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J Appl Physiol 94: 278-284, 2003. First published September 13, 2002; doi:10.1152/japplphysiol.00140.2002
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Vol. 94, Issue 1, 278-284, January 2003

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

Brian W. Timmons1, Oded Bar-Or1, and Michael C. Riddell2

1 Children's Exercise and Nutrition Centre, McMaster University, Hamilton L8N 3Z5; and 2 Department of Kinesiology and Health Science, Faculty of Pure and Applied Sciences, York University, Toronto, Ontario, Canada M3J 1P3


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

To determine whether the relative utilization of exogenous carbohydrate (CHOexo) differs between children and adults, substrate utilization during 60 min of cycling at 70% peak O2 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 13C-enriched 6% CHOexo 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 · kg-1 · min-1, P < 0.001) and total CHO oxidation lower (27.4 ± 1.5 vs. 34.8 ± 1.2 mg · kg-1 · min-1, P < 0.001) in boys than in men, respectively. During the CHOexo trial, CHOexo oxidation was higher (P < 0.001) in boys (8.8 ± 0.5 mg · kg-1 · min-1) than in men (6.2 ± 0.5 mg · kg-1 · 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 CHOexo in boys may be important in preserving endogenous fuels and may be related to pubertal status.

children; adults; carbon-13 isotope; substrate utilization


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

DURING ENDURANCE EXERCISE, carbohydrate (CHO) and fat are the primary nutrients utilized as fuel. Their relative use depends largely on the intensity and duration of exercise (35). However, the metabolic response to exercise can also be manipulated in adults (3, 11) and in children (31-33) by ingesting CHO before and during the activity. In adult men, exogenous CHO (CHOexo) fed during prolonged exercise increases, or at least maintains, plasma glucose levels (1, 12) and high rates of plasma glucose oxidation late in exercise (8), thereby resulting in improved endurance. These effects of CHOexo have also been documented in adolescent boys (31-33). In adults, the apparent maximal rate of CHOexo oxidation during prolonged exercise is ~1 g/min (17), and the contribution of CHOexo oxidation to total energy expenditure ranges from 10 to 20% during exercise lasting 60-120 min (10, 21, 29). Several factors seem to affect the rate of CHOexo oxidation, including the rate of CHOexo ingestion (25, 30) and exercise intensity (22, 29). However, maturity status has not been investigated as a possible factor influencing the rate of CHOexo oxidation during exercise.

Considerable evidence suggests that children have a lower respiratory exchange ratio (RER) than adolescents and adults during submaximal exercise performed at the same absolute (34, 36) and relative (19, 20) intensity. The lower RER in children indicates that they utilize more fat and less CHO for energy at a given intensity of exercise. The relative utilization of CHOexo during exercise, however, has not been directly compared between children and adults, and it is also unclear how CHOexo may affect endogenous substrate utilization during exercise in children compared with adults. Recent work from our laboratory suggests that the contribution of CHOexo to total energy metabolism may be higher in children and adolescents compared with what has been reported for adults (31-33). However, these studies were not designed to directly compare children with adults, and definitive proof of age- or maturity-related differences in CHOexo oxidation and energy metabolism is, therefore, lacking.

Considering our present understanding of CHOexo oxidation in adults and of substrate utilization in children, we wished to directly compare the effects of CHOexo on energy metabolism between children and adults during exercise performed at the same relative intensity. On the basis of our earlier observations, we hypothesized that, compared with adults, young children would oxidize relatively more CHOexo during exercise and that CHOexo would contribute proportionally more to total energy metabolism. To test this hypothesis, we compared the rate of CHOexo oxidation (by 13C stable isotope methodology) in a group of pre- and early pubertal boys with a group of adult men exercising at a similar intensity and ingesting CHOexo at identical rates, relative to their body weight (BW).


