hprod) in men exposed to cold for 2-h (liquid-conditioned suit perfused with 10 degrees C water). Measurements were made after low-CHO diet and exercise (Lo) and high-CHO diet without exercise (Hi). The size of CHO reserves had no effect on Hprod but a major impact on fuel selection before and during shivering. In the cold, a complete shift from lipid oxidation for Lo (53, 28, and 19% Hprod for lipids, CHO, and proteins, respectively) to CHO-based metabolism for Hi (23, 65, and 12% Hprod for lipids, CHO, and proteins, respectively) was observed. Plasma glucose oxidation remains a minor fuel under all conditions (<13% Hprod), falling to 7% Hprod for Lo. Therefore, adjusting plasma glucose oxidation to compensate for changes in muscle glycogen oxidation is not a strategy used for maintaining heat production. Instead, proteins and lipids share responsibility for this compensation. We conclude that humans can show remarkable flexibility in oxidative fuel selection to ensure that heat production is not compromised during sustained cold exposure.

Oxidation of exogenous glucose, sucrose, and maltose during prolonged cycling exercise

The purpose of the present study was to investigate whether combined ingestion of two carbohydrates (CHO) that are absorbed by different intestinal transport mechanisms would lead to exogenous CHO oxidation rates of >1.0 g/min. Nine trained male cyclists (maximal O(2) consumption: 64 +/- 2 ml x kg body wt(-1) x min(-1)) performed four exercise trials, which were randomly assigned and separated by at least 1 wk. Each trial consisted of 150 min of cycling at 50% of maximal power output (60 +/- 1% maximal O(2) consumption), while subjects received a solution providing either 1.8 g/min of glucose (Glu), 1.2 g/min of glucose + 0.6 g/min of sucrose (Glu+Suc), 1.2 g/min of glucose + 0.6 g/min of maltose (Glu+Mal), or water. Peak exogenous CHO oxidation rates were significantly higher (P < 0.05) in the Glu+Suc trial (1.25 +/- 0.07 g/min) compared with the Glu and Glu+Mal trials (1.06 +/- 0.08 and 1.06 +/- 0.06 g/min, respectively). No difference was found in (peak) exogenous CHO oxidation rates between Glu and Glu+Mal. These results demonstrate that, when a mixture of glucose and sucrose is ingested at high rates (1.8 g/min) during cycling exercise, exogenous CHO oxidation rates reach peak values of approximately 1.25 g/min.

Oxidation of combined ingestion of glucose and fructose during exercise

The purpose of the present study was to examine whether combined ingestion of a large amount of fructose and glucose during cycling exercise would lead to exogenous carbohydrate oxidation rates >1 g/min. Eight trained cyclists (maximal O(2) consumption: 62 +/- 3 ml x kg(-1) x min(-1)) performed four exercise trials in random order. Each trial consisted of 120 min of cycling at 50% maximum power output (63 +/- 2% maximal O(2) consumption), while subjects received a solution providing either 1.2 g/min of glucose (Med-Glu), 1.8 g/min of glucose (High-Glu), 0.6 g/min of fructose + 1.2 g/min of glucose (Fruc+Glu), or water. The ingested fructose was labeled with [U-(13)C]fructose, and the ingested glucose was labeled with [U-(14)C]glucose. Peak exogenous carbohydrate oxidation rates were approximately 55% higher (P < 0.001) in Fruc+Glu (1.26 +/- 0.07 g/min) compared with Med-Glu and High-Glu (0.80 +/- 0.04 and 0.83 +/- 0.05 g/min, respectively). Furthermore, the average exogenous carbohydrate oxidation rates over the 60- to 120-min exercise period were higher (P < 0.001) in Fruc+Glu compared with Med-Glu and High-Glu (1.16 +/- 0.06, 0.75 +/- 0.04, and 0.75 +/- 0.04 g/min, respectively). There was a trend toward a lower endogenous carbohydrate oxidation in Fruc+Glu compared with the other two carbohydrate trials, but this failed to reach statistical significance (P = 0.075). The present results demonstrate that, when fructose and glucose are ingested simultaneously at high rates during cycling exercise, exogenous carbohydrate oxidation rates can reach peak values of approximately 1.3 g/min.

