Pre-exercise carbohydrate meal and endurance running capacity when carbohydrates are ingested during exercise.
ABSTRACT This study examined whether combining a pre-exercise carbohydrate meal with the ingestion of a carbohydrate-electrolyte solution during exercise is better in improving endurance running capacity than a carbohydrate-electrolyte solution alone. Ten men completed three treadmill runs at 70% VO2max to exhaustion. They consumed 1.) a carbohydrate meal three hours before exercise and a carbohydrate-electrolyte solution during exercise (M + C), or 2.) a liquid placebo three hours before exercise and the carbohydrate-electrolyte solution during exercise (P + C), or 3.) a placebo three hours before exercise and placebo during exercise (P + P). When the meal was consumed (M + C) serum insulin concentrations were higher at the start of exercise, and carbohydrate oxidation rates were higher during the first 60 min of exercise compared with the values found in the P + C and P + P trials (p < 0.01). Exercise time was longer in the M + C (147.4+/-9.6 min) compared with the P + C (125.3+/-7 min) (p < 0.01). Also, exercise time was longer in M + C and P + C compared with the P + P (115.1+/-7.6 min) (p < 0.01 and p < 0.05 respectively). These results indicate that the combination of a pre-exercise carbohydrate meal and a carbohydrate-electrolyte solution further improves endurance running capacity than the carbohydrate-electrolyte solution alone.
- SourceAvailable from: Shiou‐Liang Wee[Show abstract] [Hide abstract]
ABSTRACT: Influence of high and low glycemic index meals on endurance running capacity. Med Purpose: The purpose of this study was to examine the effect of high and low glycemic index (GI) carbohydrate (CHO) pre-exercise meals on endurance running capacity. Methods: Eight active subjects (five male and three female) ran on a treadmill at ~70% [latin capital V with dot above]O 2max to exhaustion on two occasions separated by 7 d. Three hours before the run after an overnight fast, each subject was given in a single-blind, random order, isoenergetic meal of 850 ± 21 kcal (mean ± SEM; 67% carbohydrate, 30% protein, and 3% fat) containing either high (HGI) or low (LGI) GI carbohydrate foods providing 2.0 g CHO·kg -1 body weight. Results: Ingestion of the HGI meal resulted in a 580% and 330% greater incremental area under the 3-h blood glucose and serum insulin response curves, respectively. Performance times were not different between the HGI and LGI trials (113 ± 4 min and 111 ± 5 min, respectively). During the first 80 min of exercise in the LGI trial, CHO oxidation was 12% lower and fat oxidation was 118% higher than in the HGI trial. Although serum insulin concentrations did not differ between trials, blood glucose at 20 min into exercise in the HGI trial was lower than that during the LGI trial at the same time (3.6 ± 0.3 mmol·L -1 vs 4.3 ± 0.3 mmol·L -1 ; P < 0.05). During exercise, plasma glycerol and serum free fatty acid concentrations were lower in the HGI trial than in the LGI trial. Conclusions: This results demonstrate that although there is a relative shift in substrate utilization from CHO to fat when a low GI meal is ingested before exercise compared with that for a high GI meal, there is no difference in endurance running capacity. Acarbohydrate (CHO) meal ingested 3-4 h before exercise can increase liver (20) and muscle glycogen concentrations (9) as well as provide an absorbable source of CHO as it empties from the stomach (31). Pre-exercise CHO meals also affect the metabolic response and substrate utilization during exercise. Different methods have been used to study the influence of pre-exercise feeding on energy metabolism during exercise. These include using different monosaccharides, whole foods with different GI values, foods that are processed differently, and the addition of other macro-nutrients to a CHO source. A few studies have considered the GI of foods when studying the effect of pre-exercise ingestion of CHO meals (10,14,24,25). Despite differences in the metabolism of fast and slowly digested starches, the benefits from eating starch on endurance performance remain unclear. To provide high and low GI CHO meals, Thomas and Febbraio (10,24,25) used lentils and potatoes in their studies. The amount of potatoes and lentils consumed was calculated to provide 1 g CHO·kg -1 body weight (BW). However, in lentils there is significantly more protein than in potatoes. From values provided by Thomas et al. (25) the protein and energy content of the lentil meal was 208% and 36% higher, respectively, than in the potato meal. Therefore, their preexercise meals were not isoenergetic nor of the same macronutrient composition. Because of the energy differences between the test meals, the results of these studies should be interpreted with caution. Furthermore, it is not known how much of the enhanced insulinemic, depressed glycemic, and other metabolic response can be attributed to the increased protein content of the lentil meal.Medicine and science in sports and exercise 01/1999; 31:393-399. · 4.48 Impact Factor