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Subjects. Twelve boys and ten men volunteered to participate in this study, which was approved by the McMaster University Research Ethics Review Board. Their physical and fitness characteristics are summarized in Table 1. Pubertal status of the boys was determined according to pubic hair development by the criteria of Tanner (39) assessed by a parent and the child. Boys were either Tanner stage 1 (n = 8) or Tanner stage 2 (n = 4). All subjects were healthy, nonobese, not taking any medication, and were recreationally active but not competitive athletes. After the purpose, procedures, and risks of the study were explained to each subject, the men signed an informed consent, and the boys assented verbally to participate. Each boy's parent then signed an informed consent on his or her son's behalf.

                              
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  		Table 1.   Subjects' physical and fitness characteristics

Initial testing. Each subject visited the Children's Exercise and Nutrition Centre for an initial session to collect anthropometric data, including height (Harpenden Stadiometer, CMS Weighing Equipment), naked BW (BWB-800, Tanita), and percent body fat (Bioimpedance-Analyzer-101A, RJL Systems) and to conduct a maximal exercise test on a mechanically braked cycle ergometer (Fleisch-Metabo) for determination of peak O2 uptake (VO2 peak). Measurements of O2 uptake (VO2) and CO2 production (VCO2) were made continuously by using a metabolic cart (Vmax29, SensorMedics). Heart rate (HR) was recorded continuously throughout the test by using a Polar HR monitor (Polar Vantage XL, Polar Electro). The initial testing was completed at least 1 wk before the experimental trials.

Experimental trials. On two occasions, separated by 1-2 wk, subjects cycled at 70% of their individual VO2 peak for two 30-min periods separated by a 5-min rest period. Trials were identical except for CHO intake before and during exercise. In the CHO trial (CT), each subject consumed a 6% CHO-electrolyte solution (4% sucrose, 2% glucose, ~18 mmol/l Na+, ~3 mmol/l K+) and, in the placebo trial (PT), an artificially sweetened beverage (identical in flavor and electrolyte concentration but without CHO) before and intermittently throughout exercise. The total volume of beverage consumed was 24 ml/kg BW per trial. Both beverages were prepared in powder form by the Gatorade Sports Science Institute (Barrington, IL). The CHO drink was artificially enriched with uniformly labeled [13C]sucrose and [13C]glucose (in a 2:1 ratio) to an isotopic composition of +20.883 change per 1,000 difference vs. the 13C/12C ratio from the international standard 13C Pee Dee Belemnitella-1 (PDB-1) (+20.883per thousand [delta -13C]PDB-1). The sequence of the experimental trials was counterbalanced, and only the subjects were blinded to the contents of their drink.

Experimental protocol. On the day before each trial, activity level and nutrient intake were standardized for each subject according to their habitual routines. This was achieved by having physical activity and dietary intake recorded the day before their first trial and then repeated the day before the next visit. In addition, subjects avoided corn products, or food derived from corn, to reduce background enrichment of expired CO2 from naturally derived 13C (37). Subjects arrived at the laboratory in the morning (~0730) after an overnight fast and were given a small standardized breakfast (boys: 125 ml tap water and 1 slice of toast with sugar-free jam ~90 kcal; men: twice that amount). After eating, subjects emptied their bladder, and a naked weight was taken (BWB-800) to calculate the volume of fluid intake for that session. After the subjects sat quietly for ~20 min, a resting expired gas sample was collected for 3 min, and a preexercise blood sample was drawn from an arm vein through a "butterfly" winged infusion set (Terumo). Subjects were then given their first drink (4 ml/kg BW) 30 min before the start of exercise [time (t) = -30 min] and consumed the same volume at five subsequent time points (t = -15, 0, 15, 30, and 50 min). Exercise began 30 min after the resting blood sample, and the target exercise intensity was verified within the first 5 min. The pedaling rate remained constant at 60 rpm throughout exercise. Subsequent expired gas samples were collected for a 3-min period starting at the 12th and 27th min of each exercise bout. Immediately after exercise, a second blood sample was drawn while subjects remained seated on the cycle ergometer. HR was monitored throughout exercise.

