Muscle groups in the upper body are involved in several sports (swimming, rowing, kayaking, cross-country skiing, etc.), in repeated lifting and loading tasks, which are frequent in military, industrial, and domestic activities and in locomotion in disabled individuals. For this reason, several studies have compared the cardiorespiratory responses to arm and leg exercises (see Richardson et al. (32) for a review). Fuel selection during arm exercise has also been described in several studies (2-5,9,10,12,19,20,22,23,35,36,39,40). However, few of them have been conducted during exercise durations of more than 30 min (3,12,19,35,36) or with a comparison with leg exercise at similar percentages of the mode-specific V˙O2max (%MS-V˙O2max) (3-5,9,10,12,20,22,23) or of the ventilatory threshold (V T) (19,35,36,39,40). Data compiled from these studies show that the %En from CHO oxidation (reported or estimated from the RER) was generally higher during arm than leg exercise. Ahlborg et al. (3) also showed that the fractional extraction of glucose across the exercising muscles and the contribution of plasma glucose to the energy yield were both higher during arm than leg exercise. In this respect, numerous studies have shown that CHO ingested immediately before and during leg exercise are readily available for oxidation, significantly contribute to the energy yield, and increase endurance performance (14). However, there is a paucity of data on the effect of CHO supplementation on the metabolic response to prolonged upper body exercise. Glucose ingestion immediately before arm exercise (19,35,36) and canoeing (8) in able-bodied subjects and before wheelchair exercise in paraplegic athletes (19,35,36) has been shown to increase RER and CHO oxidation (albeit not significantly) and to slightly improve performance (19,35,36). However, in these studies, no comparison was made with leg exercise and the metabolic fate of exogenous glucose, and possible changes in endogenous glucose oxidation were not measured.
The purpose of the present study was to compare fuel selection during prolonged moderate arm and leg exercise (120 min at 50% of the corresponding maximal aerobic power output), in 10 male subjects without and with glucose ingestion (2 g·kg−1). Total CHO and fat oxidation were measured using indirect respiratory calorimetry corrected for protein oxidation and the glucose ingested during the exercise period was artificially labeled with 13C to compute its oxidation rate. On the basis of data from the literature (see above), we hypothesized that the contribution of CHO oxidation to the energy yield (%En) would be slightly higher in arm than leg exercise. We also hypothesized that, as regularly observed during leg exercise (14), when glucose is ingested during arm exercise, exogenous glucose would be oxidized thus decreasing endogenous glucose oxidation. Finally, based on the data from Ahlborg et al. (3) showing a higher %En from plasma glucose during arm than leg exercise, we hypothesized that when glucose is ingested, the %En from exogenous glucose would be higher during arm than leg exercise.
The experiment was conducted on 10 active male subjects (∼4-6 h·wk−1 of exercise = mainly cycling and running with occasional swimming and weightlifting, 25.3 ± 1.4 yr old, height = 176.3 ± 2.8 cm, weight = 75.6 ± 3.6 kg (mean ± SEM)) who gave their informed written consent to participate in this study, which was approved by the University of Montreal ethics committee on the use of human subjects in research. All the subjects had a normal glucose tolerance as shown by plasma glucose concentration after a 12-h fast (5.0 ± 0.1 mmol·L−1) and 120 min after ingestion of 75 g of glucose in 300 mL of water (5.4 ± 0.3 mmol·L−1). None of the subjects were smokers, heavy drinkers (<3 drinks per week), under medication, or taking recreational drugs.