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ABSTRACT: In the last decade, research has begun to investigate the efficacy of carbohydrate supplementation for improving aspects of physical capacity and skill performance during sport-specific exercise in adolescent team games players. This research remains in its infancy, and further study would be beneficial considering the large youth population actively involved in team games. Literature on the influence of carbohydrate supplementation on skill performance is scarce, limited to shooting accuracy in adolescent basketball players and conflicting in its findings. Between-study differences in the exercise protocol, volume of fluid and carbohydrate consumed, use of prior fatiguing exercise and timing of skill tests may contribute to the different findings. Conversely, initial data supports carbohydrate supplementation in solution and gel form for improving intermittent endurance running capacity following soccer-specific shuttle running. These studies produced reliable data, but were subject to limitations including lack of quantification of the metabolic response of participants, limited generalization of data due to narrow participant age and maturation ranges, use of males and females within the same sample and non-standardized pre-exercise nutritional status between participants. There is a lack of consensus regarding the influence of frequently consuming carbohydrate-containing products on tooth enamel erosion and the development of obesity or being overweight in adolescent athletes and non-athletes. These discrepancies mean that the initiation or exacerbation of health issues due to frequent consumption of carbohydrate-containing products by adolescents cannot be conclusively refuted. Coupled with the knowledge that consuming a natural, high-carbohydrate diet ∼3–8 hours before exercise can significantly alter substrate use and improve exercise performance in adults, a moral and ethical concern is raised regarding the direction of future research in order to further knowledge while safeguarding the health and well-being of young participants. It could be deemed unethical to continue study into carbohydrate supplementation while ignoring the potential health concerns and the possibility of generating similar performance enhancements using natural dietary interventions. Therefore, future work should investigate the influence of pre-exercise dietary intake on the prolonged intermittent, high-intensity exercise performance of adolescents. This would enable quantification of whether pre-exercise nutrition can modulate exercise performance and, if so, the optimum dietary composition to achieve this. Research could then combine this knowledge with ingestion of carbohydrate-containing products during exercise to facilitate ethical and healthy nutritional guidelines for enhancing the exercise performance of adolescents. This article addresses the available evidence regarding carbohydrate supplementation and prolonged intermittent, high-intensity exercise in adolescent team games players. It discusses the potential health concerns associated with the frequent use of carbohydrate-containing products by adolescents and how this affects the research ethics of the field, and considers directions for future work.Sports Medicine 10/2012; 42(10). · 5.32 Impact Factor
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ABSTRACT: utilization during subsequent exercise muscle glycogen storage at rest but augments its Ingestion of a high-glycemic index meal increases You might find this additional information useful...2/707#BIBL 1 other HighWire hosted article: This article has been cited by [PDF] [Full Text] [Abstract] , August 1, 2006; 84 (2): 354-360. Am. J. utilization during subsequent exercise in women Influence of high-carbohydrate mixed meals with different glycemic indexes on substrate including high-resolution figures, can be found at: Updated information and services http://jap.physiology.org/cgi/content/full/99/2/707 can be found at: Journal of Applied Physiology about Additional material and information http://www.the-aps.org/publications/jappl This information is current as of August 7, 2006 .. those papers emphasizing adaptive and integrative mechanisms. It is published 12 times a year (monthly) by the American publishes original papers that deal with diverse areas of research in applied physiology, especiallyJournal of Applied Physiology 01/2005; 99(7):707-714. · 3.43 Impact Factor
Pre-Exercise Carbohydrate Meal and Endurance Running Capacity
when Carbohydrates are Ingested During Exercise
C. Chryssanthopoulos, C. Williams
Department of Physical Education. Sports Science and Recreation Management, Loughborough University. Loughborough. England
C. Chryssanthopoulos, C . Williams. Pre-Exercise Carbohydrate
Meal and Endurance Running Capacity when Carbohydrates
are Ingested During Exercise. Int. 1. Sports Med.. Vol. 18,
pp. 543 - 548,1997.
