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1 Children's Exercise and Nutrition Centre and 2 Department of Medicine, McMaster University, Hamilton, Ontario, Canada L8N 3Z5
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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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 O2 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 13CO2, 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 (~4.5 mmol/l) trials but were elevated by ~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 ~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 ~25 and 40%, respectively, after 90 min of moderate-intensity exercise.
children; adolescents; carbohydrate
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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IN ADULTS, EXOGENOUS
CARBOHYDRATE (CHOexo) ingestion just before, or
during, exercise improves performance and delays fatigue (5). CHOexo is thought to limit
fatigue by maintaining high rates of total carbohydrate
(CHOtot) oxidation, sparing endogenous glycogen stores, and
elevating blood glucose concentrations late in exercise (4,
30). With the use of 13C labeling, the oxidation
rates of a variety of CHOexo sources (e.g., glucose and
glucose polymers, fructose, sucrose, maltodextrins, and cornstarch)
have been reported for various exercise intensities [see
Péronnet et al. (33) for a review]. The most common
form of CHOexo investigated is exogenous glucose (G), which
has been reported to be oxidized at a peak rate of ~1.0-1.2
g/min during prolonged, high-intensity [~60-70% peak
O2 uptake (
O2 peak)] exercise (14, 41). On average, glucose oxidation provides somewhere between 7 and 9% of total energy provision during 120 min of
moderate- to high-intensity exercise (24) and appears to
be the main substrate utilized during the later stages of endurance exercise (26, 27).
In contrast to G during exercise, exogenous fructose (F) has a delayed rate of intestinal absorption (35) and often causes gastrointestinal distress during exercise (29). In addition, compared with G, F has a lower rate of oxidation during exercise (13, 17, 25, 26), possibly as a result of its slower absorption rate and the necessity for its conversion to glucose by the liver before oxidation (17). The combination of F and G (FG), however, is well absorbed during exercise (12) and may facilitate a higher oxidation rate than either of the two monosaccharides ingested separately (1). The reason for the higher rate of oxidation during exercise after FG ingestion is currently unknown, but it may be related to enhanced intestinal absorption through the activation of additional transport mechanisms (7, 12, 36).
Less is known about CHOexo utilization during exercise in
children and adolescents. During exercise performed in a fasting state,
children seem to have a lower respiratory exchange ratio (RER) than
adults, indicating that they oxidize more endogenous fat and less
carbohydrate (CHO) (21-23). In addition, preexercise CHO snacks (i.e., fig or candy bar) do not appear to alter RER or
increase exercise time to exhaustion in adolescent boys
(15). In contrast, our laboratory has shown that
intermittent feedings of glucose (~1.4 g glucose · kg body
mass
1 · h1) during exercise
increase blood glucose and insulin levels while suppressing plasma free
fatty acid and glycerol release in healthy adolescent boys
(38). In addition, by using
13C-labeled glucose, our laboratory found that this feeding
pattern reduces endogenous CHO (CHOendo) and lipid
utilization, lowers the rating of perceived exertion (RPE)
(38a) and can contribute up to 25% of the total energy
provision during prolonged exercise in healthy adolescent boys
(37, 38).
Although G and F are commonly found in natural foods and sport beverages and are frequently consumed by youth, no studies exist that have examined substrate utilization or performance during exercise performed with FG intake in this segment of the population. The primary purpose of this study, therefore, was to compare the oxidation rates of G with FG during prolonged exercise in healthy, untrained adolescent boys. On the basis of the previous observations in adults (1), we hypothesized that FG oxidation would be higher than G oxidation in adolescent boys. In addition, we also investigated endogenous substrate sparing, metabolic responses, and exercise performance during exercise in these subjects when they ingested W, G, or FG.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Subjects.
