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ARTICLE The performance effect of early versus late carbohydrate feedings during prolonged exercise Matthew William Sinclair Heesch, Molly Elizabeth Mieras, and Dustin Russel Slivka
Abstract: The purpose of this study was to determine how the timing of isoenergetic carbohydrate feedings during prolonged cycling affects performance in a subsequent 10-km cycling time trial. Recreationally trained male cyclists (n = 8; age, 34.5 ± 8.3 years; mass, 80.0 ± 6.3 kg; body fat, 16.0% ± 3.8%, peak oxygen uptake, 4.54 ± 0.42 L·min−1) completed 4 experimental trials consisting of cycling continuously for 2 h at 62.4% ± 1.9% of peak oxygen uptake, followed immediately by a self-paced 10-km time trial. The 4 conditions included no carbohydrate ingestion (PP), early carbohydrate ingestion (CP), late carbohydrate ingestion (PC), or carbohydrate ingestion throughout (CC). Blood samples were obtained at 0, 60, and 120 min of cycling as well as at the conclusion of the time trial. The 10-km time trial time to completion was faster in trials CC (17.70 ± 0.52 min) and PC (17.60 ± 0.62 min) as compared with trial PP (18.13 ± 0.52 min, p = 0.028 and p = 0.007, respectively) while trial CP (17.85 ± 0.58 min, p = 0.178) was not. Serum glucose increased with carbohydrate feedings (p < 0.05), while serum free fatty acid concentrations were lower in trials PC and CC than trials CP and PP (p < 0.05). There was no difference in oxygen uptake, heart rate, rating of perceived exertion, or substrate use between trials (p > 0.05). These data indicate that carbohydrate ingestion throughout or late during a 2-h cycling bout can improve subsequent 10-km time trial performance. Key words: cycling, blood glucose, free fatty acids, time trial. Résumé : Cette étude se propose de vérifier l’effet du moment de l’apport de sucre isoénergétique au cours d’un exercice prolongé a` vélo sur la performance lors d’une épreuve subséquente de 10 km contre-la-montre. Des cyclistes entraînés sur le plan récréatif (n = 8, 34,5 ± 8,3 ans, 80,0 ± 6,3 kg, 16,0 ± 3,8 % de gras, consommation d’oxygène de pointe de 4,54 ± 0,42 L·min−1) participent a` quatre essais expérimentaux consistant a` pédaler 2 h a` une intensité sollicitant 62,4 ± 1,9 % du consommation d’oxygène de pointe, puis a` effectuer selon un rythme autodéterminé l’épreuve de 10 km contre-la-montre. Les quatre conditions sont aucun apport de sucre (« PP »), apport hâtif de sucre (« CP »), apport retardé de sucre (« PC ») et apport de sucre tout au long de l’essai (« CC »). On prélève des échantillons de sang au début, a` la 60e et a` la 120e minute ainsi qu’a` la fin de l’épreuve contre-la-montre. La performance au 10 km contre-la-montre est plus rapide dans les conditions CC (17,70 ± 0,52 min) et PC (17,60 ± 0,62 min) comparativement a` la condition PP (18,13 ± 0,52 min, p = 0,028 et p = 0,007, respectivement); la performance dans la condition CP n’est pas plus rapide (17,85 ± 0,58 min, p = 0,178). La concentration sérique de glucose augmente avec l’apport de sucre (p < 0,05) et la concentration sérique des acides gras libres est plus faible dans les conditions PC et CC que dans les conditions CP et PP (p < 0,05). D’une condition a` l’autre, on n’observe aucune différence de consommation d’oxygène, de fréquence cardiaque, de perception de l’intensité de l’effort et d’utilisation de substrats (p > 0,05). D’après ces résultats, l’apport retardé de sucre ou tout au long des 2 h de l’exercice a` vélo peut contribuer a` l’amélioration de la performance a` l’épreuve subséquente de 10 km contre- la- montre. [Traduit par la Rédaction] Mots-clés : cyclisme, glucose sanguin, acides gras libres, contre-la-montre.
Introduction Ingesting carbohydrate during prolonged exercise can increase time to fatigue (Coggan and Coyle 1989) and improve time-trial performance (Febbraio et al. 2000; McConell et al. 1996; Moodley et al. 1992). However, the mechanisms of this benefit are still not completely understood (Karelis et al. 2010). Possible mechanisms include alteration of neural drive via carbohydrate sensing in the mouth (Carter et al. 2004; Chambers et al. 2009), maintenance of blood glucose levels when endogenous carbohydrate stores are exhausted (Pallikarakis et al. 1986), depression of free fatty acid (FFA) concentrations (Hargreaves et al. 1991), better maintenance of carbohydrate oxidation rates late in exercise (Massicotte et al. 1990), and sparing of endogenous carbohydrate stores (Stellingwerff et al. 2007). The timing of carbohydrate ingestion may affect these mechanisms and therefore be an important factor in attaining
performance benefits during exercise; however, the timing of carbohydrate ingestion has received relatively little attention. Ingesting carbohydrate throughout exercise improves timetrial performance after a bout of prolonged cycling; however, ingesting carbohydrate late in an extended exercise bout was shown to not significantly improve time-trial performance versus a placebo (McConell et al. 1996). However, it should be noted that ingesting carbohydrate late in exercise was also not significantly different than ingesting carbohydrate throughout, possibly because of the limited statistical power observed with only 8 participants (McConell et al. 1996). This absence of a performance benefit with carbohydrate consumption late in exercise occurred despite higher blood glucose levels, similar blood insulin levels, and a similar rate of carbohydrate oxidation compared with a trial where subjects received carbohydrate throughout. One possible explanation for this observation was that plasma FFA levels may
Received 29 January 2013. Accepted 25 June 2013. M.W.S. Heesch, M.E. Mieras, and D.R. Slivka. School of Health, Physical Education, and Recreation, University of Nebraska at Omaha, Omaha, NE 68182, USA. Corresponding author: Dustin Russel Slivka (e-mail: [email protected]). Appl. Physiol. Nutr. Metab. 39: 58–63 (2014) dx.doi.org/10.1139/apnm-2013-0034
Published at www.nrcresearchpress.com/apnm on 3 July 2013.
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have been elevated by the time carbohydrate was ingested late in exercise. Higher plasma FFA levels have been shown to inhibit glucose uptake into the muscle during moderate exercise (Hargreaves et al. 1991). Additionally, carbohydrate ingestion during exercise blunts the normally observed increase of plasma FFA during prolonged exercise (Davis et al. 1992; Deuster et al. 1992; Murray et al. 1991). Another plausible explanation for the previous findings was that the large amount of carbohydrate (157.5 g) received late in exercise (30 min prior to the performance trial) was unable to be absorbed by the gastrointestinal tract before performance measures were assessed. Absorption and oxidation rates of exogenous carbohydrate appears to plateau for a single transportable carbohydrate at 1.0 to 1.1 g·min−1 (Jeukendrup and Jentjens 2000). The benefit of carbohydrate feedings early in exercise has been demonstrated by Tsintzas et al. (1996) who reported that ingestion of a 5.5% carbohydrate–electrolyte solution during the first hour of exercise, followed by ingestion of water until exhaustion, improved time to exhaustion when compared with ingestion of water throughout the entire protocol. However, it has been suggested by others that “if carbohydrate feeding is begun during an event, it should be continued throughout the event” (Coyle 2004). Because of conflicting findings, the impact of the timing of carbohydrate ingestion during exercise remains unclear. Therefore, the purpose of this study was to identify how the timing of isoenergetic carbohydrate feedings during prolonged exercise affects performance in a time trial immediately following prolonged exercise. Based on previous findings (McConell et al. 1996; Tsintzas et al. 1996), it was hypothesized that time trial performance would be improved when carbohydrate was ingested throughout exercise, as well as when carbohydrate was only ingested during the early stages of exercise, but that performance would not be improved when carbohydrate was only ingested during the late stages of exercise.