This study examined whether combining a pre-exerctse
carbohydrate meal with the ingestion of a carbohydrate-elec-
trolyte solution during exercise is better in improving endur-
ance running capacity than a carbohydrate-electrolyte solution
alone. Ten men completed three treadmill runs at 70% ~0,max
to exhaustion. They consumed 1.) a carbohydrate meal three
hours before exercise and a carbohydrate-electrolyte solution
during exercise (M + C), or 2.) a llquid placebo three hours be-
fore exercise and the carbohydrate-electrolyte solution during
exercise (P+ C), or 3.) a placebo three hours before exercise
and placebo during exercise (P + P). When the meal was con-
sumed (M + C) serum insulin concentrations were higher at the
start of exercise, and carbohydrate oxidation rates were higher
during the first 60min of exercise compared with the values
found in the P+ C and P + P trials (p ~0.01). Exercise time was
longer in the M + C (147.4 f 9.6 rnin) compared with the P + C
(125.3 + 7 min) (p < 0.01). Also, exercise time was longer in
M + C and P + C compared with the P + P (115.1 f 7.6 min)
(p < 0.01 and p < 0.05 respectively). These results indicate that
the combination of a pre-exercise carbohydrate meal and a car-
bohydrate-electrolyte solution further improves endurance run-
ning capacity than the carbohydrate-electrolyte solution alone.
Key words: Pre-exercise meal. carbohydrate-electrolyte solu-
tion, running capacity
Consuming a carbohydrate meal 3-4 hours before prolonged
exercise improves cycling performance (21.29) and running
capacity (1) compared to an overnight fast. Furthermore, the
combination of a pre-exercise meal and a carbohydrate-elec-
trolyte solution during exercise improves endurance running
capacity to a greater extent than a pre-exercise meal alone
(1). However, pre-exercise feeding elevates serum insulin con-
centration before exercise (1.5.29). accelerates carbohydrate
oxidation (1,7,14,15,27,29). and depresses the blood born FFA
concentration (1,5,7,29). Collectively, all these metabolic per-
turbations may not be the most favourable to the endurance
athlete, since they may accelerate the use of the limited mus-
cle glycogen stores (5,22).
Ingesting carbohydrates during exercise delays the onset of fa-
tigue in cycling (4.6,8.29), as well as in running (16.20.24-
26). Also. the ingestion of carbohydrates during exercise does
not alter the carbohydrate oxidation rates, at least during the
first hour of exercise (126.96.36.199-26). and spares muscle glyco-
gen stores during running (23,24).
Therefore, the aim of the present study was to examine whe-
ther the ingestion of carbohydrates before and during exercise
would improve endurance running capacity to a greater extent
than simply ingesting carbohydrates during exercise.
Ten male reaeational/club level runners volunteered to take
part in this study. Their age, height, body weight. \io,rnax,
and maximum heart rate were 34.9 ? 2.5 years, 174.5 * 2.9 cm,
72.4 r 3.6 kg, 58.6 f 1 . 9 ml x k g x min-I. and 186 24 b x min-I
respectively (mean 2 SE). Nine subjects completed all three
trials, and one completed only two trials. All subjects were
fully jnformed about the nature of the experiments and what
was required of them before giving formal consent. The study
had the approval of the Ethical Advisory Committee of Lough-
After subjects became familiar with treadmill running and ex-
perimental procedures, they performed two preliminary tests:
a) a 16-rnin incremental submaximal running test to deter-
mine the relationship between running speed and oxygen
b) an uphill treadmill running test to determine each subject's
maximum oxygen uptake.