Twelve 11- to 14-yr-old boys volunteered through local public service
announcements. Table 1 shows their
anthropometric and functional characteristics. All subjects
were healthy, nonobese, and habitually active but were not competitive
athletes. The purpose, nature, and possible risks of the experiment
were explained to the boys and their parent(s). Subjects gave a verbal
agreement to participate, and a parent then signed an informed consent. The Research Ethics Board of the Faculty of Health Sciences, McMaster University, approved the study.
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Pretesting.
Height, weight, percent body fat by bioimpedance, and
O2 peak were measured during a
preliminary session. O2 peakwas determined for each subject during an "all-out" progressive
exercise test on an electronically braked cycle ergometer (Corival 400, Lode, Gronigen, Netherlands), each stage lasting 2 min
(2). Measurements of O2 uptake
(O2) and CO2 production
(CO2) were made continuously using a
Quinton metabolic cart (Quinton Q-plex 1, Quinton Instrument, Seattle,
WA) and averaged over the final 30 s of each workload.
O2 peak was considered to have been
reached if at least two of the following criteria were met: heart
rate (HR) within 10 beats/min of age-predicted maximum, RER 1.0, and
leveling of O2 with increasing intensity
or volitional exhaustion (subject unable to maintain cadence above 60 rpm for 5 consecutive seconds, despite encouragement by the investigator).
O2
and power output during the test, a least squares regression curve was
generated using Statistica for Windows (StatSoft, Tulsa, OK), taking
the average O2 during the final 30 s of each stage for each power output. The corresponding power output
for 55% O2 peak was determined from
this curve. Peak power was determined as the final power output
generated during the last 2-min stage of the test. Partial completion
of the final stage was credited using the method of Bar-Or
(2). HR was measured throughout the test using a Sports
Tester PE3000 system (Polar Electro, Kempele, Finland).
Experimental trials.
Each subject attended three experimental trials spaced 1-2 wk
apart. Trials were identical except for postbreakfast fluid and CHO
intake, and the order was counterbalanced among the subjects. Subjects
were blinded to the content of drinks in each trial. During the trials,
subjects drank either water (W trial), 6% glucose (G trial), or 3%
glucose plus 3% fructose (FG trial) beverages intermittently for a
total of 25 ml/kg body mass. All three beverages were grape flavored
and contained 18 mmol/l NaCl. The W was also artificially sweetened
with Aspartame. Each experimental trial consisted of three
30-min bouts of cycling at 55% of their predicted O2 peak, separated by 5-min
rest periods. After the last bout, a 10-min rest was provided that was
followed by an all-out performance ride to volitional exhaustion at
90% of their predetermined peak power. Exhaustion was considered to
have been achieved when the subject could no longer maintain a cycling
cadence of 60 rpm for 10 s despite encouragement from an investigator.
Protocol.
Subjects were asked to eat their usual meals but refrain from consuming
corn and corn-derived food items during the study period to reduce the
amount of naturally enriched [13C]glycogen in muscle and
liver. In addition, they were asked to avoid excessive physical
activity the day before a trial. They arrived fasted to the laboratory
on the morning of the trial (~0800). Breakfast was provided
by the investigators and consisted of one slice of white bread,
toasted, with one-half tablespoon peanut butter and 100 ml orange
juice. After breakfast, an indwelling venous catheter was inserted into
an arm vein for subsequent blood sampling. The start of the first bout
of exercise (time = 0 min) on the cycle ergometer occurred ~90
min after the start of breakfast. Subjects were instructed to ingest
the provided beverages within 30 s at 30 and 15 min before the
start of the first exercise bout and at 0, 15, and 30 min during each
bout (for a total of 9 times). The glucose in the G and FG trials was
derived from corn (BDH-Chemical, Toronto, ON) and artificially enriched
with uniformly labeled [13C]glucose (99 atom %excess,
Isotec, Miamisburg, OH) to an isotopic composition of +47.729 delta per
1,000 difference (
) vs. the 13C-to-12C ratio from the international
standard 13C Pee Dee Beleminitella-1 (PDB-1; i.e., +47.729
[
-13C]PDB-1), as measured by dual-inlet isotope
ratio mass spectrometry (VG-Sira 10, series II, Manchester,
UK). The F in the FG beverage was derived from corn and
enriched with uniformly labeled [13C]fructose (99 atom
%excess, Isotec, Miamisburg, OH) to an isotopic composition of +48.011
[-13C]PDB-1. The resulting enrichment of the FG
beverage was +47.888 [-13C]PDB-1. These high levels
of enrichment, compared with that of normal expired gas (Fig.