Materials and methods Participants Participants were recruited from the local community and were healthy males between the ages of 19 and 45 years who cycled for exercise at least 3 times per week, and had at least 1 cycling bout in excess of 2 h within the past 2 weeks. All participants reviewed and signed an Institutional Review Board approved Informed Consent document. Aerobic capacity Peak oxygen uptake (V˙O2peak) was assessed to measure aerobic capacity and to establish cycling intensity for the prolonged cycling trials. Participants rode on their own bicycle mounted on a Computrainer electronically braked cycle trainer ergometer (Racermate, Seattle, Wash., USA). The test began at a workload of 95 W, and the workload was increased by 35 W every 3 min until volitional fatigue. Power at V˙O2peak (Wmax) was calculated by adding the power output of the last completed stage to the proportion of time in the final stage multiplied by 35 W. Expired gases were analyzed using a flow and gas concentration calibrated TrueOne 2400 metabolic cart (ParvoMedics, Sandy, Utah, USA). A familiarization with the 10-km performance trial was conducted approximately 30 min after the V˙O2peak test. Body composition Body composition was assessed by hydrostatic weighing using an electronic load cell-based system (Exertech, Dresbach, Minn., USA) corrected for estimated residual lung volume. Residual lung volume was estimated by the Exertech software using the formula previously described by Quanjer et al. (1993). Body density from hydrostatic weighing was converted to percent body fat using the Siri equation (Siri 1993).
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Experimental trials Participants performed 4 separate trials, with no fewer than 3 days between each. The average washout period was 6 ± 2 days. All trials were completed within 28 days of the first. In each trial, participants cycled continuously at 60% of the Wmax for 2 h, followed immediately by a 10-km cycling performance trial, on their own bicycle mounted on a Computrainer ergometer (Racermate). The time trial was a simulated flat, straight, 10-km course. Participants started each time trial in the same gear, and were allowed to shift at their discretion throughout the time trial. Trials were performed in a randomized, counterbalanced order, and there was no evidence of an order effect between trials (p = 0.672). In the control condition (PP), participants received 250 mL of an artificially sweetened placebo beverage every 15 min, starting immediately upon the start of exercise, for the duration of the 2-h cycling bout. In a second condition (CC), participants received 250 mL of a 3% carbohydrate solution every 15 min for the duration of the 2-h cycling bout. In a third condition (CP), participants received 250 mL of a 6% carbohydrate solution every 15 min during the first hour of cycling, followed by 250 mL of an artificially sweetened placebo beverage every 15 min during the second hour of cycling. In the fourth condition (PC), participants received 250 mL of an artificially sweetened placebo beverage every 15 min during the first hour of cycling, followed by 250 mL of a 6% carbohydrate solution every 15 min during the second hour of cycling. All 3 trials in which carbohydrate was given were isoenergetic, and the same amount of fluid was ingested in each trial. Maltodextrin was used to prepare the beverages from raw ingredients by the investigators. Crystal Light (Kraft Foods Inc.; sweetened with aspartame) was used as the artificial sweetener in the placebo beverage, as well as added to all beverages to mask flavor. All testing was performed in the same laboratory, where temperature was kept consistent between 21–22 °C, and humidity was between 24%–43% for all trials. The pressure between the wheel of the bicycle and the ergometer was kept consistent between all trials for each individual participant (within 0.05 lbs (0.02 kg)), and was always between 2.70 and 3.00 lbs (1.2 and 1.4 kg) for all participants. For each trail, participants reported to the lab after an overnight fast, having been instructed not to consume any alcohol, caffeine, or tobacco and to avoid strenuous exercise for the previous 24 h. Prior to the participants arriving at the lab for their first trial, they were instructed to carefully record their self-selected diet over the previous 24 h to establish a diet to repeat in the 24 h prior to all other trials. Participants self-reported adherence to these instructions through the use of dietary and exercise logs. Metabolic gasses Metabolic gasses were collected during the last 5 min of each 30 min during the 2 h of cycling. The gas samples for the last 3 min of this collection were averaged and used to report exercise intensity and calculate substrate use utilizing the formula previously reported (Jeukendrup and Wallis 2005). Metabolic gasses were also collected continuously during the 10-km performance trial. All gas samples were collected using a calibrated TrueOne 2400 metabolic cart (ParvoMedics). Heart rate was monitored throughout the protocol using a Polar Heart Rate Monitor (Polar, Lake Success, N.Y., USA), and averaged over 15-min intervals. RPE Subjects were asked to report rating of perceived exertion (RPE) according to the 15-point Borg Scale (Borg 1973) every 15 min during the prolonged cycling trials, and at 5 km and the conclusion of the 10-km performance trials. RPE was averaged for each hour of the 2-h cycling bout. Blood sampling and analysis Five milliliters of blood was drawn from an antecubital vein before the start of the each prolonged cycling trial, at 60 min, Published by NRC Research Press
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120 min, and at the conclusion of the 10-km performance time trial to determine serum concentrations of glucose, insulin, lactate, and FFAs. Blood samples were obtained using BD Vacutainer tubes with clot activator. Samples were allowed to clot for 30 min, then spun for 15 min at 2000g. Serum was separated and frozen at −80 °C for later analysis. Serum glucose concentrations were assessed using Infinity Glucose reagent (Thermo Scientific, Middletown, Va., USA), read on a Nanodrop 2000c spectrophotometer (Thermo Scientific), and calculated using the extinction coefficient for NADH. Serum lactate concentrations were assessed using a 0.32-mol·L–1 glycine, 0.32-mol·L–1 hydrazine hydrate, 2.4-mmol·L–1 NAD, and 8-U·mL–1 solution (all reagents from Thermo Scientific), read on a Nanodrop 2000c spectrophotometer and calculated using the extinction coefficient for NADH. Serum insulin concentrations were assessed using an ELISA kit from Calbiotech (Spring Valley, Calif., USA) according to manufacturer instructions. Serum FFA concentrations were assessed using a commercially available ELISA kit from EIAab (Wuhan, China) according to manufacturer instructions.
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Fig. 1. (A) Ten-kilometer performance trial time to completion. a, p < 0.05 from PP. Values are means ± SE. (B) Individual 10-km performance trial time to completion. PP, placebo control; CC, carbohydrate ingestion throughout exercise; CP, carbohydrate ingestion early only; PC, carbohydrate ingestion late only.
Statistical analysis Serum glucose, insulin, FFAs, lactate, substrate use, RPE, and heart rate during the 2-h prolonged cycling bout were analyzed with a repeated measures 2-way (trial × time) ANOVA. The 10-km cycling time trial performance, oxygen uptake (V˙O2), heart rate, and RPE during the 10-km performance trial were analyzed using a 1-way (trial) repeated measures ANOVA. When a significant F ratio was found, Fishers protected least significant difference method was used to detect where differences occurred (Hayter 1986; Seaman et al. 1991). A probability of type I error of less than 5% was considered significant (p < 0.05). Based on previously published data using similar methods (McConell et al. 1996), an effect size of 0.47 was observed in performance trials between carbohydrate supplementation and placebo control conditions. If a similar effect size were observed in the present study (approximately a 30 s difference in 10-km performance trials), a statistical power of 0.86 would be obtained with 8 participants.