Int. 1. Sports Med. 18 (1997) 543-548
O Georg Thieme Verlag Stuttgart . New York
Int.]. Sports Med. 18 (1997)
C. Chryssonthopoulos. C . Williams
All preliminary tests were conducted according to the proce-
dures previously described (28). Subjects also undertook a
60 min treadmill run at 70% ~0,max, about one week before
the first experimental trial in order to become fully familiar-
ised with the drinking pattern and the measurements used
during the main trails.
Subjects were required to record training and to weigh their
food intake during the two days prior to the first trial, and to
replicate these in the next two trials. The dietary information
obtained was then analyzed (18). There were no significant dif-
ferences between the three trials in the average daily energy
intake, or composition in terms of carbohydrate, fat, or protein
consumed during the two days prior to each trial. Subjects
were also required to avoid training the day before each main
Each subject was required to run to fatigue at 70%irOzmaxon a
motorised level treadmill (Quinton, Seattle, USA) on three dif-
ferent occasions separated by one week. On the first occasion a
high carbohydrate meal was eaten three hours before exercise,
and during exercise a carbohydrate-electrolyte solution was
ingested (M + C). On the second and third trials 10 ml x kg1
B W of a liquid placebo were ingested three hours before exer-
cise (P + C and P+ P). During exercise, in the second trial sub-
jects ingested a carbohydrate-electrolyte solution (P+C),
whereas in the third trial a liquid placebo was administered
(P+ P). The order of the three trials was random. Since solid
food was used before exercise a double blind design was im-
possible. In order to make the desjgn single blind subjects
were told that the purpose of the experiment was to compare
solid and liquid carbohydrate food in different combinations
and concentrations before and/or duringexercise and that dur-
ing the three trials they ingested the same amount of carbohy-
drate by adjusting the concentration of the ljquid parts (i.e. li-
quid placebo. or orange juice) of the meals.
After a 12-hour overnight fast subjects arrived at the labora-
tory at 8 : 00 a.m. Duplicate 20 p1 capillary blood samples were
taken from the thumb of a pre-warmed hand after they had
been sitting quietly for 15 min. Following this, subjects ingest-
ed either the liquid placebo (P +C and P+ P), or the high carbo-
hydrate meal (M +C). During the 3-hour postprandial period
subjects remained in the laboratory reading and writing, or
were involved with low intensity physical activities (e.g. at-
tending lectures, doing office work etc.) outside the laboratory.
These activities were very similar in all experimental trials. Be-
fore the initiation of exercise, each subject's nude body weight
was recorded. Thereafter. subjects were seated and a venous
blood sample was taken from an antecubital vein as well as an-
other capillary blood sample from a pre-warmed hand. Sub-
jects warmed up for 5 rnin on the treadmill at 60% W,max.
Following this. the speed of the treadmill was increased to
70% m2max and the subjects continued running until exhaus-
tion. They were encouraged to run as long as possible. How-
ever, when they were unable to maintain their prescribed run-
ning speed, and in order to ensure that fatigue had occurred,
the speed was reduced to 60% ~0,max for 2 min. Thereafter.
the speed was returned to the prescribed speed and the sub-
jects were encouraged to continue for as long as possible.
One minute expired air samples and duplicate 2 0 ~ 1
samples were collected at 10min and 20min into exercise
and every 20 min thereafter. Expired air and capillary blood
samples were also collected at the last minute of the run. Also.
each subject's rate of perceived exertion (RPE) was obtained
using the Borg's scale. Furthermore, two additional scales were
used, one to monitor the subject's abdominal discomfort (AD)
and the other was used to assess their sensation of gut fullness
(CF). Both scales ranged from 0 (AD: "Completely comforta-
ble"; GF: "Empty") up to 10 (AD: "Unbearable pain": GF:
"Bloated"). Heart rate was monitored throughout exercise by
short-range telemetry (Polar Electro Sports Tester P E 3000).