1), provide a strong measurement signal and reduce the error associated with a small shift in the isotopic composition of CO2 arising from the oxidation of endogenous
substrates during exercise (24).
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Respiratory gas and substrate oxidation.
Resting O2 and
CO2 were determined with subjects
breathing into a mouth piece connected to the metabolic cart while
sitting quietly in a chair during a 5-min collection period at 
40
min. In addition, O2 and
CO2 were determined during exercise from 5-min sampling periods at 5 and 25 min in each bout. CHOtot
and total fat (fattot) oxidation rates were calculated at
each time point from RER and O2 averaged
over the collection period using a table of nonprotein respiratory
quotients (34). During gas sampling, 10-ml expired gas
samples were drawn from Douglas bags connected to the exhaust port of
the metabolic cart and stored in vacutainer tubes for subsequent
determination of 13C/12C in expired
CO2 and CHOexo oxidation. The isotopic
composition of CO2 in expired gas samples was determined
using an isotope ratio mass spectrometer (BreathMat Plus, Finnigan MAT,
Bremen, Germany) and expressed in [-13C]PDB-1.
](/content/vol90/issue3/fulltext/903/img001.gif)
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CO2 is in liters per minute
STPD, Rexp is the isotopic composition of
expired CO2 during exercise, Rref is the
isotopic composition of expired CO2 at rest
before CHOexo ingestion, Rexo is the isotopic
composition of the CHOexo beverage, and k
(0.7426 l/g) is the volume of CO2 provided by the complete
oxidation of glucose (34). This method of determining
CHOexo oxidation assumes that 13CO2
recovery in expired gas during exercise is complete or almost complete
(6, 16, 19), although there is a delay in recovery due to
the large labile bicarbonate pool (31). This delay in 13CO2 recovery appears to be less in children,
however (43). In North American studies, this method has
been shown to overestimate the actual exogenous substrate oxidation,
because of shifts in the isotopic composition of endogenous substrates
caused by exercise (24). This methodological limitation
can be avoided, as in this study protocol, if CHOexo
enrichment is several magnitudes higher than the natural
13C enrichment of foods found in North American diets
(32). Finally, CHOendo was determined from the
following equation
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Blood variables
Ten of the twelve subjects volunteered to give venous blood samples
during the trials. From these volunteers, whole blood samples were
drawn from an indwelling catheter inserted into a forearm vein into
heparinized syringes at 40 min (baseline) and at 0, 2, 5, 10, 20, 30, 65, 90, and 100 min, as well as immediately after the performance test.
Each sample was centrifuged at 15,900 g for 2 min, and the
plasma supernatant was stored at 20°C and subsequently analyzed for
glucose and lactate (model 2300 Select Analyzer, Yellow Springs
Instruments, Yellow Springs, OH) and insulin (Coat-A-Count
radioimmunoassay, DPC Diagnostics) concentrations.
HR, RPE, and stomach fullness scale.
HR was monitored continuously throughout the
O2 peak test and the
experimental trials using a Sports Tester PE3000 system (Polar Electro,
Kempele, Finland). During the experimental trials, 30-s HR averages
were determined at rest immediately before the ingestion of the first
dose of CHOexo and at 5 and 25 min in each bout. Before
exercise, Borg's 6-20 RPE category scale (3) was
introduced to the subjects by using a set of standardized instructions
modified for children by Bar-Or (2). This scale was used
to rate whole body perceived exertion at 5 and 25 min in each bout.