Breath [13CO2] recovery from an oral glucose load during exercise: comparison between [U-13C] and [1,2-13C]glucose

The purpose of the present experiment was to compare 13CO2 recovery at the mouth, and the corresponding exogenous glucose oxidation computed, during a 100-min exercise at 63 +/- 3% maximal O2 uptake with ingestion of glucose (1.75 g/kg) in six active male subjects, by use of [U-13C] and [1,2-13C]glucose. We hypothesized that 13C recovery and exogenous glucose oxidation could be lower with [1,2-13C] than [U-13C]glucose because both tracers provide [13C]acetate, with possible loss of 13C in the tricarboxylic acid (TCA) cycle, but decarboxylation of pyruvate from [U-13C]glucose also provides 13CO2, which is entirely recovered at the mouth during exercise. The recovery of 13C (25.8 +/- 2.3 and 27.4 +/- 1.2% over the exercise period) and the amounts of exogenous glucose oxidized computed were not significantly different with [1,2-13C] and [U-13C]glucose (28.9 +/- 2.6 and 30.7 +/- 1.3 g, between minutes 40 and 100), suggesting that no significant loss of 13C occurred in the TCA cycle. This stems from the fact that, during exercise, the rate of exogenous glucose oxidation is probably much larger than the flux of the metabolic pathways fueled from TCA cycle intermediates. It is thus unlikely that a significant portion of the 13C entering the TCA cycle could be diverted to these pathways. From a methodological standpoint, this result indicates that when a large amount of [13C]glucose is ingested and oxidized during exercise, 13CO2 production at the mouth accurately reflects the rate of glucose entry in the TCA cycle and that no correction factor is needed to compute the oxidative flux of exogenous glucose.

Oxidation rate of exogenous carbohydrate during exercise is higher in boys than in men

To determine whether the relative utilization of exogenous carbohydrate (CHO(exo)) differs between children and adults, substrate utilization during 60 min of cycling at 70% peak O(2) uptake was studied in 12 pre- and early pubertal boys (9.8 +/- 0.1 yr) and 10 men (22.1 +/- 0.5 yr) on two occasions. Subjects consumed either a placebo or a (13)C-enriched 6% CHO(exo) beverage (total volume per trial: 24 ml/kg). Substrate utilization was calculated for the final 30 min of exercise. During both trials, total fat oxidation was higher (5.4 +/- 0.5 vs. 3.0 +/- 0.4 mg x kg(-1) x min(-1), P < 0.001) and total CHO oxidation lower (27.4 +/- 1.5 vs. 34.8 +/- 1.2 mg x kg(-1) x min(-1), P < 0.001) in boys than in men, respectively. During the CHO(exo) trial, CHO(exo) oxidation was higher (P < 0.001) in boys (8.8 +/- 0.5 mg x kg(-1) x min(-1)) than in men (6.2 +/- 0.5 mg x kg(-1) x min(-1)) and provided a greater (P < 0.001) relative proportion of total energy in boys (21.8 +/- 1.4%) than in men (14.6 +/- 0.9%). These results suggest that, although endogenous CHO utilization during exercise is lower, the relative oxidation of ingested CHO is considerably higher in boys than in men. The greater reliance on CHO(exo) in boys may be important in preserving endogenous fuels and may be related to pubertal status.

Differential metabolic fate of the carbon skeleton and amino-N of [13C]alanine and [15N]alanine ingested during prolonged exercise

The decarboxylation/oxidation and the deamination of 13C- and [15N]alanine ingested (1 g/kg or 73.7 +/- 2 g) during prolonged exercise at low workload (180 min at 53 +/- 2% maximal O2 uptake) was measured in six healthy male subjects from V13CO2 at the mouth and [15N]urea excretion in urine and sweat. Over the exercise period, 50.6 +/- 3.5 g of exogenous alanine were oxidized (68.7 +/- 4.5% of the load), providing 10.0 +/- 0.6% of the energy yield vs. 4.8 +/- 0.4, 47.6 +/- 4.3, and 37.4 +/- 4.7% for endogenous proteins, glucose, and lipids, respectively. Alanine could have been oxidized after conversion into glucose in the liver and/or directly in peripheral tissues. In contrast, only 13.0 +/- 3.2 mmol of [(15)N]urea were excreted in urine and sweat (10.6 +/- 0.4 and 2.4 +/- 0.5 mmol, respectively), corresponding to the deamination of 2.3 +/- 0.3 g of exogenous alanine (3.1 +/- 0.4% of the load). These results confirm that the metabolic fate of the carbon skeleton and the amino-N moiety of exogenous alanine ingested during prolonged exercise at low workload are markedly different. The large positive nitrogen balance (8.5 +/- 0.3 g) suggests that in this situation protein synthesis could be increased when a large amount of a single amino acid is ingested.