Mode-specific maximal power output and %MS-V˙O2max [cycling (Lifecycle 9500 HR; Lifefitness, Schiller Park, IL) and arm cranking (Angio; Lode BV, Groningen, the Netherlands)] were determined, 1 wk apart, in a randomized counterbalanced order using open-circuit spirometry (MOXUS Metabolic Cart; AEI Technologies, Naperville, IL) with incremental continuous protocols. The subjects were positioned on the arm cranking ergometer as described by Bhambhani et al. (6) and were allowed a familiarization period on both ergometers. After a 2-min warm-up (25 and 50 W for arm cranking and cycling, respectively), the workload was incremented every 2 min by 15-20 W for arm exercise and 30-45 W for leg exercise until volitional exhaustion. The subjects were then studied four times (at 1-wk intervals) during 120-min exercise periods at 50% of the mode-specific maximal power output. During the exercise period, the subjects ingested either 2 g·kg−1 body mass of glucose dissolved in 20.1 mL·kg−1 of water or 20.1 mL·kg−1 of water only. The drinks were given in seven doses (5.7 mL·kg−1 20 min before the beginning of exercise and 2.9 mL·kg−1 every 20 min thereafter up to minute 100). The total amounts of water and glucose administered were 1521 ± 73 mL and 153 ± 7 g, respectively. Two days before each experiment, the subjects followed a standardized diet (45 kcal·kg−1: 55% CHO, 30% fat, and 15% protein) and were fed prepackaged meals for the dinner and breakfast preceding the experiment. The prepackaged meals did not contain any food with a high 13C content (e.g., corn, sugar cane), which may modify the background 13C enrichment of expired CO2. In addition, during the 3 d preceding the tests, the subjects refrained from exercising and from drinking alcohol. The experiments, which were presented in a balanced random order among the subjects, were performed between 9:00 and 11:30 a.m., 2 h after the breakfast (∼850 kcal: 55% CHO, 30% fat, and 15% proteins) ingested at 7:00 a.m.
The glucose ingested during the exercise period (Biopharm, Laval, Quebec, Canada) derived from corn (13C/12C = −11.0‰ δ13C PDB (Peedee Belemnite)) and was artificially enriched with U13C-glucose (13C/C > 99%; Isotec, Miamisburg, OH) to achieve a final isotopic composition close to 70‰ δ13C PDB: the actual value measured by mass spectrometry (see below) was 69.0‰ δ13C PDB.
Measures and computations.
Measurements were made at rest before ingestion of exogenous glucose and every 20 min during the exercise period. Fat and CHO oxidation were computed from indirect respiratory calorimetry corrected for protein oxidation. For this purpose, V˙O2 and carbon dioxide production (V˙CO2) were measured using open-circuit spirometry, and urea production was estimated from its concentration in urine and sweat, and from urine and sweat loss during the 120 min of exercise (24). Sweat loss was estimated from changes in body mass, taking into account fluid intake, mass loss through CO2 production, and water loss from the lungs. For the measurement of 13C/12C in expired CO2, 10-mL samples of expired gases were collected in vacutainers (Becton Dickinson, Franklin Lakes, NJ). Finally, 10-mL blood samples were withdrawn through a catheter (Baxter Healthcare, Corp., Valencia, CA) inserted into an antecubital vein at the beginning of the experiment for the measurement of plasma glucose, free fatty acid, and insulin concentrations at rest before ingestion of the first dose of water or glucose and after 60 and 120 min. Plasma, urine, and sweat samples were stored at −80°C until analysis.
Protein oxidation and the associated amount of energy provided were estimated from the amount of urea excreted taking into account that 1 g of urea excreted corresponds to 2.9 g of protein oxidized, and that the energy potential of protein is 4.70 kcal·g−1 (25). Fat and CHO oxidation were then computed from V˙O2 and V˙CO2 (L·min−1) corrected for the volumes of O2 and CO2 corresponding to protein oxidation (1.010 and 0.843 L·g−1, respectively) (25):
The amount of energy provided by the oxidation of CHO and fat were computed from their respective energy potential (18).
Measurements of 13C/12C in expired CO2 were performed by mass spectrometry (Prism, VG, Manchester, United Kingdom). The isotopic composition of ingested glucose (after combustion) and expired CO2 was expressed in per mil difference by comparison with the PDB Chicago Standard: δ13C PDB = 1000[(R spl / R std) − 1], where R spl and R std are the 13C/12C ratio in the sample and standard (1.1237%), respectively.
The oxidation rate of exogenous glucose (g·min−1) was computed as follows:
where V˙CO2 (not corrected for protein oxidation) is in liters per minute, R exp is the observed isotopic composition of expired CO2, R ref is the isotopic composition of expired CO2 when only water was ingested, R exo is the isotopic composition of the exogenous glucose ingested, and k (0.7426 L·g−1) is the volume of CO2 provided by the complete oxidation of glucose. This computation is made based on the observation that in response to exercise, 13C provided from 13C-glucose is not irreversibly lost in pools of tricarboxylic acid cycle intermediates and/or bicarbonate and that 13CO2 recovery in expired gases is, thus, complete or almost complete (34,37). However, the 13C/12C in expired CO2 only slowly equilibrates with 13C/12C in the CO2 produced in tissues (28). To take into account this delay between 13CO2 production in tissues and at the mouth, the computations were only made during the last 80 min of the observation period, thus allowing for a 40-min equilibration period at the beginning of exercise.