Results Participants Eight (n = 8) recreationally trained male cyclists (age, 34.5 ± 8.3 years; height, 179.5 ± 9.3 cm; weight, 80.0 ± 6.3 kg; body composition, 16.0% ± 3.8% body fat; V˙O2peak, 4.54 ± 0.42 L·min−1; and Wmax, 312.0 ± 43.2 W) completed the testing associated with this investigation. Performance The 10-km performance trial time to completion was faster in trials CC (p = 0.028) and PC (p = 0.007) as compared with trial PP, while trial CP was not different from trial PP (p = 0.178, Fig. 1). There were no significant differences in average power output between trials (p = 0.067, Table 1), although means reflected time to completion results, as trials with higher average power output also yielded faster 10-km performance trials. There were no differences between performance trials for V˙O2 (p = 0.222), HR (p = 0.568), or RPE (p = 0.347, Table 1). V˙O2, HR, RPE, substrate use During the prolonged cycling trials at 60% of Wmax (62.4% ± 1.9% V˙O2peak, 181 ± 24 W), V˙O2 (p = 0.244), HR (p = 0.190), RPE (p = 0.703), and fat oxidation (p = 0.130) were not different between trials, but were all higher during the second hour of cycling regardless of trial (p = 0.004, p = 0.029, p < 0.001, and p = 0.044, respectively, Table 2). Carbohydrate oxidation was not different between prolonged cycling trials (p = 0.171) or between the first and second hours of cycling (p = 0.888, Table 2).
Table 1. Average power output, oxygen uptake (V˙O2), heart rate (HR), and rating of perceived exertion (RPE) during 10-km time trials.
Average power (W) V˙O2 (L·min−1) HR (beats·min−1) RPE
PP
CC
CP
PC
237±16 3.57±0.26 157±4 17.4±0.3
250±13 3.85±0.12 158±5 17.8±0.2
249±17 3.67±0.22 155±6 17.6±0.3
254±20 3.72±0.16 155±5 17.0±0.5
Note: Values are means ± SE.
Glucose Serum glucose concentrations were not different between trials at rest (p = 0.261). After 60 min of cycling, serum glucose concentrations were higher in trials CC and CP when compared with trial PP (p = 0.027 and p = 0.009, respectively) as well as higher in CP compared with PC (p = 0.046, Fig. 2A). After 120 min of cycling, serum glucose concentrations were higher in trials CC and PC than in trials PP (p = 0.015 and p = 0.016, respectively) and CP (p = 0.016 and p = 0.014, respectively, Fig. 2A). There were no significant differences between serum glucose concentrations at the conclusion of the time trial (p = 0.089). Insulin There were no differences in serum insulin concentrations between trials or over time during the 2-h cycling bout (p = 0.810, Published by NRC Research Press
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Table 2. Oxygen uptake (V˙O2), heart rate (HR), carbohydrate oxidation (CHO Ox), fat oxidation (Fat Ox), and rating of perceived exertion (RPE) during prolonged cycling trials.
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PP V˙O2 (L·min−1) HR (beats·min−1) CHO Ox (g·min−1) Fat Ox (g·min−1) RPE
CC
CP
PC
Hour 1
Hour 2
Hour 1
Hour 2
Hour 1
Hour 2
Hour 1
Hour 2
2.81±0.14 137±6 2.29±0.25 0.55±0.06 11.7±0.7
2.90±0.17a 139±6a 2.22±0.30 0.62±0.06a 13.2±0.8a
2.83±0.12 133±5 2.54±0.29 0.47±0.07 11.4±0.6
2.95±0.14a 137±6a 2.58±0.21 0.51±0.06a 12.8±0.6a
2.79±0.14 134±6 2.50±0.32 0.46±0.07 11.6±0.8
2.88±0.14a 135±6a 2.46±0.27 0.52±0.06a 13.3±0.9a
2.80±0.14 134±6 2.26±0.28 0.55±0.06 11.4±0.7
2.90±0.13a 138±7a 2.30±0.22 0.60±0.04a 13.1±0.7a
Note: Values are means ± SE. ap < 0.05 between hour 1 and hour 2 (main effect of time).
Fig. 2. (A) Serum glucose concentrations. (B) Serum insulin concentrations. (C) Serum lactate concentrations. (D) Serum FFA concentrations. a, p < 0.05 between CC and PP; b, p < 0.05 between PC and PP; c, p < 0.05 between PC and CP; d, p < 0.05 between CP and PP; e, p < 0.05 between CC; f, p < 0.05 from between all other time points. PP, placebo control; CC, carbohydrate ingestion throughout exercise; CP, carbohydrate ingestion early only; PC, carbohydrate ingestion late only.
Fig. 2B). There were also no differences in serum insulin following the 10-km performance trial (p = 0.550). Lactate Serum lactate concentrations were not different between trials (p = 0.821), but increased with time during the 2-h cycling bout (p < 0.001, Fig. 2C). There were no differences in serum lactate following the 10-km performance trial (p = 0.675). FFA Serum FFA concentrations were higher at rest in trial PC than trial PP (p = 0.040), but not different between any other trials (p > 0.05). After 60 min of cycling, serum FFA concentrations were lower in trial CP than in trials PC and PP (p = 0.008 and p = 0.027, respectively). After 120 min of cycling, serum FFA concentrations were lower in trial PC than trials CP and PP (p = 0.008 and p = 0.018, respectively). Following the conclusion of the time trial, serum
FFA concentrations were lower in trial PC than trials CP and PP (p = 0.018 and p = 0.014, respectively). Serum FFA data are presented in Fig. 2D.