Effect of cold exposure on fuel utilization in humans: plasma glucose, muscle glycogen, and lipids

The relative roles of circulatory glucose, muscle glycogen, and lipids in shivering thermogenesis are unclear. Using a combination of indirect calorimetry and stable isotope methodology ([U-13C]glucose ingestion), we have quantified the oxidation rates of these substrates in men acutely exposed to cold for 2 h (liquid conditioned suit perfused with 10 degrees C water). Cold exposure stimulated heat production by 2.6-fold and increased the oxidation of plasma glucose from 39.4 +/- 2.4 to 93.9 +/- 5.5 mg/min (+138%), of muscle glycogen from 126.6 +/- 7.8 to 264.2 +/- 36.9 mg glucosyl units/min (+109%), and of lipids from 46.9 +/- 3.2 to 176.5 +/- 17.3 mg/min (+376%). Despite the observed increase in plasma glucose oxidation, this fuel only supplied 10% of the energy for heat generation. The major source of carbohydrate was muscle glycogen (75% of all glucose oxidized), and lipids produced as much heat as all other fuels combined. During prolonged, low-intensity shivering, we conclude that total heat production is unequally shared among lipids (50%), muscle glycogen (30%), plasma glucose (10%), and proteins (10%). Therefore, future research should focus on lipids and muscle glycogen that provide most of the energy for heat production.

Oral [(13)C]glucose and endogenous energy substrate oxidation during prolonged treadmill running

Six male subjects were studied during running exercise (120 min, 69% maximal oxygen consumption) with ingestion of a placebo or 3.5 g/kg of [(13)C]glucose (approximately 2 g/min). Indirect respiratory calorimetry corrected for urea excretion in urine and sweat, production of (13)CO(2) at the mouth, and changes in plasma glucose (13)C/(12)C were used to compute energy substrate oxidation. The oxidation rate of exogenous glucose increased from 1.02 at minute 60 to 1.22 g/min at minute 120 providing approximately 24 and 33% of the energy yield (%En). Glucose ingestion did not modify protein oxidation, which provided approximately 4-5%En, but significantly increased glucose oxidation by approximately 7%, reduced lipid oxidation by approximately 16%, and markedly reduced endogenous glucose oxidation (1.25 vs. 2.21 g/min between minutes 80 and 120, respectively). The oxidation rate of glucose released from the liver (0.38 and 0.47 g/min, or 10-13%En at minutes 60 and 120, respectively), and of plasma glucose (1.30-1.69 g/min, or 34 and 45%En and 50 and 75% of glucose oxidation) significantly increased from minutes 60 to 120, whereas the oxidation of muscle glycogen significantly decreased (1.28 to 0.58 g of glucose/min, or 34 and 16%En and 50 and 25% of glucose oxidation). These results indicate that, during moderate prolonged running exercise, ingestion of a very large amount of glucose significantly reduces endogenous glucose oxidation, thus sparing muscle and/or liver glycogen stores.

First-pass effect of an intravenous bolus of [13C]bicarbonate displayed breath-by-breath

The dilution of an intravenous bolus dose of [13C]bicarbonate is used as an estimate for the metabolic rate under certain conditions. It is a consistent finding in all studies that the total amount of intravenous [13C]bicarbonate cannot be recovered as breath 13CO2. In this study, we used a breath-by-breath analysis of 13CO2 to depict the washout of 13CO2 at a high temporal resolution to analyze the extent to which a probable first-pass effect is responsible for the reduced recovery. Eight healthy men were tested at seated rest and with bicycle exercise at a constant load relative to 40 and 75% maximal O2 consumption (V˙o 2 max). [13C]bicarbonate (0.0125 g/kg body wt) was administered as an intravenous bolus in each test. Respiratory mass spectrometry was used to derive the course of the end-tidal13CO2-to-12CO2 ratio from the breath-by-breath data. Approximately 2 min after13C administration, the washout curve could be fitted well by a two-exponential curve describing a two-compartment mammillary model. Immediately after administration of the bolus dose, an excess peak in the end-tidal13CO2-to-12CO2 ratio appeared. This peak could not be included in the two-exponential fitting. The area under the first peak resulted in 3.8 ± 1.3% of the total [13C]bicarbonate dose at rest, 11.5 ± 2.9% at moderate exercise (40%V˙o 2 max), and 16.9 ± 4.0% at intensive exercise (75% V˙o 2 max). The first-pass effect had an increasing impact of up to about two-thirds of the lacking bicarbonate with higher exercise intensity. The “loss” of tracer via this first-pass effect must be considered when the results of studies with parenteral administration of [13C]bicarbonate are considered, especially when it is given as a bolus dose and during exercise.