Plasma glucose and free fatty acid concentrations were measured using spectrophotometric automated assays (Boehringer, Mannheim, Germany), whereas plasma insulin concentration was measured using an automated radioimmunoassay (KTSP-11001; Immunocorp Sciences, Montreal, Quebec, Canada). Urine and sweat urea concentrations were measured using a Synchron Clinical System (CX7; Beckman, Anaheim, CA).
Data are presented as mean ± SEM. Comparisons were made using two-way (trial × time) ANOVA for repeated measurements (Statistica package; StatSoft, Tulsa, OK). The location of significant differences (P < 0.05) was identified using post hoc comparisons (Tukey's honestly significantly different test) when ANOVA yielded a significant F ratio.
The maximal power output (159.5 ± 8.6 vs 352.5 ± 17.6 W) and V˙O2max (3.36 ± 0.15 vs 4.77 ± 0.20 L·min−1) observed during arm exercise were lower than during leg exercise. In response to the 120-min exercise at 50% of the mode-specific maximal power output, the V˙O2 was stable during the exercise period and was not significantly modified when glucose was ingested (Table 1). The average V˙O2 sustained during the 120-min exercise period was significantly higher during leg than arm exercise and corresponded to 51.6 ± 1.5% and 53.9 ± 1.2% MS-V˙O2max, respectively (not significantly different). The RER significantly decreased during the exercise period and was not significantly different when water was ingested in the two modes of exercise. Ingestion of glucose significantly increased RER during arm but not during leg exercise, and in this situation, the RER was significantly higher during arm than leg exercise. The amount of urea excreted in sweat and urine during the 120-min exercise period was not significantly different in the four experimental situations.
As shown in Table 2, the amount of protein oxidized during the 120-min exercise period was not significantly different in the four experimental situations. However, the %En from protein oxidation was significantly higher during arm than leg exercise (Fig. 1). The amounts of CHO and fat oxidized, which were significantly lower during arm than leg exercise because of the significantly lower energy expenditure, decreased and increased, respectively, during the exercise period in all the experimental situations (Table 2). The %En from CHO oxidation, which was not significantly different between arm and leg exercises when water was ingested, significantly increased when glucose was ingested in arm exercise only (from 66.3 ± 1.9 to 78.6 ± 2.6%En; Fig. 1). In this situation, the %En from CHO oxidation was significantly higher in arm than leg exercise. The %En from fat oxidation was significantly lower in arm than leg exercise with both ingestion of water and glucose and was significantly decreased with glucose ingestion in arm but not leg exercise.
No significant difference was observed in 13C/12C in expired CO2 at rest before ingestion of the first dose of 13C-glucose in the two trials (background enrichment of expired CO2: average value = −22.3 ± 0.7‰ δ13C PDB, n = 20; Fig. 2). The 13C/12C in expired CO2 progressively increased during the 120-min period of exercise, with higher values observed during arm than leg exercise during the last 60 min of exercise. When glucose was ingested, the peak oxidation rates of exogenous glucose (0.83 ± 0.05 and 0.92 ± 0.13 g·min−1 in arm and leg exercises, respectively) and the total amount of exogenous glucose oxidized during the last 80 min of exercise were significantly lower during arm than leg exercise (Table 2). However, because of the lower energy expenditure during arm exercise, the %En from exogenous glucose oxidation during the last 80 min of the exercise period was significantly higher during arm than leg exercise (Fig. 1).
Because of the lower energy expenditure, endogenous CHO oxidation was significantly lower during arm than leg exercise with and without glucose ingestion (Table 2). Glucose ingestion markedly reduced endogenous CHO oxidation in both arm and leg exercises (∼26% reduction in both modes of exercise). When expressed in %En, the oxidation of endogenous CHO significantly decreased with glucose ingestion and was not significantly different in the two modes of exercise (Fig. 1).