Substrate utilization. For each period of gas collection, oxidation rates of total CHO (CHOtotal) and total fat (Fattotal) were calculated according to the following equations (28)
CHO<SUB>total</SUB> (g/min) = 4.59 · <A><AC>V</AC><AC>˙</AC></A><SC>co</SC><SUB>2</SUB> (l/min) − 3.23 · <A><AC>V</AC><AC>˙</AC></A><SC>o</SC><SUB>2</SUB> (l/min)

Fat<SUB>total</SUB> (g/min) = −1.70 · <A><AC>V</AC><AC>˙</AC></A><SC>co</SC><SUB>2</SUB> (l/min) + 1.69 · <A><AC>V</AC><AC>˙</AC></A><SC>o</SC><SUB>2</SUB> (l/min)
The energy provided from CHO and fat oxidation was calculated from their energy potentials (3.87 and 9.75 kcal/g, respectively). To measure the ratio of 13C/12C in the expired CO2, a 20-ml syringe was used to draw a sample of the expired gas directly from the tube connecting the subject's mouthpiece to the metabolic cart. Sampling did not interfere with the subject exercising. Duplicate samples (10 ml) were emptied from the syringe into vacutainer tubes (Becton Dickinson) and subsequently analyzed for the ratio of 13C/12C in the expired CO2 (BreathMat Plus, Finnigan MAT). CHOexo oxidation was then calculated for the sampling periods according to the following equation modified from Mosora et al. (24)
CHO<SUB>exo</SUB> (g/min) = <A><AC>V</AC><AC>˙</AC></A><SC>co</SC><SUB>2</SUB> [(R<SUB>exp</SUB> − R<SUB>ref</SUB>)/(R<SUB>exo</SUB> − R<SUB>ref</SUB>)] (l/<IT>k</IT>)
where VCO2 (l/min) is in STPD, Rexp is the isotopic composition of expired CO2 during CT, Rref is the isotopic composition of expired CO2 during PT at the corresponding time point, Rexo is the isotopic composition of the CHOexo, and k (0.7426 l/g) is the volume of CO2 produced by the complete oxidation of 1 g of glucose. Endogenous CHO (CHOendo) oxidation was calculated by subtracting CHOexo from CHOtotal. Because of the presence of a large bicarbonate pool in the body and because of the delay in measuring 13CO2 production by the tissues at the mouth (26), computations of CHOexo oxidation were made for the last 30 min of exercise only.

Plasma glucose and lactate. Whole blood (~2 ml) was added to an EDTA-containing vacutainer (Becton Dickinson) and centrifuged (2,000 g for 10 min at 5°C). The supernatant was removed and stored at -70°C for subsequent analysis of plasma glucose and lactate (YSI 2300L STAT, Yellow Springs Instruments). Values were corrected for changes in plasma volume according to Dill and Costill (13).

Statistical analyses. Data are presented as means ± SE and were analyzed by using a statistical software package (STATISTICA for Windows 5.0, StatSoft). Differences in physical and fitness characteristics were compared with independent t-tests. The averaged substrate oxidation responses of the two groups were compared with a two-way (group × trial) ANOVA. Significance in all other variables was determined by using a three-way ANOVA with one between factor (group) and two within factors (trial and time). Where appropriate, Tukey's honestly significant difference post hoc test for unequal sample size was used to determine the location of significance among means. The threshold for statistical significance was set at P < 0.05 for all tests.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

HR and VO2. As expected, the average resting HR in both trials was higher (P < 0.001) in the boys (73 ± 3 beats/min) than in the men (57 ± 2 beats/min). During exercise, there were no differences between groups for average HR or the percentage of maximal HR (HRmax) achieved in either trial. However, the average HR for both groups during PT (156 ± 2 beats/min) was slightly lower (P = 0.02) than during CT (160 ± 2 beats/min). Figure 1 displays the percentage of HRmax achieved during exercise in PT and CT for both groups. The percentage of HRmax achieved increased over time (main effect, P < 0.001), and this increase was slower in the boys compared with the men (group × time interaction, P < 0.001) during the first 30 min of exercise only. Table 2 summarizes the VO2 during exercise for the boys and the men in both trials. VO2 expressed relative to BW did not differ between the groups, but the percentage of VO2 peak averaged across trials was slightly lower (P = 0.01) in the boys than in the men. However, when exercise intensity was expressed as a percentage of each subject's ventilatory threshold, there was no difference between boys and men (data not shown).