Oxidation of [(13)C]glycerol ingested along with glucose during prolonged exercise

The respective oxidation of glycerol and glucose (0.36 g/kg each) ingested simultaneously immediately before exercise (120 min at 68 +/- 2% maximal oxygen uptake) was measured in six subjects using (13)C labeling. Indirect respiratory calorimetry corrected for protein and glycerol oxidation was used to evaluate the effect of glucose + glycerol ingestion on the oxidation of glucose and fat. Over the last 80 min of exercise, 10.0 +/- 0.8 g of exogenous glycerol were oxidized (43% of the load), while exogenous glucose oxidation was 21% higher (12.1 +/- 0.7 g or 52% of the load). However, because the energy potential of glycerol is 18% higher than that of glucose (4.57 vs. 3.87 kcal/g), the contribution of both exogenous substrates to the energy yield was similar (4.0-4.1%). Total glucose and fat oxidation were similar in the placebo (144.4 +/- 13.0 and 60.5 +/- 4.2 g, respectively) and the glucose + glycerol (135.2 +/- 12.0 and 59.4 +/- 6.5 g, respectively) trials, whereas endogenous glucose oxidation was significantly lower than in the placebo trial (123.7 +/- 11.7 vs. 144.4 +/- 13.0 g). These results indicate that exogenous glycerol can be oxidized during prolonged exercise, presumably following conversion into glucose in the liver, although direct oxidation in peripheral tissues cannot be ruled out.

Substrate utilization during exercise with glucose and glucose plus fructose ingestion in boys ages 10--14 yr

We measured substrate utilization during exercise performed with water (W), exogenous glucose (G), and exogenous fructose plus glucose (FG) ingestion in boys age 10-14 yr. Subjects (n = 12) cycled for 90 min at 55% maximal O(2) uptake while ingesting either W (25 ml/kg), 6% G (1.5 g/kg), or 3% F plus 3% G (1.5 g/kg). Fat oxidation increased during exercise in all trials but was higher in the W (0.28 +/- 0.023 g/min) than in the G (0.24 +/- 0.023 g/min) and FG (0.25 +/- 0.029 g/min) trials (P = 0.04). Conversely, total carbohydrate (CHO) oxidation decreased in all trials and was lower in the W (0.63 +/- 0.05 g/min) than in the G (0.78 +/- 0.051 g/min) and FG (0.74 +/- 0.056 g/min) trials (P = 0.009). Exogenous CHO oxidation, as determined by expired (13)CO(2), reached a maximum of 0.36 +/- 0.032 and 0.31 +/- 0.030 g/min at 90 min in G and FG, respectively (P = 0.04). Plasma insulin levels decrease during exercise in all trials but were twofold higher in G than in W and FG (P < 0.001). Plasma glucose levels decreased transiently after the onset of exercise in all trials and then returned to preexercise values in the W and FG (approximately 4.5 mmol/l) trials but were elevated by approximately 1.0 mmol/l in the G trial (P < 0.001). Plasma lactate concentrations decreased after the onset of exercise in all trials but were lower by approximately 0.5 mmol/l in W than in G and FG (P = 0.02). Thus, in boys exercising at a moderate intensity, the oxidation rate of G plus F is slightly less than G alone, but both spare endogenous CHO and fat to a similar extent. In addition, compared with flavored W, the ingestion of G alone and of G plus F delays exhaustion at 90% peak power by approximately 25 and 40%, respectively, after 90 min of moderate-intensity exercise.

Toward office-based measurement of gastric emptying in symptomatic diabetics using [13C]octanoic acid breath test

OBJECTIVE

Current methods for measuring gastric emptying by breath test require sampling over several hours and are too inaccurate for clinical use. The aim of this study was to develop an office-based method for measuring gastric emptying of solids in patients with diabetes using a [13C]octanoic acid breath test.