Wet sponges were available for the subjects to use ad libitum
throughout each of the runs, Immediately after subjects stop-
ped exercise, a further venous sample was taken while they
were seated: thereafter subjects dried themselves, and post-
exercise nude body weight was recorded.
All trials were conducted under similar laboratory conditions:
temperatures: M + C: 19.8 + 0.8 "C, P + C: 20.3 + 0.6 "C, and P + P:
20.1 ? 0.8 "C; relative humidity: M + C: 59.6 r 2.8 %. P + C:
53.3 r 2.4%, and P+ P: 51.9 &2.3% (mean +SE; n.s.).
Three hours before each run subjects consumed either
10 m x kg-' B W of a liquid placebo (dilute sugar free orange
juice; less than I kcal per 700 ml undiluted) (P+ C. and P + P
trials), or the high carbohydrate meal (M + C) (2.5 g carbohy-
drates per kg BW). The pre-exercise meal (M + C) consisted of
white bread, jam, cornflakes, sugar, skimmed milk and orange
juice which amounted to 86% of energy intake from carbohy-
drate, 11 % from protein and less than 3% from fat. About 88% of
the carbohydrate included in the meal was obtained from food
classified as having a high glycaemic index. whereas the rest
was obtained from food with moderate and low glycaemic in-
dex (11). During exercise in the M + C and P + C trials an isoton-
ic lemon and lime carbohydrate-electrolyte solution was in-
gested, whereas in the P + P trial equivalent amount of a liquid
placebo (dilute sugar free lemon and lime juice) was given. The
carbohydrate-electrolyte solution was a commercially avail-
able sports drink (Lucozade Sport) which contained 6.9% car-
bohydrate (dextrose, maltodextrin and glucose syrup) and four
electrolytes (24 mmol x I- ' sodium. 2.5 mmol x I-' potassium.
1.2 mmol x I-
calcium, and 0.8 mmol x 1-
mediately prior to the start of exercise subjects ingested
5 ml x k g BW of the carbohydrate solution (M + C and P + C
trials) or equivalent amount of placebo (P+ P), and 2 ml x kg-'
BW of the assigned fluid every 20min thereafter. The total
amount of carbohydrate ingested in the M+C trial was
259 k 13.8 g (181 ? 9 g as meal and 78 + 6 g during exercise),
whereas in the P + C trial was 70? 4 g.
The method of collection and analysis of expired air samples
was the same as previousfy described (28). Venous blood sam-
ples were collected into lithium heparin and serum tubes.
Blood glucose, blood lactate, haemoglobin, haematocrit, per-
centage changes in plasma volume, plasma FFA, and plasma
Pre-Exercise Carbohydrate Meal and Endurance Running Capacity
glycerol, were measured as previously described (28). Serum
sodium and potassium were anaIysed by flame photometry
(Coming 435 flame photometer). Serum samples were also
stored at - 70°C and analysed at a later date for serum insulin
radioimmunoassay; Coat-A-Count Insulin. DPC kit) using
a gamma counter (Packard. Cobra 5000).
A two-way analysis of variance (ANOVA) for repeated meas-
ures on two factors (treatment by time) was used to compare
cardiovascular changes, blood glucose. and blood lactate re-
sponses between trials. The remaining responses were exam-
ined using a two tailed Student's t-test for dependent samples.
When significant differences were revealed using the ANOVA,
a Tukey post hoc test was performed. The accepted level of sig-
nificance was set at p < 0,05. Data are reported as mean f SE.
Exercise time was longer in the M + C (147.4 + 9.6 rnin; n =9)
and P + C (125.3 + 7.0 min: n = 10) trials compared with the
P+ P (115.1 r7.6 min; n= 10) trial (p < 0.01 and p< 0.05 respec-
tively). Also, exercise time was longer in the M +C compared
with the P + C trial (p < 0.07 ). No difference was found. how-
ever, when exercise time to exhaustion was analysed by order
(TI : 126.3 + 10.8 min vs T2: 129.8 29.3 min vs T3: 130.0f
6.5 min; n.s.).