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  		Fig. 1.   Percentage of maximal heart rate (HR) achieved during exercise in placebo (PT; open symbols) and carbohydrate trials (CT; solid symbols) for boys (circles) and men (squares). Values are means ± SE. Hatched bars represent exercise. * Significant group × time interaction, P < 0.05.


                              
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  		Table 2.   VO2 during exercise in placebo and carbohydrate trials for boys and men

RER and breath enrichment. The average resting RER for both trials was not different between the boys (0.84 ± 0.01) and the men (0.84 ± 0.03). The average RER during exercise in PT was significantly lower (P < 0.001) in the boys than in the men and decreased over time (P < 0.001) from 0.91 ± 0.01 to 0.88 ± 0.01 in the boys and from 0.96 ± 0.01 to 0.93 ± 0.01 in the men (Fig. 2). In CT, the average RER during exercise was also significantly lower (P < 0.001) in the boys than in the men and decreased from 0.96 ± 0.01 to 0.94 ± 0.01 (P < 0.05) in the men but remained relatively constant at 0.91 ± 0.01 in the boys.


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  		Fig. 2.   Respiratory exchange ratio (RER) during exercise in PT (open symbols) and CT (solid symbols) for boys (circles) and men (squares). Values are means ± SE. Hatched bars represent exercise. * Significant difference between PT and CT, P < 0.05. dagger  Main effect for group, P < 0.05. Dagger  Sixty-five-minute value for boys and men in PT and men in CT significantly different from 15-min value in same trial, P < 0.05.

There were no differences in the isotopic composition of expired CO2 at rest between groups or between trials (pooled average = -22.1 ± 0.4per thousand [delta -13C]PDB-1). Figure 3 shows the changes in breath enrichment during exercise in PT and CT. During PT, the ratio of 13C/12C in expired CO2 increased slightly but significantly (P < 0.05) with exercise in the men but remained relatively unchanged in the boys. During CT, the 13C/12C in expired CO2 increased markedly with exercise in both groups, indicating a strong measurement signal compared with rest, but was significantly higher (P < 0.001) in the boys at all time points.


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  		Fig. 3.   Ratio of 13C/12C in expired air [delta per thousand vs. Pee Dee Bellemnitella (PDB)] at rest and during exercise in PT (open symbols) and CT trials (solid symbols) for boys (circles) and men (squares). Values are means ± SE. Hatched bars represent exercise. * Significant difference between boys and men within trial, P < 0.05. dagger  Significant main effect for trial, P < 0.05. Dagger  Thirty-minute and subsequent values during PT are significantly higher than rest in men, P < 0.05.