METHODS

In 22 symptomatic diabetic patients (17 insulin-dependent diabetes, 5 non-insulin-dependent diabetes) and 6 controls, we simultaneously measured gastric emptying of an egg meal (420 kcal) by scintigraphy and [13C]octanoic acid breath test. Conventional (nonlinear) methods for scintigraphic and [13C]octanoic acid breath test emptying and generalized linear regression method to predict scintigraphic half-life (t(1/2)) using four breath samples obtained during the first 3 h.

RESULTS

Despite 8 h of breath sampling, the t(1/2) estimate using the conventional method was markedly different from the scintigraphic value (delta t(1/2): median, 113 min; range, 19-282 min). The generalized linear model (using samples at baseline, 30, and 120 or 150 min) yielded predicted scintigraphic tLAG and t(1/2) that were more accurate than the conventional method; mean standard deviations of differences were 16 and 27 min, respectively. Breath test correctly assessed normal or prolonged emptying in 21 of 22 patients.

CONCLUSIONS

The [13C]octanoic acid breath test can be simplified to measure gastric tLAG and t(1/2) and can be expected to correctly identify normal t(1/2) in symptomatic diabetics. Further refinement of the model will need to include studies of patients with markedly delayed t(1/2).

Effect of training status on fuel selection during submaximal exercise with glucose ingestion

In this study, an oral glucose load was enriched with a [U-(13)C]glucose tracer to determine differences in substrate utilization between endurance-trained (T) and untrained (UT) subjects during submaximal exercise at the same relative and absolute workload when glucose is ingested. Six highly trained cyclists/triathletes [maximal workload (Wmax), 400 +/- 9 W] and seven UT subjects (Wmax, 296 +/- 8 W) were studied during 120 min of cycling exercise at 50% Wmax ( approximately 55% maximal O(2) consumption). The T subjects performed a second trial at the mean workload of the UT group (148 +/- 4 W). Before exercise, 8.0 ml/kg of a (13)C-enriched glucose solution (80 g/l) was ingested. During exercise, boluses of 2.0 ml/kg of the same solution were administered every 15 min. Measurements were made in the 90- to 120-min period when a steady state was present in breath (13)CO(2) and plasma glucose (13)C enrichment. Energy expenditure was higher in T than in UT subjects (58 vs. 47 kJ/min, respectively; P < 0.001) at the same relative intensity. This was completely accounted for by an increased fat oxidation (0.57 vs. 0.40 g/min; P < 0.01). At the same absolute intensity, fat oxidation contributed more to energy expenditure in the T compared with the UT group (44 vs. 33%, respectively; P < 0.01). The reduction in carbohydrate oxidation in the T group was explained by a diminished oxidation rate of muscle glycogen (indirectly assessed by using tracer methodology at 0.72 +/- 0.1 and 1.03 +/- 0.1 g/min, respectively; P < 0.01) and liver-derived glucose (0.15 +/- 0.03 and 0.22 +/- 0.02 g/min, respectively; P < 0.05). Exogenous glucose oxidation rates were similar during all trials (+/-0.70 g/min).

Oxidation of an oral [13C]glucose load at rest and prolonged exercise in trained and sedentary subjects

The purpose of this study was to compare the oxidation of [13C]glucose (100 g) ingested at rest or during exercise in six trained (TS) and six sedentary (SS) male subjects. The oxidation of plasma glucose was also computed from the volume of 13CO2 and 13C/12C in plasma glucose to compute the oxidation rate of glucose released from the liver and from glycogen stores in periphery (mainly muscle glycogen stores during exercise). At rest, oxidative disposal of both exogenous (8.3 +/- 0.3 vs. 6.6 +/- 0.8 g/h) and liver glucose (4.4 +/- 0.5 vs. 2.6 +/- 0.4 g/h) was higher in TS than in SS. This could contribute to the better glucose tolerance observed at rest in TS. During exercise, for the same absolute workload [140 +/- 5 W: TS = 47 +/- 2.5; SS = 68 +/- 3 %maximal oxygen uptake (VO2 max)], [13C]glucose oxidation was higher in TS than in SS (39.0 +/- 2.6 vs. 33.6 +/- 1.2 g/h), whereas both liver glucose (16.8 +/- 2.4 vs. 24.0 +/- 1.8 g/h) and muscle glycogen oxidation (36.0 +/- 3.0 vs. 51.0 +/- 5.4 g/h) were lower. For the same relative workload (68 +/- 3% VO2 max: TS = 3.13 +/- 0.96; SS = 2.34 +/- 0.60 l O2/min), exogenous glucose (44.4 +/- 1.8 vs. 33.6 +/- 1.2 g/h) and muscle glycogen oxidation (73.8 +/- 7.2 vs. 51.0 +/- 5.4 g/h) were higher in TS. However, despite a higher energy expenditure in TS, liver glucose oxidation was similar in both groups (22.2 +/- 3.0 vs. 24.0 +/- 1.8 g/h). Thus exogenous glucose oxidation was selectively favored in TS during exercise, reducing both liver glucose and muscle glycogen oxidation.