Stomach fullness was also assessed at 5 and 25 min in each bout using a
five-point category scale that included the following categories:
1) not full at all, 2) somewhat full,
3) full, 4) very full, and 5)
extremely full.
Statistical analyses. Data are presented as means ± SE. For measurements taken repeatedly during the trials, a two-way ANOVA was used. Tukey's honest significant difference post hoc test for equal cell size was used to determine significance among mean values. Comparisons between performance times among the trials were also made using the Wilcoxon matched-pairs test. Significance was set at P < 0.05 for all statistical tests.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Two subjects were unable to complete the third bout of exercise during the W trial. All subjects completed the three exercise bouts in G and FG trials. For measurement taken during all three trials (i.e., repeated-measures ANOVA), statistical operations were performed for the 10 subjects who completed them. For visual comparisons, figures and tables are shown, however, with n = 12 subjects, except for the measurements made in the final bout of the W trial.
Subjects successfully consumed the provided beverages in the allotted time periods and did not complain of gastric upset. CHOexo intake was identical during the G and FG trials, averaging 67.3 ± 15.4 g in each trial.
Preexercise.
Resting HR (Fig. 2, top) and
RER (Fig. 2, bottom); plasma insulin, glucose, and
lactate (see Fig. 5); and expired 13CO2 PDB-1 (Fig. 1, top) values were similar among the trials, averaging 82 ± 3 beats/min, 0.85 ± 0.001, and 23.7 ± 0.41 [-13C]PDB-1, respectively.
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Prolonged exercise.
Work rate was identical during exercise during all time points and in
all three trials, averaging 50 ± 4.48 W. O2, CO2, and expired minute ventilation (E) during exercise
are given in Table 2. No
differences in O2,
CO2, or E were found either between trials or among time points. On average,
subjects exercised at 53 ± 1.17%
O2 peak during the
trials.
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-13C/PDB values during the trials
are shown in Fig. 1, top. During the W trial,
expired 13CO2 increased slightly, but
significantly, from 23.52 ± 0.325 [-13C]PDB-1 at rest to a maximum of 22.23 ± 0.282 [-13C]PDB-1 at 60 min and then decreased
slightly, but significantly, to 22.48 ± 0.231 [-13C]PDB-1 of exercise [time effect;
F = 8.81; degrees of freedom (df) = 6,54, P < 0.001]. In the G and FG trials, expired
13CO2 values were 23.89 ± 0.416 [-13C]PDB-1 and 23.66 ± 0.500 [-13C]PDB-1 at rest (not significantly different),
increasing to 3.51 ± 0.836 and 5.26 ± 0.939 [-13C]PDB-1 at 90 min of exercise (not
significantly different), respectively.
CHOexo oxidation rates during G and FG are shown in Fig. 1,
bottom. CHOexo oxidation rates were similar
between trials at 10 min of exercise and increased throughout exercise
to maximal rates of 0.36 ± 0.032 and 0.31 ± 0.030 g/min at
90 min of exercise in the G and FG trials, respectively (trial-by-time
interaction; F = 2.45; df = 5,55;
P = 0.04). CHOexo oxidation during the
entire 90 min of exercise (i.e., area under the curves in Fig. 1)
averaged 0.24 ± 0.020 and 0.22 ± 0.022 g/min in the G and
FG trials, respectively (group effect; F = 2.05;
df = 1,11; P = 0.18).
HR levels are shown in Fig. 2, top. HR increased with
exercise in all trials (time effect; F = 5.07; df = 5,45; P < 0.001) and was slightly higher in the G
trial than in W and FG trials (trial effect; F = 6.83;
df = 2,18; P = 0.01). RER values as shown in Fig.
2, bottom, decreased in all trials (time effect;
F = 35.14; df = 5,45; P < 0.001)
and were lower in the W than in the G and FG trials (trial effect;
F = 6.03; df = 2,18; P = 0.010).