Substrate utilization. Oxidation rates and percent energy contributions for CHOtotal, CHOendo, CHOexo, and Fattotal are reported for the final 30 min of exercise only. To control for the effect of body size on absolute values of substrate utilization, oxidation rates were expressed relative to BW (i.e., mg · kg-1 · min-1, Table 3). In both trials, average CHOtotal oxidation was significantly lower (P < 0.001), and average Fattotal oxidation was significantly higher (P < 0.001), in the boys compared with the men. Main effects for trial approached significance, with CHOtotal oxidation tending to be lower (P = 0.07) and Fattotal oxidation tending to be higher (P = 0.08) in PT compared with CT. During CT, CHOendo oxidation was lower (P < 0.001) and CHOexo oxidation higher (P < 0.01) in the boys compared with the men. Compared with PT, CHOendo oxidation during the last 30 min of exercise in CT was reduced by 24.2 ± 6.3% in the boys and by 14.7 ± 3.0% in the men. However, because of within-group variability, this difference did not reach statistical significance (P = 0.2). We, therefore, examined the individual data and found a similar pattern whereby 10 out of 12 boys and 9 out of 10 men had a reduced CHOendo oxidation in CT compared with PT. The percent reduction for this subgroup of subjects who spared CHOendo was 32.5 ± 3.2% in the boys and 16.5 ± 2.5% in the men (P < 0.01). Similar to the CHOendo oxidation data, within-group variability for Fattotal oxidation during CT precluded a statistically significant (P = 0.5) difference in Fattotal sparing between boys (10.5 ± 8.4%) and men (-4.8 ± 23.0%). Examination of the individual data revealed that Fattotal oxidation was unchanged in 1 boy and 2 men but was reduced in 7 out of 12 boys and 5 out of 10 men in CT compared with PT. There was no difference between the boys and men who reduced Fattotal oxidation during CT (pooled average = 33.7 ± 6.3%). CHOexo oxidation increased over time (P < 0.001) during CT in both groups but was significantly higher (P < 0.01) in the boys at all time points (Table 3). The total amount of CHOexo oxidized over the last 30 min of exercise was 9.2 ± 0.6 and 15.5 ± 1.2 g in the boys and men, respectively (P < 0.001). To determine the oxidation efficiency of the ingested CHOexo, the rate of CHOexo oxidation was divided by the CHOexo ingestion rate and expressed as a percentage. The oxidation efficiency was 36.8 ± 2.0% in the boys vs. 26.0 ± 2.1% in the men (P < 0.01).

                              
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  		Table 3.   Substrate utilization during last 30 min of exercise in PT and CT for boys and men

To account for intergroup differences in energy expenditure, the percentage of total energy provided from CHOendo, CHOexo, and Fattotal oxidation was calculated. The relative contributions of CHOendo, CHOexo, and Fattotal oxidation over the last 30 min of exercise in both trials are shown in Fig. 4. During PT, the contribution of Fattotal oxidation to total energy increased significantly (P < 0.001) from 32.3 ± 2.3% at 30 min to 37.9 ± 2.6% at 60 min of exercise in the boys and from 15.0 ± 1.8 to 21.6 ± 2.4%, respectively, in the men. The average energy contribution from Fattotal was significantly higher (P < 0.001) in the boys (35.5 ± 2.3%) than in the men (19.0 ± 1.8%). During CT, the percent contribution from Fattotal did not change over time in the boys and increased slightly (13.6 ± 2.2 to 18.9 ± 1.7%) in the men (P = 0.08). However, the average percent contribution remained significantly higher (P < 0.001) in the boys (30.5 ± 2.6%) than in the men (16.7 ± 1.8%). During PT, the percent energy contribution of CHOtotal decreased over time (P < 0.05) from 67.7 ± 2.3 to 62.1 ± 2.6% in the boys and from 85.0 ± 1.8 to 78.4 ± 2.4% in the men. The average percentage of energy provided was 64.5 ± 2.2% in the boys and 81.1 ± 1.8% in the men (P < 0.001). During CT, the contribution of CHOtotal remained stable in the boys at 69.5 ± 2.7% but decreased slightly in the men (86.4 ± 2.2 to 81.1 ± 1.7%, P = 0.08). The percentage of energy derived from CHOendo during CT decreased over time (P < 0.001) from 54.1 ± 3.1 to 42.6 ± 3.3% in the boys and from 77.5 ± 2.5 to 61.5 ± 2.2% in the men. CHOendo provided, on average, 47.7 ± 3.1% of the total energy in the boys and 68.8 ± 2.2% in the men (P < 0.001). The contribution of CHOexo oxidation to total energy metabolism increased over time (P < 0.001) in the boys (15.7 ± 1.3 to 26.5 ± 1.4%) and in the men (9.0 ± 0.6 to 19.6 ± 1.4%), but the average contribution was significantly higher (P < 0.001) in the boys (21.8 ± 1.4%) than in the men (14.6 ± 0.9%).