Oral [13C]glucose oxidation during prolonged exercise after high- and low-carbohydrate diets

The effect of a diet either high or low in carbohydrates (CHO) on exogenous 13C-labeled glucose oxidation (200 g) during exercise (ergocycle: 120 min at 64.0 +/- 0.5% maximal oxygen uptake) was studied in six subjects. Between 40 and 80 min, exogenous glucose oxidation was significantly higher after the diet low in CHO (0.63 +/- 0.05 vs. 0.52 +/- 0.04 g/min), but this difference disappeared between 80 and 120 min (0.71 +/- 0.03 vs. 0.69 +/- 0.04 g/min). The oxidation rate of plasma glucose, computed from the volume of 13CO2 produced the 13C-to-12C ratio in plasma glucose at 80 min, and of glucose released from the liver, computed from the difference between plasma glucose and exogenous glucose oxidation, was higher after the diet low in CHO (1.68 +/- 0.26 vs. 1.41 +/- 0.17 and 1.02 +/- 0.20 vs. 0.81 +/- 0.14 g/min, respectively). In contrast the oxidation rate of glucose plus lactate from muscle glycogen (computed from the difference between total CHO oxidation and plasma glucose oxidation) was lower (0.31 +/- 0.35 vs. 1.59 +/- 0.20 g/min). After a diet low in CHO, the oxidation of exogenous glucose and of glucose released from the liver is increased and partly compensates for the reduction in muscle glycogen availability and oxidation.

Importance of measuring CO2-production rate when using 13C-breath tests to measure fat digestion

Stable isotope breath tests offer a safe, repeatable non-invasive method of measuring fat digestion. They involve the ingestion of a substrate labelled with 13C followed by serial measurements of the 13C:12C ratio in expired CO2, from which the percentage of the 13C dose recovered (PDR) can be calculated. However PDR depends on the CO2-production rate. Our aim was to compare results obtained using directly measured CO2-production rates with those calculated from two predicted values. Twelve normal healthy children and twenty-four children with cystic fibrosis (CF) (without or with the normal dose of enzyme supplementation) were studied with 1,3-distearyl, 2[carboxyl-13C] octanoyl glycerol. The volume of CO2 produced (litres/min) was measured at rest for 30 min approximately 3 h after substrate ingestion, and the results were converted to mmol/min. For each subject the expected BMR was calculated from the equation of Schofield (1985), based on sex, age, weight and height, and from these values, CO2-production rate was derived. Surface area was calculated and an estimated value of 5 mmol/m2 per min (Shreeve et al. 1970) was used. Using these three CO2-production rates, three different PDR were calculated and compared. In healthy children there was a close concordance between measured and predicted CO2-production rates, but children with CF had a mean measured CO2-production rate 39% higher than normal children. This use of normal data for predicted CO2-production rates in children with CF underestimates cumulative PDR. If direct measurements of CO2-production rate are not available or impossible to perform the VCO2 obtained from the BMR calculated using the equations of Schofield (1985) or Shreeve et al. (1970) can be used in normal children. However, if accurate results for PDR are to be obtained, CO2-production rates should be measured when performing breath tests in conditions where energy expenditure and/or CO2-production rate are not expected to be normal.