RER values in the G trial did not differ significantly from the FG trial.
Substrate utilization rates at the various measurement time points are
shown in Table 3.
Fattot oxidation increased during exercise in all three
trials (time effect; F = 21.4; df = 5,45, P < 0.001) but was higher in the W trial (0.28 ± 0.023 g/min) than in the G trial (0.24 ± 0.023 g/min) and the FG
trial (0.25 ± 0.029 g/min) (trial effect; F = 3.9; df = 2,18; P = 0.04). No difference in
fattot oxidation was found between the G and FG trials.
CHOtot oxidation decreased during exercise in all three trials (time effect; F = 18.2; df = 5,45;
P < 0.001) but was lower in the W trial (0.63 ± 0.05 g/min) than in the G trial (0.78 ± 0.051 g/min) and the FG
trial (0.74 ± 0.056 g/min) (trial effect; F = 6.0; df = 2,18, P = 0.009). No difference in
CHOtot oxidation was found between the G and FG trials.
CHOendo oxidation decreased with exercise in all trials
(time effect; F = 47.4; df = 5,45; P < 0.001) and tended to be higher in the W trial
(0.63 ± 0.048 g/min) than in the G trial (0.55 ± 0.04 g/min) and FG (0.52 ± 0.05 g/min) (group effect;
F = 1.98; df = 2,18; P = 0.17).
Post hoc analysis indicated that CHOendo oxidation was
significantly lower by 95 min in the G trial (P = 0.04)
and in the FG trial (P = 0.03) than in the W trial. No
differences in CHOendo oxidation was found between the G
and FG trials.
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Performance test.
Figure 6 shows individual exercise time
to exhaustion during the performance test in each trial. Performance
times averaged 142 ± 37, 177 ± 33, and 202 ± 40 s in the W, G, and FG trials, respectively (trial effect;
F = 3.2; df = 2,22; P = 0.059).
Post hoc analysis indicates that the performance time in the W trial was significantly less than in the FG trial (P = 0.049)
but not significantly different from the G trial (P = 0.29). Seven of twelve subjects had higher performance times in the G
than in W trial (Wilcoxon matched-pair test; P = 0.19),
and 9 of the 12 had higher times in the FG than in the W trial
(Wilcoxon matched-pair test; P = 0.03).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The main finding of this study is that G and FG are oxidized at a similar rate during prolonged moderate-intensity exercise, contributing to ~16% of the total energy provision in boys ages 11-14 yr (Figs. 1 and 3, Table 3). We also found that both beverages similarly spare endogenous fat and CHO by ~17 and ~14%, respectively in these subjects (Fig. 3, Table 3). Finally, compared with flavored W, the ingestion of FG delays exhaustion at 90% peak power by ~40% after 90 min of moderate-intensity exercise (Fig. 6).
Our finding that FG ingested intermittently during exercise is oxidized at a similar rate in adolescents compared with G contrasts with the previous study in adults by Adopo et al. (1). Indeed, we found that FG oxidation appears to be slightly, but significantly, lower than G oxidation by the end of 90 min of moderate-intensity intermittent exercise in boys (Fig. 1).
The reason for the contrasting findings between our study and the
previous experiment by Adopo et al. (1) is currently unclear but may be related to subject selection or study design. In
both studies, subjects performed exercise at a similar relative intensity (55 vs. 60% O2 peak) and for
a similar duration (90 vs. 120 min) after a standardized breakfast. The
exercise protocols differed somewhat, in that ours consisted of
intermittent, rather than continuous, cycling. It seems unlikely,
however, that the 5-min rest periods given in our study would alter the
responses to one form of CHOexo but not the other. It is
possible, however, that the contradictory findings between studies may
be explained by differences in subjects' age or maturation level. In
general, our subjects were pre- and early pubescent and untrained,
whereas in the Adopo et al. study, subjects were trained adults.