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  		Fig. 4.   Percentage of energy contribution from substrate during last 30 min of exercise in placebo (A and C) and carbohydrate (B and D) trials for boys (A and B) and men (C and D). CHOtotal, total carbohydrate; CHOexo, exogenous carbohydrate; CHOendo, endogenous carbohydrate; Fattotal, total fat. See RESULTS for statistical differences.

Glucose and lactate. Plasma glucose ([glucose]) and lactate concentration ([lactate]) are shown in Table 4. Preexercise [glucose], averaged across trials, was slightly lower in the boys (5.99 ± 0.18 mmol/l) than in the men (6.67 ± 0.39 mmol/l), but this difference did not reach statistical significance (P = 0.10). Regardless of group, the postexercise [glucose] was lower (P < 0.001) in PT than in CT but was not different between groups in either trial.

                              
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  		Table 4.   Glucose and lactate responses to exercise in placebo and carbohydrate trials for boys and men

There were no differences in preexercise [lactate] between groups or trials. The increase in [lactate] postexercise in CT was not different from PT for men, but it tended to be higher (P = 0.06) in CT than in PT for the boys. Postexercise [lactate], averaged across trials, was lower (P < 0.001) in the boys (1.78 ± 0.16 mmol/l) than in the men (3.81 ± 0.51 mmol/l).


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Previous investigations have shown that children utilize proportionally more fat and less CHO compared with adults during exercise performed at the same relative intensity (19, 20). Our data support these findings in that, compared with the men, the boys utilized ~70% more fat and ~23% less CHO during exercise performed without CHOexo (Table 3). We also extend previous findings by showing that this pattern of fuel preference is maintained when exercise is performed with CHOexo feeding (Table 3, Fig. 4). The main finding, however, is that the oxidation rate of CHOexo, relative to BW, is considerably higher (~37%) in young boys compared with adult men during exercise performed at ~70% VO2 peak (Table 3). Consequently, the relative contribution of CHOexo oxidation to total energy metabolism is also considerably higher in boys (~22%) compared with men (~15%, Fig. 4).

Two hypotheses may help explain the preferential use of fat as fuel during submaximal exercise in young children compared with adults. First, children may have a relatively higher endogenous fat oxidation due to a higher intramuscular triglyceride (IMTG) availability compared with adults (7). This is supported by a recent study (38) of adult women in whom a higher resting concentration of IMTG resulted in a greater utilization of this endogenous fuel during subsequent submaximal exercise. Alternatively, the higher fat oxidation in boys may have been a default mechanism due to an underdeveloped glycogenolytic and/or glycolytic system. However, because of ethical limitations in measuring muscle enzyme activity and endogenous substrate concentrations, innovative approaches will be required to resolve the above issues in the pediatric population.

That children rely more on fat than CHO during exercise, compared with adults, parallels the observation that adult women also utilize proportionally more fat during exercise than do adult men (16, 40). Although the mechanisms responsible for gender differences in substrate metabolism in adults are debatable (9), higher concentrations of IMTG have been implicated (38). Interestingly, higher rates of CHOexo oxidation have also been reported for women compared with men, despite their higher relative utilization of fat (23, 27). Therefore, future studies should include both boys and girls to determine possible gender differences in substrate metabolism among children.