Exogenous carbohydrate utilisation: effects on metabolism and exercise performance

It is generally recognized that a decrease in carbohydrate availability can lead to the development of fatigue during prolonged exercise in humans. Administration of glucose or other carbohydrates before or during exercise has been shown to postpone fatigue, conserve muscle glycogen and improve performance. Carbohydrates can be categorised according to their ability to increase blood glucose concentration (known as glycaemic index) and by the extent they stimulate the release of insulin. The glycaemic index is reflected in the rate at which consumed carbohydrate is made available in the blood. Glucose is the only type of carbohydrate that can readily be oxidised by skeletal muscle for energy production. Gastric emptying is the primary factor limiting the rate of carbohydrate delivery to the blood and therefore influences the utilisation of exogenous carbohydrate ingested before or during exercise. Various methods have been used to assess the oxidation of exogenous carbohydrates during exercise. Peak rates of CHO oxidation during exercise have been reported between 0.4 and 1.0 g/min, and the rates of oxidation do not appear to be influenced to a major extent by the use of multiple drinking schedule in comparison with a single bolus schedule. Previous studies also suggest that the ingestion of fructose during exercise does not offer any additional benefits over ingestion of glucose or glucose polymer solutions of similar concentration. The hormones insulin, glucagon and adrenaline together with cortisol and growth hormone play key roles in the regulation of carbohydrate metabolism during exercise. Ingestion of moderately concentrated carbohydrate solutions (4-8%) enhances prolonged exercise performance and is appropriate for optimising energy and fluid delivery without causing adverse effects. The ergogenic effects of carbohydrate ingestion on performance during intermittent exercise such as competitive sports are less well established, although the evidence to date suggests diminished performance when carbohydrate are limiting.

Exogenous glucose oxidation during exercise in endurance-trained and untrained subjects

To investigate the effect of training status on the fuel mixture used during exercise with glucose ingestion, seven endurance-trained cyclists (Tr; maximum O2 uptake 67 +/- 2.3 ml.kg-1.min-1) and eight untrained subjects (UTr; 48 +/- 2 ml.kg-1.min-1) were studied during 120 min of exercise at approximately 60% maximum O2 uptake. At the onset of exercise, 8 ml.kg-1.min-1 of an 8% naturally enriched [13C]glucose solution was ingested and 2 ml/kg every 15 min thereafter. Energy expenditure was higher in Tr subjects compared with UTr subjects (3,404 vs. 2,630 kJ; P < 0.01). During the second hour, fat oxidation was higher in Tr subjects (37 +/- 2 g) compared with UTr subjects (23 +/- 1 g), whereas carbohydrate oxidation was similar (116 +/- 8 g in Tr subjects vs. 114 +/- 4 g in UTr subjects). No differences were observed in exogenous glucose oxidation (50 +/- 2 g in Tr subjects and 45 +/- 3 g in UTr subjects, respectively). Peak exogenous glucose oxidation rates were similar in the two groups (0.95 +/- 0.07 g/min in Tr subjects and 0.96 +/- 0.03 g/min in UTr subjects). It is concluded that the higher energy expenditure in Tr subjects during exercise at the same relative exercise intensity is entirely met by a higher rate of fat oxidation without changes in the rates of exogenous and endogenous carbohydrates.

Respective oxidation of 13C-labeled lactate and glucose ingested simultaneously during exercise

The purpose of this experiment was to measure, by using 13C labeling, the oxidation rate of exogenous lactate (25 g, as Na+, K+, Ca2+, and Mg2+ salts) and glucose (75 g) ingested simultaneously (in 1,000 ml of water) during prolonged exercise (120 min, 65 +/- 3% maximum oxygen uptake in 6 male subjects). The percentage of exogenous glucose and lactate oxidized were similar (48 +/-3 vs. 45 +/- 5%, respectively). However, because of the small amount of oral lactate that could be tolerated without gastrointestinal discomfort, the amount of exogenous lactate oxidized was much smaller than that of exogenous glucose (11.1 +/- 0.5 vs. 36.3 +/- 1.3 g, respectively) and contributed to only 2.6 +/- 0.4% of the energy yield (vs. 8.4 +/- 1.9% for exogenous glucose). The cumulative amount of exogenous glucose and lactate oxidized was similar to that observed when 100 g of [13C]glucose were ingested (47.3 +/- 1.8 vs. 50.9 +/- 1.2 g, respectively). When [13C]glucose was ingested, changes in the plasma glucose 13C/12C ratio indicated that between 39 and 61% of plasma glucose derived from exogenous glucose. On the other hand, the plasma glucose 13C/12C ratio remained unchanged when [13C]lactate was ingested, suggesting no prior conversion into glucose before oxidation.