Discussion The main finding of this study was that carbohydrate ingested either throughout or late during a prolonged exercise bout improved subsequent time-trial performance when compared with a placebo. Conversely, ingestion of carbohydrate early, but not later in exercise, did not improve time-trial performance when compared with a placebo control trial. The exercise performance outcomes of the current study are in contrast to previously reported findings that ingestion of carbohydrate throughout prolonged exercise improved the ability to produce work in a 15-min performance ride, while ingestion of the same amount of carbohydrate late in exercise did not (McConell et al. 1996). The present findings Published by NRC Research Press
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agree with the original hypothesis from that study that carbohydrate ingestion late in exercise would improve performance to a greater or equal degree than ingestion of the same quantity of carbohydrate throughout exercise. There are several possible explanations for the differences between the findings of these 2 studies. During the previous study, the late carbohydrate ingestion trial consisted of ingesting 157.5 g of glucose polymer within 30 min before the start of the performance trial, a glucose ingestion rate of 5.25 g·min−1. All of the carbohydrate ingested may not have been able to be absorbed and available for oxidation in that time period, and such a high rate of glucose ingestion may have led to gastrointestinal distress. Several studies have explored absorption and oxidation rates of exogenous carbohydrate, and there appears to be a plateau of oxidation rates for a single transportable carbohydrate at 1.0 to 1.1 g·min−1 (Jeukendrup and Jentjens 2000). Ingestion of exogenous carbohydrate at a rate greater than 1.1 g·min−1 provides no further benefit, and can become detrimental in some cases because of gastrointestinal distress. The present study limited carbohydrate ingestion to 1.0 g·min−1 for any given period throughout the 2-h cycling bout, allowing ingested carbohydrate to be absorbed. Therefore, the carbohydrate ingested in the current study late in exercise should have been available for oxidation by the exercising muscle during the 10-km time trial. In the present study, participants received a total of 60 g of carbohydrate. This amount was chosen so that regardless of the timing of ingestion, all carbohydrate could be absorbed. Additionally, this amount of carbohydrate may limit the gastrointestinal distress associated with consuming highly concentrated carbohydrate beverages during exercise. The overall rate of carbohydrate ingestion in the current study for the 3 trials in which carbohydrate was given, 0.5 g·min−1, has previously been shown to be sufficient to induce performance benefits (Murray et al. 1991; Mitchell et al. 1989), and is confirmed by the present study. It should also be noted that differences in the outcomes of the 2 studies may be due to limited statistical power. While there were not significant differences in performance between trial CP and any other trials, it is possible that with additional subjects, statistical power would have been increased, and significant differences in performance between trial CP and trials PP, PC, and CC could have been observed. It has been previously suggested that maintenance of blood glucose levels late in exercise is 1 mechanism of increased performance with carbohydrate ingestion throughout exercise (Coggan and Coyle 1987; Coyle et al. 1986). The data from the present study supports this assertion. In the 2 trials (CC and PC) where serum glucose concentrations were significantly higher than the control trial (PP) at the start of the 10-km time trial, performance was also significantly better than in the control trial. In the trial (CP) where serum glucose concentrations were not different than the control trial at the start of the 10-km time trial, performance in the 10-km time trial was also not significantly different. Participants were not hypoglycemic in any of the trials. It has been previously stated by Coyle (2004) that carbohydrate feeding should not cease once it has been started during exercise. The findings of this study support this viewpoint, as during trial CP, when carbohydrate ingestion was stopped after the first hour of exercise, blood glucose levels fell during the second hour of exercise, and the 10-km time trial performance was not significantly different than a placebo trial. There were no differences in serum insulin concentrations between the trials. This finding is likely due to the relatively small amount of carbohydrate ingested (never more than 1 g·min−1) not being sufficient to elicit an insulin response, as well as the normally observed blunting of insulin response during exercise. Serum FFA levels were higher in trial PC than in trial PP at rest; however, this observation did not persist throughout exercise. In fact, FFA concentrations were significantly lower in trial PC than PP at 120 min and post-time trial. FFA concentrations were ele-
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vated in trials PP and CP compared with trials CC and PC at the start of the 10-km time trial, which may have resulted in an inhibition of glucose uptake into the exercising muscles. Elevated FFA levels have been shown to inhibit glucose uptake into muscle by 33% during 1 h of moderate-intensity dynamic leg extension exercise (Hargreaves et al. 1991). However, ingestion of medium-chain tryglycerol during a 2-h cycling bout at 63% of maximal V˙O2 increased serum FFA concentration, but did not alter performance trial results or substrate use (Goedecke et al. 1999). Although the current investigation did not measure FFA inhibition of glucose transport, this mechanism may be supported by the lower performance results in the trials where FFA was elevated at the start of the 10-km performance trial. Serum lactate concentration did not differ between trials, but did increase at each time point during the 2-h cycling bout, regardless of trial. Therefore, lactate concentration cannot explain the observed differences in time trial performance. During the 2-h cycling bout preceding the time trial, there were no differences between trials in V˙O2, HR, carbohydrate oxidation, fat oxidation, or RPE. However, V˙O2, HR, fat oxidation, and RPE all drifted upwards during the second hour of exercise, as commonly observed during prolonged exercise. Total carbohydrate oxidation was not affected by carbohydrate ingestion. The carbohydrate oxidation findings in this study are in agreement with the findings of others. The amount of carbohydrate and fat oxidation is most dependent on exercise intensity and the training status of the participant (Coggan et al. 1992; Deuster et al. 1992). While exogenous carbohydrate oxidation can make up a significant portion of metabolism in carbohydrate supplemented exercising subjects, total carbohydrate oxidation does not increase (Hawley et al. 1992; Jeukendrup et al. 1997; Jeukendrup and Jentjens 2000; Massicotte et al. 1990; Moodley et al. 1992; Saris et al. 1993; Wagenmakers et al. 1993). Since oxidized carbohydrate may come from exogenous carbohydrate, blood glucose, liver glycogen, or muscle glycogen, these findings contribute to the proposed mechanism that ingestion of carbohydrate during exercise may spare endogenous carbohydrate sources. Some studies have found no evidence of muscle glycogen sparing when exogenous carbohydrate is ingested during exercise (Coyle et al. 1986; Hargreaves et al. 1984), while others have reported its occurrence (Stellingwerff et al. 2007; Tsintzas et al. 1996; Tsintzas and Williams 1998). Sparing of liver glycogen, on the other hand, has been clearly demonstrated, and offers a more concrete explanation of the differences in endogenous carbohydrate oxidation when carbohydrate is ingested during exercise (Jeukendrup et al. 1999). The present study did not measure endogenous glycogen stores, although sparing of muscle glycogen may have occurred with carbohydrate feedings. In conclusion, the ingestion of carbohydrate either throughout or late during prolonged exercise improves 10-km time trial performance compared with a placebo following a 2-h cycling bout. However, early carbohydrate ingestion without later carbohydrate ingestion does not improve performance when compared with a placebo. Observed performance benefits coincided with elevated serum glucose levels and lower serum FFA concentrations at the start of the 10-km time trial. These findings demonstrate that carbohydrate ingestion either throughout or late during prolonged cycling can improve time trial performance, possibly through alteration of blood glucose and FFA levels.