Further investigation is necessary, therefore, to determine whether
various CHOexo beverages have altered absorption or
oxidation rates in children or adolescents compared with adults.
More important differences in study protocol that may explain the contrasting findings are the concentration and the timing of the CHOexo beverages. In the Adopo et al. (1) study, subjects consumed a 20% CHOexo solution at the onset of exercise, whereas, in our study, subjects drank 6% CHOexo solutions intermittently during exercise. It is possible that the high G concentration in their study saturated intestinal glucose transport, thereby potentially limiting its oxidation rate. In contrast, the ingestion of FG in their study, however, was thought by Adopo et al. to stimulate absorption of the CHOexo through additional transport mechanisms recruited by the addition of F, as has been shown previously (39). In other words, the lower oxidation rate of G relative to FG oxidation, in their study, may have been explained by a saturation of glucose transport mechanisms in the G trial that may not have occurred during the FG trial. Indeed, increasing CHO concentration from 8 to 25% markedly decreases glycemic responses in healthy adults, likely because of reduced CHO absorption (9). In our laboratory's study, subjects ingested 6% CHOexo, 18 mmol/l NaCl solutions that have been shown to have high rates of intestinal absorption compared with more concentrated CHO beverages (40). It is unlikely, therefore, that intestinal absorption was limiting in either of the CHOexo trials in our study. Thus FG oxidation may only exceed G oxidation if the CHOexo is provided in a concentrated bolus at the onset of exercise. Further investigation is required, however, to test this hypothesis.
Another important difference between our study and the previous study by Adopo et al. (1) is the timing of CHOexo ingestion. It may be that the bolus ingestion of CHOexo at the onset of exercise in the study by Adopo et al. allowed for sufficient time for the absorbed F to be converted to G by the liver before its oxidation. This conversion of F to G appears to be obligatory before oxidation by muscle when F is ingested during exercise (17). In our study, the ingestion of FG during the exercise may not have allowed sufficient time for the F to be converted to G before its oxidation, which may explain why FG oxidation was slightly impaired during exercise in our study. This hypothesis appears unlikely, however, because rates of oxidation of the two CHOexo beverages were similar for the initial 60 min of exercise (Fig. 1).
Plasma glucose levels decreased after the onset of exercise for a brief period regardless of the beverages that were consumed (Fig. 5). This pattern, which has also been observed previously by Delamarche et al. (11) in 8- to 11-yr-old boys and girls, is thought to indicate some impairment in the glucoregulatory response to exercise in youth. Interestingly, the 11% decrease in glycemia during the initial 20 min of exercise, which was also observed previously by Delamarche et al., was not attenuated by CHOexo in our study (Fig. 5). After this hypoglycemic response, levels returned to baseline in W and in FG trials and were elevated by ~ 1 mmol/l above baseline during the G trial. These data appear to agree with our laboratory's previous observation that blood glucose levels are higher during exercise with G vs. W ingestion in healthy adolescent boys (38) and boys with insulin-dependent diabetes mellitus (37).
In addition to a blunted glucose response to FG, we found that the plasma insulin levels were twofold higher during exercise with G than with FG or W (Fig. 5). A blunted insulin response to F compared with G intake has previously been observed in adults during exercise (18, 20). We extend these findings, for children and adolescents, by demonstrating that the insulin response to FG is considerably less than an isocaloric intake of G alone. Indeed, the failure of FG to elevate plasma glucose above baseline may help to explain why plasma insulin concentrations were considerably lower in the FG than in the G trial, even though the amount of CHOexo consumed was identical between the two trials. In addition, the observation that insulin levels increase dramatically after G intake in our study may support the previous hypothesis that, compared with adults, children and adolescents may have a decreased insulin sensitivity, which is usually compensated by a greater insulin secretion (43).
Plasma lactate concentrations increased at the onset of exercise during the initial 20 min, after which time they decreased gradually during exercise (Fig. 5). A decrease in lactate levels during prolonged moderate-intensity exercise has previously been shown by our laboratory (38) and others (15) in adolescents; however, it is currently unclear whether this indicates a reduction in lactate production or an increase in lactate clearance.