Postprandial thermogenesis and substrate utilization after ingestion of different dietary carbohydrates

Whole-body thermogenesis, substrate utilization (open-circuit ventilated-hood system), and exogenous carbohydrate oxidation were evaluated in 10 healthy lean male volunteers (aged 27.8 +/- 2.5 years) for 6 hours after oral ingestion of 75 g naturally enriched fructose, glucose (both derived from corn starch), cane sugar, and a good digestible corn starch (all mixed with 400 mL water). The integrated areas under the glucose and insulin response curves above baseline were highest with glucose and starch, intermediate with sucrose, and lowest with fructose, whereas there were no significant differences in the integrated nonesterified fatty acid (NEFA) response between carbohydrates. The total increment in energy expenditure (EE) above baseline was similar with fructose (130 +/- 24 kJ/6 h) and sucrose (141 +/- 17 kJ/6 h), was higher with sucrose as compared with starch (108 +/- 24 kJ/6 h, P < .05) and glucose (94 +/- 20 kJ/6 h, P < .05), and tended to be higher with fructose as compared with glucose (P = .059). Both the increment in total carbohydrate oxidation (P < .05) and the increment in exogenous carbohydrate oxidation (P < .01) were significantly higher with fructose and sucrose compared with glucose and starch. The initial inhibition of lipid oxidation was higher with sucrose and fructose than with glucose and starch, whereas the integrated decrement in lipid oxidation over 6 hours was only higher with fructose compared with glucose and starch (P < .05). In conclusion, thermogenesis and substrate utilization vary considerably after ingestion of different types of carbohydrate in young lean males, indicating that the carbohydrate composition of the diet may have important consequences for energy and macronutrient balance.

Effects of carbohydrate (CHO) and fat supplementation on CHO metabolism during prolonged exercise

The aim of the study was to examine carbohydrate (CHO) utilization in subjects receiving CHO or CHO + medium-chain triglycerides (MCT) supplements during 180 minutes of exercise at 50% maximal aerobic work rate ([Wmax] 57% maximal oxygen consumption [VO2max]). In a double-blind crossover design, nine trained athletes cycled four times. Subjects received a bolus of 4 mL x kg(-1) at the start and 2 mL x kg(-1) every 20 minutes during exercise of either a 150-g x L(-1) CHO solution (CHO trial), an equicaloric 70 energy% (en%) CHO-30 en% MCT suspension containing 29 g MCT (CHO + MCT trial), or a 150-g x L(-1) CHO (high-CHO [HCHO]) solution plus 20 g MCT (HCHO + MCT trial). A fourth trial consisted of a 13C-background control trial (CON). The four trials were randomized. Before and after the exercise bout, muscle biopsies were taken from the quadriceps muscle and muscle glycogen levels were determined. During exercise, breath samples were collected for estimation of exogenous and endogenous CHO oxidation. No significant differences were detected in glycogen breakdown among the trials (277 +/- 14 mmol x kg dry weight(-1) CHO, 249 +/- 20 CHO + MCT, and 240 +/- 18 HCHO + MCT) or in the respiratory exchange ratio during exercise. Mean exogenous CHO oxidation rates during the final hour of exercise were 0.79, 0.63, and 0.73 g x min(-1), respectively. No differences were observed between the trials regarding exogenous or endogenous CHO oxidation. Plasma free fatty acid (FFA) concentrations were elevated during exercise to a level of approximately 500 micromol x L(-1) and were comparable in all trials, whereas plasma ketone concentrations significantly increased after MCT ingestion as compared with the CHO trial. It is concluded that 29 g MCT co-ingested with CHO during 180 minutes of exercise does not influence CHO utilization or glycogen breakdown.

A computer program for estimating the influence of the body bicarbonate pool during CO2 breath tests

Mathematical representation of physiological systems is a useful means of monitoring various parameters and factors of such systems. The use of compartmental models for the study of physiological systems has led to the introduction of several new techniques for solving problems, concerning the kinetics between the individual sites of these systems. The computer program, presented here, allows the calculation of the influence of a mammillary model, representing the bicarbonate pool, on the expiration rate of the label, administered during a breath test. It consists of two modules, one for performing non-linear least squares regression fitting of discrete measurements from a breath test (model output) and a second one for estimating the corresponding label released into the bicarbonate pool (model input), based on the measurements of expired air. Data, derived from metabolic studies using oral administration of naturally labelled 13C-glucose in man, under various metabolic conditions, was used as an input to this program.