References Borg, G.A. 1973. Perceived exertion: a note on history and methods. Med. Sci. Sports 5(2): 90–93. Carter, J.M., Jeukendrup, A.E., and Jones, D.A. 2004. The effect of carbohydrate mouth rinse on 1-h cycle time trial performance. Med. Sci. Sports Exerc. 36(12): 2107–2111. doi:10.1249/01.MSS.0000147585.65709.6F. PMID:15570147. Chambers, E.S., Bridge, M.W., and Jones, D.A. 2009. Carbohydrate sensing in the human mouth: effects on exercise performance and brain activity. J. Physiol. 587(Pt 8): 1779–1794. doi:10.1113/jphysiol.2008.164285. Coggan, A.R., and Coyle, E.F. 1987. Reversal of fatigue during prolonged exercise Published by NRC Research Press
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by carbohydrate infusion or ingestion. J. Appl. Physiol. 63(6): 2388–2395. PMID:3325488. Coggan, A.R., and Coyle, E.F. 1989. Metabolism and performance following carbohydrate ingestion late in exercise. Med. Sci. Sports Exerc. 21(1): 59–65. doi:10.1249/00005768-198902000-00011. PMID:2927302. Coggan, A.R., Kohrt, W.M., Spina, R.J., Kirwan, J.P., Bier, D.M., and Holloszy, J.O. 1992. Plasma glucose kinetics during exercise in subjects with high and low lactate thresholds. J. Appl. Physiol. 73(5): 1873–1880. PMID:1474063. Coyle, E.F. 2004. Fluid and fuel intake during exercise. J. Sports Sci. 22(1): 39–55 Coyle, E.F., Coggan, A.R., Hemmert, M.K., and Ivy, J.L. 1986. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. J. Appl. Physiol. 61(1): 165–172. PMID:3525502. Davis, J.M., Bailey, S.P., Woods, J.A., Galiano, F.J., Hamilton, M.T., and Bartoli, W.P. 1992. Effects of carbohydrate feedings on plasma free tryptophan and branched-chain amino acids during prolonged cycling. Eur. J. Appl. Physiol. Occup. Physiol. 65(6): 513–519. doi:10.1007/BF00602357. PMID: 1483439. Deuster, P.A., Singh, A., Hofmann, A., Moses, F.M., and Chrousos, G.C. 1992. Hormonal responses to ingesting water or a carbohydrate beverage during a 2 h run. Med. Sci. Sports Exerc. 24(1): 72–79. doi:10.1249/00005768-19920100000013. PMID:1549000. Febbraio, M.A., Chiu, A., Angus, D.J., Arkinstall, M.J., and Hawley, J.A. 2000. Effects of carbohydrate ingestion before and during exercise on glucose kinetics and performance. J. Appl. Physiol. 89(6): 2220–2226. PMID: 11090571. Goedecke, J.H., Elmer-English, R., Dennis, S.C., Schloss, I., Noakes, T.D., and Lambert, E.V. 1999. Effects of medium-chain triaclyglycerol ingested with carbohydrate on metabolism and exercise performance. Int. J. Sport Nutr. 9(1): 35–47. PMID:10036340. Hargreaves, M., Costill, D.L., Coggan, A., Fink, W.J., and Nishibata, I. 1984. Effect of carbohydrate feedings on muscle glycogen utilization and exercise performance. Med. Sci. Sports Exerc. 16(3): 219–222. doi:10.1249/00005768198406000-00004. PMID:6748917. Hargreaves, M., Kiens, B., and Richter, E.A. 1991. Effect of increased plasma free fatty acid concentrations on muscle metabolism in exercising men. J. Appl. Physiol. 70(1): 194–201. PMID:2010376. Hawley, J.A., Dennis, S.C., and Noakes, T.D. 1992. Oxidation of carbohydrate ingested during prolonged endurance exercise. Sports Med. 14(1): 27–42. doi: 10.2165/00007256-199214010-00003. PMID:1641541. Hayter, A.J. 1986. The maximum familywise error rate of Fisher’s least significant difference test. J. Am. Stat. Assoc. 81: 1001–1004. doi:10.2307/2289074. Jeukendrup, A.E., and Jentjens, R. 2000. Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research. Sports Med. 29(6): 407–424. doi:10.2165/00007256-20002906000004. PMID:10870867. Jeukendrup, A.E., and Wallis, G.A. 2005. Measurement of substrate oxidation during exercise by means of gas exchange measurements. Int. J. Sports Med. 26(Suppl. 1): S28–S37. doi:10.1055/s-2004-830512. Jeukendrup, A.E., Mensink, M., Saris, W.H., and Wagenmakers, A.J. 1997. Exogenous glucose oxidation during exercise in endurance-trained and untrained subjects. J. Appl. Physiol. 82(3): 835–840. doi:10.1097/00005768-19960500100576. PMID:9074971. Jeukendrup, A.E., Wagenmakers, A.J., Stegen, J.H., Gijsen, A.P., Brouns, F., and Saris, W.H. 1999. Carbohydrate ingestion can completely suppress endogenous glucose production during exercise. Am. J. Physiol. 276(4 Pt 1): E672–E683.
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Karelis, A.D., Smith, J.W., Passe, D.H., and Péronnet, F. 2010. Carbohydrate administration and exercise performance: What are the potential mechanisms involved? Sports Med. 40(9): 747–763. doi:10.2165/11533080-000000000-00000. PMID:20726621. Massicotte, D., Péronnet, F., Brisson, G., Boivin, L., and Hillaire-Marcel, C. 1990. Oxidation of exogenous carbohydrate during prolonged exercise in fed and fasted conditions. Int. J. Sports Med. 11(4): 253–258. doi:10.1055/s-20071024802. PMID:2228353. McConell, G., Kloot, K., and Hargreaves, M. 1996. Effect of timing of carbohydrate ingestion on endurance exercise performance. Med. Sci. Sports Exerc. 28(10): 1300–1304. doi:10.1097/00005768-199610000-00014. PMID:8897388. Mitchell, J.B., Costill, D.L., Houmard, J.A., Fink, W.J., Pascoe, D.D., and Pearson, D.R. 1989. Influence of carbohydrate dosage on exercise performance and glycogen metabolism. J. Appl. Physiol. 67(5): 1843–1849. PMID: 2600017. Moodley, D., Noakes, T.D., Bosch, A.N., Hawley, J.A., Schall, R., and Dennis, S.C. 1992. Oxidation of exogenous carbohydrate during prolonged exercise: the effects of the carbohydrate type and its concentration. Eur. J. Appl. Physiol. Occup. Physiol. 64(4): 328–334. doi:10.1007/BF00636220. PMID:1592058. Murray, R., Paul, G.L., Seifert, J.G., and Eddy, D.E. 1991. Responses to varying rates of carbohydrate ingestion during exercise. Med. Sci. Sports Exerc. 23(6): 713– 718. doi:10.1249/00005768-199106000-00010. PMID:1886479. Pallikarakis, N., Jandrain, B., Pirnay, F., Mosora, F., Lacroix, M., Luyckx, A.S., and Lefebvre, P.J. 1986. Remarkable metabolic availability of oral glucose during long-run duration exercise in humans. J. Appl. Physiol. 60(3): 1035–1042. PMID:3514570. Quanjer, Ph.H., Tammeling, G.J., Cotes, J.E., Pedersen, O.F., Peslin, R., and Yernault, J.-C. 1993. Lung volumes and forced ventilatory flows. Eur. Respir. J. 6(Suppl. 16): 5–40. PMID:8499054. Saris, W.H., Goodpaster, B.H., Jeukendrup, A.E., Brouns, F., Halliday, D., and Wagenmakers, A.J. 1993. Exogenous carbohydrate oxidation from different carbohydrate sources during exercise. J. Appl. Physiol. 75(5): 2168–2172. PMID:8307875. Seaman, M.A., Levin, J.R., and Serlin, R.C. 1991. New developments in pairwise multiple comparisons some powerful and practical procedures. Psychol. Bull. 110: 557–586. doi:10.1037//0033-2909.110.3.577. Siri, W.E. 1993. Body composition from fluid spaces and density: analysis of methods. 1961. Nutrition, 9(5): 480–491; discussion 480, 492. PMID:8286893. Stellingwerff, T., Boon, H., Gijsen, A.P., Stegen, J.H., Kuipers, H., and van Loon, L.J. 2007. Carbohydrate supplementation during prolonged cycling exercise spares muscle glycogen but does not affect intramyocellular lipid use. Pflugers Arch. 454(4): 635–647. [Epub 2007 Feb 27.] doi:10.1007/s00424007-0236-0. PMID:17333244. Tsintzas, K., and Williams, C. 1998. Human muscle glycogen metabolism during exercise. Effect of carbohydrate supplementation. Sports Med.25(1): 7–23. doi:10.2165/00007256-199825010-00002. PMID:9458524. Tsintzas, O.K., Williams, C., Wilson, W., and Burrin, J. 1996. Influence of carbohydrate supplementation early in exercise on endurance running capacity. Med. Sci. Sports Exerc. 28(11): 1373–1379. doi:10.1097/00005768-19961100000005. PMID:8933487. Wagenmakers, A.J., Brouns, F., Saris, W.H., and Halliday, D. 1993. Oxidation rates of orally ingested carbohydrates during prolonged exercise in men. J. Appl. Physiol. 75(6): 2774–2780. PMID:8125902.