Compared with W ingestion, FG appears to increase the exercise time to volitional exhaustion at 90% peak power after 90 min of prolonged moderate exercise (Fig. 5). In our study, the exercise time to exhaustion was longer with FG intake (202 ± 40 s) than with W intake (142 ± 37 s) (P = 0.049), whereas the performance with G ingestion (177 ± 33 s) was not statistically significantly different from that with W intake (P = 0.29). These observations for adolescent boys are in line with others who show that, compared with W or G, F intake can postpone fatigue in exercising adults (for review, see Ref. 8) Evidence for improved high-intensity cycling performance with 6% sucrose (i.e., a disaccharide composed of G and F) compared with 6% F alone (29) or placebo (10) also supports the notion of an ergogenic effect of FG. The mechanism for the enhanced performance with FG is currently unclear but may be related to the muscle glycogen-sparing effect of F compared with either G or W intake during exercise (20). Had a muscle glycogen-sparing effect with FG ingestion occurred in our study, one might expect that CHOendo oxidation would be lower in that trial compared with the G trial. However, we found no difference in CHOendo oxidation (i.e., liver and muscle glycogen) between the G and FG trials (Fig. 3) that would support a difference in muscle or liver glycogen sparing between the two trials. It is possible that the relative proportions of muscle and liver glycogen to CHOendo may be different among the trials, which we are unable to assess with the methods used in the present study.
Although our experiment was not designed to compare substrate
utilization in younger children with older adolescents, some interesting observations should be pointed out. Previously, our laboratory found that the percent energy contribution from fat was
~30% in older boys (ages 14-17 yr) during exercise performed at
a moderate intensity (~60% O2 peak)
with W intake (37, 38). In the present study, fat
contribution was ~50% during exercise with W intake (Fig. 3) and at
a similar relative intensity (~55%
O2 peak) in younger boys (ages
10-14 yr). This finding supports previous studies that have shown
that children use more fat and less carbohydrate compared with adults during exercise performed at the same relative intensity
(21-23). In contrast to this difference in
endogenous substrate utilization between younger and older adolescents,
we found that the percent energy contribution from CHOexo
was similar between younger (17 ± 0.8%) and older (19 ± 0.8%) boys during 90 min of similar relative intensity exercise
(38). Thus it appears that the proportion of
CHOexo utilized during exercise is relatively constant
among children and adolescents but may be considerably higher than the 7-9% previously reported for adults in a similarly designed
experiment (24). The observed differences in
CHOexo oxidation between children and adolescents in our
studies compared with those conducted with adult subjects should be
viewed with caution because of potential differences in the type,
timing, and amount of CHO ingested; the duration and intensity of
exercise performed; as well as the computation procedure used to
calculate CHOexo oxidation (32, 33). Further investigation using standardized methodology is required, therefore, to
probe the influences of maturation on endogenous and exogenous fuel
utilization during exercise.
In summary, G and F plus G are oxidized at a similar rate in adolescent boys if beverages are consumed intermittently during exercise. In addition, both forms of CHOexo spare endogenous fat and CHOendo to a similar extent and contribute to ~16% of the total energy provision during moderate exercise in boys ages 10-14 yr.
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ACKNOWLEDGEMENTS |
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This work was supported by the Gatorade Sports Science Institute, the Medical Research Council of Canada, and the Natural Science and Engineering Council of Canada.
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FOOTNOTES |
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Address for reprint requests and other correspondence: O. Bar-Or, Children's Exercise and Nutrition Centre, McMaster Univ., Chedoke Hospital Division, Evel Bldg., 4th Fl., PO Box 2000, Sanatorium Road, Hamilton, ON, Canada, L8N 3Z5 (E-mail: baror{at}mcmaster.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.
Received 10 April 2000; accepted in final form 20 September 2000.
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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