Oxygen uptake ( ~ 0 ~ ) .
averaged 41.1 r 1.6 ml x k g x min-I (2.9 f 0.2 1 x min- I). 41.5 ?
1.2 ml x k g x min-I (2.9 f 0.2 I x min- I), and 41.8 * 1.7 rnl x
trjals respectively (n.s.). The average% ~ 0 ~ m a x
ing the McC, P+C, and P+P trials were 69.8 k0.6%.
70.5? 0.6%. and 71.0 +0.8%, respectively (n.s.).
was similar between the three trials and
in the M+C. P+C. and P+P
Respiratory exchange ratio (RER), was higher (p < 0.01) in the
M + C trial compared with the P + C, and P+ P trials during the
first hour of exercise (Fig.l), reflecting a higher carbohydrate
oxidation rate (Fig. 2) (p < 0.01).
No differences were found in the heart rate responses between
trials (Mean: M + C: 155 & 5 b x min- ' vs P+ C: 158 f 5 bx min- '
vs P + P: 158 2 5 b x min I ) . However, heart rates were higher
(p <0.01) in all trials at exhaustion compared with the heart
rates during the first hour of exercise. Rate of perceived exer-
tion increased from 10.7k 0.4 at lOmin of exercise to
17.7 + 0.4 at exhaustion in all trials (p< 0.01). At 80 min of exer-
cise RPE was lower in the M+C (12.7f0.6) and P+C
(13.1 k0.7) trials compared with P+P (14.350.7) trial
(p < 0.01 and p < 0.05 respectively). No difference was ob-
served, however, in the GF and AD responses between the
The volume of fluid consumed during exercise was
1137 k 90 ml, 1015 + 60 ml, and 968 ? 70 ml in the M +C. P + C,
and P + P trials respectively. These values were significantly
different only between the M +C and P+ P trials ( p i 0.05;
n = 9). The average decrease in body mass, as a result of exer-
cise, was 3.31 k 0.27 kg. 2.82 k 0.25 kg, and 2.60 50.25 kg in
M + C, P + C. and P + P trials respectively. These values were cor-
rected for the fluid ingested during running. These decreases
Int.]. Sports Med. 18 (1997)
Fig.1 Respiratory exchange ratio (RER) during the M + C, P + C. and
P + P trials (mean f SE; n = 9)
pc0.01 fromP+Cand P t P ; " p<0.05fromM+C
- b P I C
M a t
Flg. 2 Carbohydrate oxidation rate (g x min-') during the M + C, P + C,
and P + P trials (mean? SE; n = 9)
p<0.01 fromP+CandP+P;" p<0.01 frornP+P
represented body weight changes of 4.7 ? 0.4%. 3.9 a 0.3%. and
3.6?0.4% respectively. The decrease in body weight as
expressed both in kg and percent change (%) of body weight,
was higher in the M+C trial compared with the P+C
(p < 0.05, n = 9). and P + P (p < 0.01. n = 9) trials. However. no
difference was observed between the three trials in the mean
change of plasma volume (M +C: - 8.3 r 1.9% vs P+ C: - 7.5 ?
1.1 % vs P+ P: - 9.3 k 1.8%).