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ARTICLE The performance effect of early versus late carbohydrate feedings during prolonged exercise Matthew William Sinclair Heesch, Molly Elizabeth Mieras, and Dustin Russel Slivka
Abstract: The purpose of this study was to determine how the timing of isoenergetic carbohydrate feedings during prolonged cycling affects performance in a subsequent 10-km cycling time trial. Recreationally trained male cyclists (n = 8; age, 34.5 ± 8.3 years; mass, 80.0 ± 6.3 kg; body fat, 16.0% ± 3.8%, peak oxygen uptake, 4.54 ± 0.42 L·min−1) completed 4 experimental trials consisting of cycling continuously for 2 h at 62.4% ± 1.9% of peak oxygen uptake, followed immediately by a self-paced 10-km time trial. The 4 conditions included no carbohydrate ingestion (PP), early carbohydrate ingestion (CP), late carbohydrate ingestion (PC), or carbohydrate ingestion throughout (CC). Blood samples were obtained at 0, 60, and 120 min of cycling as well as at the conclusion of the time trial. The 10-km time trial time to completion was faster in trials CC (17.70 ± 0.52 min) and PC (17.60 ± 0.62 min) as compared with trial PP (18.13 ± 0.52 min, p = 0.028 and p = 0.007, respectively) while trial CP (17.85 ± 0.58 min, p = 0.178) was not. Serum glucose increased with carbohydrate feedings (p < 0.05), while serum free fatty acid concentrations were lower in trials PC and CC than trials CP and PP (p < 0.05). There was no difference in oxygen uptake, heart rate, rating of perceived exertion, or substrate use between trials (p > 0.05). These data indicate that carbohydrate ingestion throughout or late during a 2-h cycling bout can improve subsequent 10-km time trial performance. Key words: cycling, blood glucose, free fatty acids, time trial. Résumé : Cette étude se propose de vérifier l’effet du moment de l’apport de sucre isoénergétique au cours d’un exercice prolongé a` vélo sur la performance lors d’une épreuve subséquente de 10 km contre-la-montre. Des cyclistes entraînés sur le plan récréatif (n = 8, 34,5 ± 8,3 ans, 80,0 ± 6,3 kg, 16,0 ± 3,8 % de gras, consommation d’oxygène de pointe de 4,54 ± 0,42 L·min−1) participent a` quatre essais expérimentaux consistant a` pédaler 2 h a` une intensité sollicitant 62,4 ± 1,9 % du consommation d’oxygène de pointe, puis a` effectuer selon un rythme autodéterminé l’épreuve de 10 km contre-la-montre. Les quatre conditions sont aucun apport de sucre (« PP »), apport hâtif de sucre (« CP »), apport retardé de sucre (« PC ») et apport de sucre tout au long de l’essai (« CC »). On prélève des échantillons de sang au début, a` la 60e et a` la 120e minute ainsi qu’a` la fin de l’épreuve contre-la-montre. La performance au 10 km contre-la-montre est plus rapide dans les conditions CC (17,70 ± 0,52 min) et PC (17,60 ± 0,62 min) comparativement a` la condition PP (18,13 ± 0,52 min, p = 0,028 et p = 0,007, respectivement); la performance dans la condition CP n’est pas plus rapide (17,85 ± 0,58 min, p = 0,178). La concentration sérique de glucose augmente avec l’apport de sucre (p < 0,05) et la concentration sérique des acides gras libres est plus faible dans les conditions PC et CC que dans les conditions CP et PP (p < 0,05). D’une condition a` l’autre, on n’observe aucune différence de consommation d’oxygène, de fréquence cardiaque, de perception de l’intensité de l’effort et d’utilisation de substrats (p > 0,05). D’après ces résultats, l’apport retardé de sucre ou tout au long des 2 h de l’exercice a` vélo peut contribuer a` l’amélioration de la performance a` l’épreuve subséquente de 10 km contre- la- montre. [Traduit par la Rédaction] Mots-clés : cyclisme, glucose sanguin, acides gras libres, contre-la-montre.
Introduction Ingesting carbohydrate during prolonged exercise can increase time to fatigue (Coggan and Coyle 1989) and improve time-trial performance (Febbraio et al. 2000; McConell et al. 1996; Moodley et al. 1992). However, the mechanisms of this benefit are still not completely understood (Karelis et al. 2010). Possible mechanisms include alteration of neural drive via carbohydrate sensing in the mouth (Carter et al. 2004; Chambers et al. 2009), maintenance of blood glucose levels when endogenous carbohydrate stores are exhausted (Pallikarakis et al. 1986), depression of free fatty acid (FFA) concentrations (Hargreaves et al. 1991), better maintenance of carbohydrate oxidation rates late in exercise (Massicotte et al. 1990), and sparing of endogenous carbohydrate stores (Stellingwerff et al. 2007). The timing of carbohydrate ingestion may affect these mechanisms and therefore be an important factor in attaining
performance benefits during exercise; however, the timing of carbohydrate ingestion has received relatively little attention. Ingesting carbohydrate throughout exercise improves timetrial performance after a bout of prolonged cycling; however, ingesting carbohydrate late in an extended exercise bout was shown to not significantly improve time-trial performance versus a placebo (McConell et al. 1996). However, it should be noted that ingesting carbohydrate late in exercise was also not significantly different than ingesting carbohydrate throughout, possibly because of the limited statistical power observed with only 8 participants (McConell et al. 1996). This absence of a performance benefit with carbohydrate consumption late in exercise occurred despite higher blood glucose levels, similar blood insulin levels, and a similar rate of carbohydrate oxidation compared with a trial where subjects received carbohydrate throughout. One possible explanation for this observation was that plasma FFA levels may
Received 29 January 2013. Accepted 25 June 2013. M.W.S. Heesch, M.E. Mieras, and D.R. Slivka. School of Health, Physical Education, and Recreation, University of Nebraska at Omaha, Omaha, NE 68182, USA. Corresponding author: Dustin Russel Slivka (e-mail: [email protected]). Appl. Physiol. Nutr. Metab. 39: 58–63 (2014) dx.doi.org/10.1139/apnm-2013-0034
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have been elevated by the time carbohydrate was ingested late in exercise. Higher plasma FFA levels have been shown to inhibit glucose uptake into the muscle during moderate exercise (Hargreaves et al. 1991). Additionally, carbohydrate ingestion during exercise blunts the normally observed increase of plasma FFA during prolonged exercise (Davis et al. 1992; Deuster et al. 1992; Murray et al. 1991). Another plausible explanation for the previous findings was that the large amount of carbohydrate (157.5 g) received late in exercise (30 min prior to the performance trial) was unable to be absorbed by the gastrointestinal tract before performance measures were assessed. Absorption and oxidation rates of exogenous carbohydrate appears to plateau for a single transportable carbohydrate at 1.0 to 1.1 g·min−1 (Jeukendrup and Jentjens 2000). The benefit of carbohydrate feedings early in exercise has been demonstrated by Tsintzas et al. (1996) who reported that ingestion of a 5.5% carbohydrate–electrolyte solution during the first hour of exercise, followed by ingestion of water until exhaustion, improved time to exhaustion when compared with ingestion of water throughout the entire protocol. However, it has been suggested by others that “if carbohydrate feeding is begun during an event, it should be continued throughout the event” (Coyle 2004). Because of conflicting findings, the impact of the timing of carbohydrate ingestion during exercise remains unclear. Therefore, the purpose of this study was to identify how the timing of isoenergetic carbohydrate feedings during prolonged exercise affects performance in a time trial immediately following prolonged exercise. Based on previous findings (McConell et al. 1996; Tsintzas et al. 1996), it was hypothesized that time trial performance would be improved when carbohydrate was ingested throughout exercise, as well as when carbohydrate was only ingested during the early stages of exercise, but that performance would not be improved when carbohydrate was only ingested during the late stages of exercise.