Plasma glucose concentration remained stable during the exercise period when water was ingested, but significantly higher values were observed after 60 and 120 min with glucose ingestion (Fig. 3) without any significant difference between the two modes of exercise. When water was ingested, the significant increase in plasma free fatty acid concentration in response to exercise was not significantly different in the two modes of exercise. Glucose ingestion blunted the response of plasma free fatty acid concentration to exercise with no significant difference between the two modes of exercise. Glucose ingestion also blunted the reduction in plasma insulin concentration observed during exercise with water ingestion, without any effect of the mode of exercise.
A compilation of data from the literature suggests a slightly higher reliance on CHO oxidation during arm than leg exercise at similar %MS-V˙O2max or %V T in subjects exercising after a 12-h fast (average value: ∼77 vs ∼67%En) (3,4,9,10,12,20,22,23,39,40). However, in some studies, the statistical comparisons were not made or not reported (4,22,40), and when reported, the difference in fuel selection was significant (3) or not (12,23) or was not consistently significant over the range of workloads studied: at 50% but not at 30% and 75% MS-V˙O2max (5), at 60% and 70% but not at 50% MS-V˙O2max (20), or at 90% but not at 70% of the V T (39). Taken together, these results suggest that the difference in fuel selection sometimes reported between the two modes of exercise remains modest and could escape detection because of day-to-day variations in the metabolic response to exercise (18), in cross-sectional studies (e.g., (22,23)), because of interindividual variation in the %En from fat and CHO oxidation at a given %V˙O2max (1,38).
In the present experiment, the difference in fuel selection observed at similar %MS-V˙O2max during arm and leg exercise performed 2 h after a breakfast rich in CHO when water was ingested was also modest. The %En from protein oxidation was significantly higher during arm than leg exercise (7.6 ± 2.2% vs 4.7 ± 1.0%), but this was because total energy expenditure was ∼50% higher during leg than arm exercise: the amount of protein oxidized during the 2-h period of exercise was not significantly different in the two modes of exercise (∼15 g). As for the lower %En from fat oxidation observed during arm than leg exercise, although the difference was statistically significant, it remained very small, and the compensatory increases in the %En from CHO oxidation were also small and did not reach statistical significance. In line with data in the literature, which have been discussed above, these observations suggest that for a given %MS-V˙O2max, in subjects exercising ∼2 h after a breakfast rich in CHO (∼125 g of glucose), fuel selection does not differ markedly during arm and leg exercises with only a higher %En from protein oxidation during arm exercise, because of the lower absolute workload sustained, and a slight shift from fat to CHO oxidation.
Several studies have reported that plasma catecholamine concentrations at a given %MS-V˙O2max were higher during arm than leg exercise, although the statistical comparison did not reach significance (3,12) or was not reported (33). In the study by Ahlborg et al. (3) conducted at 30% MS-V˙O2max for 120 min, this was associated with a higher reliance on CHO oxidation (∼60 vs ∼30%En from CHO) during arm than leg exercise. However, in a subsequent experiment, these authors did not confirm this finding: plasma catecholamine concentrations were significantly lower during arm than leg exercise at 50% and 80% MS-V˙O2max sustained for 30 min (2). In the present study, plasma catecholamine concentrations were not measured, and no significant difference was observed in the contribution of endogenous CHO oxidation to the energy yield both with and without glucose ingestion during exercise. In addition, the respective contributions of muscle and liver glycogen to endogenous CHO oxidation were not measured. It is thus difficult to speculate on the possible role of the sympathetic system in the differential regulation of fuel selection during arm and leg exercises.
Changes in fuel selection because of glucose ingestion during arm exercise have been described by Spendiff and Campbell (35) and by Jung and Yamasaki (19). In the study by Spendiff and Campbell (35), the RER observed during a 60-min period of arm cranking at 65% MS-V˙O2max in young, upper body-trained athletes was not significantly different with ingestion of a placebo or 48 g of glucose 20 min before the exercise. However, in a subsequent study conducted in athletes in wheelchair, although this did not reach statistical significance, when compared with the value observed after ingestion of 24 g of glucose 20 min before exercise, the RER was higher when 72 g was ingested (0.91 vs 0.88) (36), suggesting an increase in CHO oxidation with the amount of glucose ingested. In the study by Jung and Yamasaki (19), 1 g·kg−1 glucose was ingested immediately before a 60-min period of arm cranking at 80% of the mode-specific lactate threshold in middle-aged able-bodied and paraplegic subjects. Although the changes did not reach statistical significance, glucose ingestion decreased fat oxidation and increased CHO oxidation in able-bodied subjects but increased fat oxidation and decreased CHO oxidation in paraplegic subjects.