Blood glucose concentration (Fig. 3) was higher at exhaustion
in the M + C trial compared with the P+ P trial ( p i 0.05). How-
ever, in the P+ P trial blood glucose concentration at exhaus-
tion (4.2 kO.2 mmol x 1- I ) was not different from the blood glu-
cose concentration at the start of exercise (Omin: 4.62
0.1 mmol x I- I). Also, blood glucose concentrarion tended to
be higher (p = 0.05) in the P + C trial at 10 min, at 60 min of
exercise compared with the P+ P and M + C, and P+ P trials,
Blood lactate concentrations were similar between trials
throughout exercise and averaged 3.2 * 0.3 mmol x I-',
Int. 1. Sports Med. 18 (1997)
C. Chryssonthopoulos, C . Williams
Table 1 Serum insulin (mu x I-'), plasma FFA, plasma glycerol, serum sodium and serum potassium concentrations (mmol x I-') before and after
exercise in the M + C. P + C, and P + P trials (mean ? SE; n = 9)
P + C
P + C
M + C
0.1 lCf 0.01
1 42d k 0.5
4.2d * 0.1
4.3a-c + 0.5
0.36aJ + 0.04
0.08b.c ir 0.01
2.7 f 0.6
0.67 f 0.1 1
0.38 f 0.06
143 k 0.6
1.9 ? 0.3
0.58b f 0.07
142 + 0.6
a: p<0.01 from M + C: b: p c 0.05 from M +C; c: pc0.01 from post exercise; d: pc0.05 from port exercise: e: p c 0.05 from P+ P
4 0 '
(M + C) compared with the ingestion of the carbohydrate-elec-
trolyte solution (P + C) alone. Furthermore. the 6.9% carbohy-
drate-electrolyte drink (P + C) produced a smaller (9%) but sig-
nificant improvement in endurance capacity compared to con-
trol conditions (P + P). However. the biggest improvement
(28 %) over control (P + P), was achieved when the pre-exercise
meal was combined with the ingestion of the carbohydrate-
electrolyte solution (M + C) during exercise.
Various studies have shown an improved endurance capacity
during cycling (6.8.29). and running (16,20,25,26), as a result
of carbohydrate ingestion during exercise. It has been suggest-
ed that the main contribution of exogenous carbohydrate is to
maintain blood glucose concentrations and a high rate of car-
bohydrate oxidation by the working muscle during endurance
cycling exercise (6,8). This is because during cycling, fatigue in
the fasted state has been associated with a decline in blood
glucose concentration, and the rate of carbohydrate oxidation
(4,6,29). In the present study the respiratory exchange ratios,
reflecting the rate of carbohydrate oxidation, were not signifi-
cantly reduced wjth time in the control trial (P+ P), although
the non ~tah'Sti~a1ly significant decline observed towards the
end of exercise (Fig. 2) may hold some physiological significance.
Furthermore, there was no significant reduction in blood glu-
cose concentration at exhaustion compared with the start of
exercise (0 min: 4.6 mmol x I-' vs Exh: 4.2 mmol x I- I). When
running is the mode of exercise blood glucose concentrations
and carbohydrate oxidation rates do not decrease with time
to the same extent as during cycling (19.24-26).
Fig. 3 Blood glucose concentration
and P + P trials (mean i:SE: n = 9)
a p=O.OSfromM+Cand P+P;
p = 0.05 from P + P;
D < 0.05 from M + C
3.4 k0.5 mmol x I-', and 3.1 f0.5 mmol r I-' in the M + C, P + C,
and P + P trials respectively.
At the start of exercise serum insulin concentration (Table 1) in
the M + C trial was about 3.2 fold and 4.4fold higher compared
with the P + C and P + P trials respectively (p < 0.01).
Plasma FFA concentrations (Table I) were lower (p<0.01) at
the beginning of exercise in the M + C trial compared with the
P + C and P + P trials. In addition, post-exercise plasma FFA con-
centrations were lower (p < 0.05) when carbohydrates were in-
gested during exercise (M + C, and P + C) compared with place-
bo ingestion (P + P). Plasma glycerol concentrations (Table 1)
were also lower in the M + C condition compared with the
P+ C (p < 0.05) and P t P (p c 0.01) conditions. Finally, pre-exer-
cise serum sodium concentration was found to be higher
(p<0.05) three hours after the consumption of the meal
(m + C), whereas serum potassium concentration was not dif-
ferent between the three conditions (Table 1).
The main finding of this study was an 18% improvement in en-
durance running capacity as a result of combining the carbo-
hydrate meal with the carbohydrate-electrolyte solution
Nevertheless, some authors have suggested that carbohydrate
intake during exercise may reduce the rate of muscle glycogen
utilisation and spare the limited muscle glycogen stores (9.23.
24). Recent studies have shown a reduced rate of glycogen
breakdown in Type I muscle fibers when carbohydrates are in-
gested during exercise compared with either water (23) or pla-
cebo (24) ingestion. Furthermore, the ingestion of carbohy-
drates during exercise may markedly reduce the hepatic glu-
cose production at 70% ~0,rnax (13).