Materials and methods Participants Participants were recruited from the local community and were healthy males between the ages of 19 and 45 years who cycled for exercise at least 3 times per week, and had at least 1 cycling bout in excess of 2 h within the past 2 weeks. All participants reviewed and signed an Institutional Review Board approved Informed Consent document. Aerobic capacity Peak oxygen uptake (V˙O2peak) was assessed to measure aerobic capacity and to establish cycling intensity for the prolonged cycling trials. Participants rode on their own bicycle mounted on a Computrainer electronically braked cycle trainer ergometer (Racermate, Seattle, Wash., USA). The test began at a workload of 95 W, and the workload was increased by 35 W every 3 min until volitional fatigue. Power at V˙O2peak (Wmax) was calculated by adding the power output of the last completed stage to the proportion of time in the final stage multiplied by 35 W. Expired gases were analyzed using a flow and gas concentration calibrated TrueOne 2400 metabolic cart (ParvoMedics, Sandy, Utah, USA). A familiarization with the 10-km performance trial was conducted approximately 30 min after the V˙O2peak test. Body composition Body composition was assessed by hydrostatic weighing using an electronic load cell-based system (Exertech, Dresbach, Minn., USA) corrected for estimated residual lung volume. Residual lung volume was estimated by the Exertech software using the formula previously described by Quanjer et al. (1993). Body density from hydrostatic weighing was converted to percent body fat using the Siri equation (Siri 1993).
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Experimental trials Participants performed 4 separate trials, with no fewer than 3 days between each. The average washout period was 6 ± 2 days. All trials were completed within 28 days of the first. In each trial, participants cycled continuously at 60% of the Wmax for 2 h, followed immediately by a 10-km cycling performance trial, on their own bicycle mounted on a Computrainer ergometer (Racermate). The time trial was a simulated flat, straight, 10-km course. Participants started each time trial in the same gear, and were allowed to shift at their discretion throughout the time trial. Trials were performed in a randomized, counterbalanced order, and there was no evidence of an order effect between trials (p = 0.672). In the control condition (PP), participants received 250 mL of an artificially sweetened placebo beverage every 15 min, starting immediately upon the start of exercise, for the duration of the 2-h cycling bout. In a second condition (CC), participants received 250 mL of a 3% carbohydrate solution every 15 min for the duration of the 2-h cycling bout. In a third condition (CP), participants received 250 mL of a 6% carbohydrate solution every 15 min during the first hour of cycling, followed by 250 mL of an artificially sweetened placebo beverage every 15 min during the second hour of cycling. In the fourth condition (PC), participants received 250 mL of an artificially sweetened placebo beverage every 15 min during the first hour of cycling, followed by 250 mL of a 6% carbohydrate solution every 15 min during the second hour of cycling. All 3 trials in which carbohydrate was given were isoenergetic, and the same amount of fluid was ingested in each trial. Maltodextrin was used to prepare the beverages from raw ingredients by the investigators. Crystal Light (Kraft Foods Inc.; sweetened with aspartame) was used as the artificial sweetener in the placebo beverage, as well as added to all beverages to mask flavor. All testing was performed in the same laboratory, where temperature was kept consistent between 21–22 °C, and humidity was between 24%–43% for all trials. The pressure between the wheel of the bicycle and the ergometer was kept consistent between all trials for each individual participant (within 0.05 lbs (0.02 kg)), and was always between 2.70 and 3.00 lbs (1.2 and 1.4 kg) for all participants. For each trail, participants reported to the lab after an overnight fast, having been instructed not to consume any alcohol, caffeine, or tobacco and to avoid strenuous exercise for the previous 24 h. Prior to the participants arriving at the lab for their first trial, they were instructed to carefully record their self-selected diet over the previous 24 h to establish a diet to repeat in the 24 h prior to all other trials. Participants self-reported adherence to these instructions through the use of dietary and exercise logs. Metabolic gasses Metabolic gasses were collected during the last 5 min of each 30 min during the 2 h of cycling. The gas samples for the last 3 min of this collection were averaged and used to report exercise intensity and calculate substrate use utilizing the formula previously reported (Jeukendrup and Wallis 2005). Metabolic gasses were also collected continuously during the 10-km performance trial. All gas samples were collected using a calibrated TrueOne 2400 metabolic cart (ParvoMedics). Heart rate was monitored throughout the protocol using a Polar Heart Rate Monitor (Polar, Lake Success, N.Y., USA), and averaged over 15-min intervals. RPE Subjects were asked to report rating of perceived exertion (RPE) according to the 15-point Borg Scale (Borg 1973) every 15 min during the prolonged cycling trials, and at 5 km and the conclusion of the 10-km performance trials. RPE was averaged for each hour of the 2-h cycling bout. Blood sampling and analysis Five milliliters of blood was drawn from an antecubital vein before the start of the each prolonged cycling trial, at 60 min, Published by NRC Research Press
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120 min, and at the conclusion of the 10-km performance time trial to determine serum concentrations of glucose, insulin, lactate, and FFAs. Blood samples were obtained using BD Vacutainer tubes with clot activator. Samples were allowed to clot for 30 min, then spun for 15 min at 2000g. Serum was separated and frozen at −80 °C for later analysis. Serum glucose concentrations were assessed using Infinity Glucose reagent (Thermo Scientific, Middletown, Va., USA), read on a Nanodrop 2000c spectrophotometer (Thermo Scientific), and calculated using the extinction coefficient for NADH. Serum lactate concentrations were assessed using a 0.32-mol·L–1 glycine, 0.32-mol·L–1 hydrazine hydrate, 2.4-mmol·L–1 NAD, and 8-U·mL–1 solution (all reagents from Thermo Scientific), read on a Nanodrop 2000c spectrophotometer and calculated using the extinction coefficient for NADH. Serum insulin concentrations were assessed using an ELISA kit from Calbiotech (Spring Valley, Calif., USA) according to manufacturer instructions. Serum FFA concentrations were assessed using a commercially available ELISA kit from EIAab (Wuhan, China) according to manufacturer instructions.
Appl. Physiol. Nutr. Metab. Vol. 39, 2014
Fig. 1. (A) Ten-kilometer performance trial time to completion. a, p < 0.05 from PP. Values are means ± SE. (B) Individual 10-km performance trial time to completion. PP, placebo control; CC, carbohydrate ingestion throughout exercise; CP, carbohydrate ingestion early only; PC, carbohydrate ingestion late only.