In the studies by Spendiff and Campbell (35,36) and Jung and Yamasaki (19), no comparison was made with leg exercise, and the metabolic fate of exogenous glucose and the possible changes in endogenous substrate oxidation were not measured. It is well documented that glucose ingested during leg exercise is readily available for oxidation and significantly contributes to the energy yield, reducing the reliance on endogenous CHO stores (14), with (13,17) or without (26) any change in total CHO oxidation. Data compiled by Jeukendrup (14) indicate that, in this mode of exercise, for an ingestion rate of ∼1.25 g·min−1 such as that used in the present experiment (150 g during a 2-h period), the average peak oxidation rate of exogenous glucose is ∼0.75 g·min−1, with values ranging from ∼0.53 to ∼1.0 g·min−1. The peak oxidation rate of 0.92 ± 0.13 g·min−1 observed in the present experiment is in good agreement with these data. During the last 80 min of leg exercise, 65.1 ± 3.5 g of exogenous glucose was oxidized contributing 24%En, increasing total CHO oxidation from 171 ± 7 to 193 ± 11 g (64-71%En) and reducing endogenous CHO from 171 ± 7 to 128 ± 10 g (64-47%En). These changes in CHO oxidation did not modify protein oxidation but decreased fat oxidation from 33 ± 4 to 27 ± 3 g (31-25%En). When compared with these figures, the amount of glucose ingested that was actually oxidized during arm exercise during the last 80 min was significantly 14% lower (56.3 ± 3.9 g or an oxidation rate of 0.83 ± 0.05 g·min−1). This could be due, at least in part, to the lower absolute workload sustained. Indeed, the oxidation rate of exogenous glucose during exercise increases not only with the ingestion rate (14) but also, for a given ingestion rate, with V˙O2 (7,29,30). However, because of the smaller energy expenditure during arm than leg exercise, the %En from exogenous glucose oxidation was significantly higher (30 ± 1 vs 24 ± 1%En). As observed during leg exercise, exogenous glucose oxidation increased total CHO oxidation (from 66 ± 2 to 78 ± 3%En) and decreased endogenous CHO (from 66 ± 2 to 48 ± 3%En) and fat oxidation (from 26 ± 2 to 14 ± 2%En) without modifying protein oxidation.
These data show that the effect of glucose ingestion on fuel selection is very similar during arm and leg exercises at the same %MS-V˙O2max. The main difference was the %En from exogenous glucose, which was slightly but significantly higher during arm than leg exercise. As recently summarized by Kiilerich et al. (21), this can hardly be explained by differences in fiber type, blood flow, and oxygen delivery or by differences in mRNA expression, protein content, and/or activity of enzymes involved in fat or CHO oxidation between arm and leg muscles (9,11,27,31). As mentioned above, this could due in part be to the lower energy expenditure during arm than leg exercise. Although this remains a matter of debate, the limiting factor for exogenous glucose oxidation could be the rate of glucose absorption (15,16). Under the hypothesis that glucose absorption was similar during the two modes of exercise, this would translate into a higher availability of plasma glucose with respect to the energy expenditure during arm than leg exercise. In support to this explanation, plasma glucose and insulin concentrations were similar in the two modes of exercise when water was ingested, an observation which is line with data from Ahlborg et al. (3) and Richter et al. (33). In contrast, although this did not reach statistical significance, both plasma glucose and insulin concentrations were higher when glucose was ingested during arm than leg exercise.
In summary, the difference in fuel selection observed between the two modes of exercise when water was ingested was modest with a slightly higher reliance on CHO oxidation during arm than leg exercise. When glucose was ingested, the amount of exogenous glucose oxidized was lower, but its contribution to the energy yield was higher during arm than leg exercise. This could simply be due to the lower energy expenditure during arm than leg exercise at the same %MS-V˙O2max.
This work was funded by the Natural Sciences and Engineering Research Council of Canada.
The results from the present study do not constitute endorsement by the ACSM.
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Keywords:©2009The American College of Sports Medicine
ARM CRANKING; 13C LABELING; INDIRECT RESPIRATORY CALORIMETRY; PROTEIN OXIDATION