This study is the first to show that Fattotal oxidation remains considerably higher (~88%) in boys than in men, even during CHOexo feeding (Table 3). This higher rate of fat oxidation, despite an increase in plasma glucose availability, has previously been shown in trained vs. sedentary adults (10, 18). However, the most novel finding of our study is that the rate of CHOexo oxidation, relative to BW, is significantly higher (~37%) in young boys compared with adult men (Table 3). The relative CHOexo oxidation rate of our men (~0.19 g/kg) is comparable to previous findings for untrained adults, ~0.20 g/kg (29) and ~0.23 g/kg (10), exercising at similar intensities as in the present study. The rate of CHOexo oxidation in our boys (~0.26 g/kg) is similar to values previously reported for trained (10) adults but is higher than for untrained (10, 29) adults. This value is also higher than our laboratory's previous findings of ~0.17 g/kg for boys aged 11-14 yr (33) and ~0.20 g/kg for boys aged 14-17 yr (31). However, the exercise intensities in the latter studies (31, 33) were lower (60 and 55% VO2 peak, respectively) than in the present study (70% VO2 peak). As a result of their high rates of CHOexo oxidation, the relative provision of CHOexo to total energy expenditure was also significantly greater (~50%) in the boys compared with the men (Fig. 4). The value for our men (~15%) is higher than the ~8% reported by Massicotte et al. (21) but is similar to other investigations where CHOexo contributed ~14% (22) and ~18% (10) to total energy expenditure in untrained adults. The percent contribution of CHOexo to total energy in our boys (~22%) is higher than the ~15% calculated for 11- to 14-yr-old boys (33) and the ~18% for 13- to 17-yr-old boys (31) in our laboratory's previously published studies. It may be, therefore, that the relative utilization of CHOexo to meet energy demands during exercise depends on age and/or pubertal status.

Puberty is associated with a period of insulin resistance (2), and insulin-stimulated glucose disposal at rest appears to be higher in prepubertal children than in pubescents and in adults (2, 5). Interestingly, insulin-stimulated translocation of glucose transporters in skeletal muscle tissue of rats appears to be maturation dependent (6, 14, 15). It may be that recruitment of the insulin-sensitive GLUT-4 protein is also higher during exercise in children compared with adults, which may help explain our observation of a higher rate of CHOexo oxidation during exercise in pre- and early pubertal boys, compared with adult men. Therefore, on the basis of our laboratory's earlier observations (31-33) and the present findings (Table 3, Fig. 4), we propose that the utilization of CHOexo as an energy source during exercise depends on maturity status, although this possibility requires further study.

Another potential explanation for the higher CHOexo oxidation rates in our boys is that the rate of intestinal absorption of the ingested CHOexo might have been greater in the boys, thus resulting in more glucose being delivered to the muscle. Because we are unaware of the rate of intestinal absorption of CHO under similar experimental conditions, this option cannot be completely discounted. However, previous studies comparing oral 13C bicarbonate dynamics between children and adults have suggested no differences in the absorption of this tracer at rest (4) or during exercise (41). Therefore, future studies utilizing isotope tracer solutions and measuring isotopic enrichment of plasma and other body fluids will be needed to elucidate possible age- or maturity-related differences in the absorption of ingested beverages during exercise.

In summary, we have shown for the first time that healthy young boys oxidize relatively more CHOexo during 60 min of exercise than do healthy young men, under identical experimental conditions. Moreover, the boys maintained higher relative rates of Fattotal oxidation compared with the men, even when fed CHOexo. Lastly, the higher rate of CHOexo oxidation in the boys contributed proportionally more to the total energy expended. The boys' preference for this exogenous fuel may result in an improved exercise performance and sparing of endogenous substrate, necessary for growth and development.


    ACKNOWLEDGEMENTS

We thank Marta Kubacki for outstanding assistance with data collection and Melanie Wolfe for technical assistance in measuring 13C/12C of expired CO2 gas samples, and we especially thank our subjects who displayed exceptional patience and commitment.


    FOOTNOTES

This study was supported, in part, by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Gatorade Sports Science Institute (GSSI). B. W. Timmons is a recipient of an Industrial NSERC Scholarship sponsored by the GSSI.

Address for reprint requests and other correspondence: M. C. Riddell, Dept. of Kinesiology and Health Science, York Univ., 4700 Keele St., Toronto, Ontario, Canada M3J 1P3 (E-mail: mriddell{at}yorku.ca).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

September 13, 2002;10.1152/japplphysiol.00140.2002

Received 25 February 2002; accepted in final form 6 September 2002.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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