The improvement in endurance capacity in the M + C trial is
consistent with other studies in which pre-exercise carbohy-
drate feedings were combined with carbohydrate ingestion
during exercise. These improvements in endurance capacity
were greater than the improvements when carbohydrate was
ingested only during exercise (1,15,29). A possible influence
on running capacity in the M + C trial could have been a higher
liver (17), and/or muscle carbohydrate stores (7) before the
Pre-Exercise Carbohydrate Meal and Endurance Running Capacity Int. 1. Sports Med. 18 (1997)
start of exercise. We have shown that this carbohydrate meal
increases glycogen concentration of the vastus lateralis muscle
by 11 % (2). However. the possibility that some portion of the
meal was still in the stomach or small intestine when exercise
begun, continuing to provide some substrate to the working
muscles during exercise, cannot be excluded. This suggestion
is supported by the higher plasma insulin levels before exer-
cise in the M + C trial compared to the P + C and P + P trials. This
observation seems to suggest that three hours after the inges-
tion of rhe meal (M + C) the process of digestionlabsorption
was still in progress.
Although insulin concentrations decrease rapidly when subjects
start to exercise its efict onfuel utilisation remains during the
exercise period (7). Elevated insulin concentrations at the start
of exercise may reduce the availability of FFA to the working
muscles, and increase the rate of muscle glycogen utilisation
(5), which may lead to premature fatigue (10). Nevertheless,
despite the elevated carbohydrate metabolism in the M+C
trial endurance capacity was improved compared to P + C and
P + P trials. It seems that in the M +C trial the meal increased
the pre-exercise insulin concentration, facilitating in this way
the uptake of the carbohydrate from the carbohydrate-electro-
lyte drink by the exercising muscle. The availability of this exo-
genous carbohydrate source to the muscle probably off-set the
elevated rate of carbohydrate oxidation.
During thefirst hour ofexercise the subjects oxidised about 46g
of carbohydra te more in the M + C trial compared to the carbohy-
drates oxidised following an overnight fast (PtC, P+P). This
amount is only equivalent to about 25%ojthe total carbohydrate
load consumed (about 180g) in the pre-exercise meal (M + C).
Therefore, it can be argued that the total pre-exercise carbohy-
drate load was more advantageous than the metabolic disad-
vantage of an occelemted carbohydmte oxidation rate.
The post-exercise FFA concentrations were lower when the
carbohydrate drink was ingested during exercise (P+C) com-
pared with the ingestion of placebo (P+ P). This is a common
observation which suggests a depression of F A mobilisation
when carbohydrates are ingested during exercise (3). However.
since the rates of fat oxidation were similar between the P+C
and P + P trials, as reflected by the RER values, it can be argued
that either the possible depression o j FFA mobiiisation in the
P+ C was not strong enough to deny this substrate to the working
muscle. or an alternative source of fat, such as intramuscular
triglycerides, was probably used (12).
In summary, the result of this study confirmed theobservation
of many investigators that a carbohydrate-electrolyte solution
ingested during exercise can improve endurance running ca-
pacity (16.20.24-26). Furthermore, the combination of both
a pre-exercise carbohydrate meal, providing 2.5g carbohy-
drate per I<g BW, and a carbohydrate-electrolyte solution in-
gested during exercise further improves endurance running
capacity. despite an elevated carbohydrate oxidation rate dur-
ing the first hour of exercise.
The authors wish to thank Dr. Alan Nevill, Department of
Sports Sciences. Liverpool John Moores University, England,
for his assistance in the statistical analyses of the results, and
Mrs. Maria Nute for her help during the experimental trials.
This study was supported by SmithKlineBeecham.
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C. Chrvssan thopoo/os. C. Williams
Professor Clyde Williams PhD
Department of Physical Education
Sports Science and Recreation Management
Leicestershire, LEI13 TU
Telephone (office): +01509-228186
Telephone (home): +01509-214151