Statistical analysis Serum glucose, insulin, FFAs, lactate, substrate use, RPE, and heart rate during the 2-h prolonged cycling bout were analyzed with a repeated measures 2-way (trial × time) ANOVA. The 10-km cycling time trial performance, oxygen uptake (V˙O2), heart rate, and RPE during the 10-km performance trial were analyzed using a 1-way (trial) repeated measures ANOVA. When a significant F ratio was found, Fishers protected least significant difference method was used to detect where differences occurred (Hayter 1986; Seaman et al. 1991). A probability of type I error of less than 5% was considered significant (p < 0.05). Based on previously published data using similar methods (McConell et al. 1996), an effect size of 0.47 was observed in performance trials between carbohydrate supplementation and placebo control conditions. If a similar effect size were observed in the present study (approximately a 30 s difference in 10-km performance trials), a statistical power of 0.86 would be obtained with 8 participants.
Results Participants Eight (n = 8) recreationally trained male cyclists (age, 34.5 ± 8.3 years; height, 179.5 ± 9.3 cm; weight, 80.0 ± 6.3 kg; body composition, 16.0% ± 3.8% body fat; V˙O2peak, 4.54 ± 0.42 L·min−1; and Wmax, 312.0 ± 43.2 W) completed the testing associated with this investigation. Performance The 10-km performance trial time to completion was faster in trials CC (p = 0.028) and PC (p = 0.007) as compared with trial PP, while trial CP was not different from trial PP (p = 0.178, Fig. 1). There were no significant differences in average power output between trials (p = 0.067, Table 1), although means reflected time to completion results, as trials with higher average power output also yielded faster 10-km performance trials. There were no differences between performance trials for V˙O2 (p = 0.222), HR (p = 0.568), or RPE (p = 0.347, Table 1). V˙O2, HR, RPE, substrate use During the prolonged cycling trials at 60% of Wmax (62.4% ± 1.9% V˙O2peak, 181 ± 24 W), V˙O2 (p = 0.244), HR (p = 0.190), RPE (p = 0.703), and fat oxidation (p = 0.130) were not different between trials, but were all higher during the second hour of cycling regardless of trial (p = 0.004, p = 0.029, p < 0.001, and p = 0.044, respectively, Table 2). Carbohydrate oxidation was not different between prolonged cycling trials (p = 0.171) or between the first and second hours of cycling (p = 0.888, Table 2).
Table 1. Average power output, oxygen uptake (V˙O2), heart rate (HR), and rating of perceived exertion (RPE) during 10-km time trials.
Average power (W) V˙O2 (L·min−1) HR (beats·min−1) RPE
PP
CC
CP
PC
237±16 3.57±0.26 157±4 17.4±0.3
250±13 3.85±0.12 158±5 17.8±0.2
249±17 3.67±0.22 155±6 17.6±0.3
254±20 3.72±0.16 155±5 17.0±0.5
Note: Values are means ± SE.
Glucose Serum glucose concentrations were not different between trials at rest (p = 0.261). After 60 min of cycling, serum glucose concentrations were higher in trials CC and CP when compared with trial PP (p = 0.027 and p = 0.009, respectively) as well as higher in CP compared with PC (p = 0.046, Fig. 2A). After 120 min of cycling, serum glucose concentrations were higher in trials CC and PC than in trials PP (p = 0.015 and p = 0.016, respectively) and CP (p = 0.016 and p = 0.014, respectively, Fig. 2A). There were no significant differences between serum glucose concentrations at the conclusion of the time trial (p = 0.089). Insulin There were no differences in serum insulin concentrations between trials or over time during the 2-h cycling bout (p = 0.810, Published by NRC Research Press
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Table 2. Oxygen uptake (V˙O2), heart rate (HR), carbohydrate oxidation (CHO Ox), fat oxidation (Fat Ox), and rating of perceived exertion (RPE) during prolonged cycling trials.
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PP V˙O2 (L·min−1) HR (beats·min−1) CHO Ox (g·min−1) Fat Ox (g·min−1) RPE
CC
CP
PC
Hour 1
Hour 2
Hour 1
Hour 2
Hour 1
Hour 2
Hour 1
Hour 2
2.81±0.14 137±6 2.29±0.25 0.55±0.06 11.7±0.7
2.90±0.17a 139±6a 2.22±0.30 0.62±0.06a 13.2±0.8a
2.83±0.12 133±5 2.54±0.29 0.47±0.07 11.4±0.6
2.95±0.14a 137±6a 2.58±0.21 0.51±0.06a 12.8±0.6a
2.79±0.14 134±6 2.50±0.32 0.46±0.07 11.6±0.8
2.88±0.14a 135±6a 2.46±0.27 0.52±0.06a 13.3±0.9a
2.80±0.14 134±6 2.26±0.28 0.55±0.06 11.4±0.7
2.90±0.13a 138±7a 2.30±0.22 0.60±0.04a 13.1±0.7a
Note: Values are means ± SE. ap < 0.05 between hour 1 and hour 2 (main effect of time).
Fig. 2. (A) Serum glucose concentrations. (B) Serum insulin concentrations. (C) Serum lactate concentrations. (D) Serum FFA concentrations. a, p < 0.05 between CC and PP; b, p < 0.05 between PC and PP; c, p < 0.05 between PC and CP; d, p < 0.05 between CP and PP; e, p < 0.05 between CC; f, p < 0.05 from between all other time points. PP, placebo control; CC, carbohydrate ingestion throughout exercise; CP, carbohydrate ingestion early only; PC, carbohydrate ingestion late only.
Fig. 2B). There were also no differences in serum insulin following the 10-km performance trial (p = 0.550). Lactate Serum lactate concentrations were not different between trials (p = 0.821), but increased with time during the 2-h cycling bout (p < 0.001, Fig. 2C). There were no differences in serum lactate following the 10-km performance trial (p = 0.675). FFA Serum FFA concentrations were higher at rest in trial PC than trial PP (p = 0.040), but not different between any other trials (p > 0.05). After 60 min of cycling, serum FFA concentrations were lower in trial CP than in trials PC and PP (p = 0.008 and p = 0.027, respectively). After 120 min of cycling, serum FFA concentrations were lower in trial PC than trials CP and PP (p = 0.008 and p = 0.018, respectively). Following the conclusion of the time trial, serum
FFA concentrations were lower in trial PC than trials CP and PP (p = 0.018 and p = 0.014, respectively). Serum FFA data are presented in Fig. 2D.
Discussion The main finding of this study was that carbohydrate ingested either throughout or late during a prolonged exercise bout improved subsequent time-trial performance when compared with a placebo. Conversely, ingestion of carbohydrate early, but not later in exercise, did not improve time-trial performance when compared with a placebo control trial. The exercise performance outcomes of the current study are in contrast to previously reported findings that ingestion of carbohydrate throughout prolonged exercise improved the ability to produce work in a 15-min performance ride, while ingestion of the same amount of carbohydrate late in exercise did not (McConell et al. 1996). The present findings Published by NRC Research Press
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agree with the original hypothesis from that study that carbohydrate ingestion late in exercise would improve performance to a greater or equal degree than ingestion of the same quantity of carbohydrate throughout exercise. There are several possible explanations for the differences between the findings of these 2 studies. During the previous study, the late carbohydrate ingestion trial consisted of ingesting 157.5 g of glucose polymer within 30 min before the start of the performance trial, a glucose ingestion rate of 5.25 g·min−1. All of the carbohydrate ingested may not have been able to be absorbed and available for oxidation in that time period, and such a high rate of glucose ingestion may have led to gastrointestinal distress. Several studies have explored absorption and oxidation rates of exogenous carbohydrate, and there appears to be a plateau of oxidation rates for a single transportable carbohydrate at 1.0 to 1.1 g·min−1 (Jeukendrup and Jentjens 2000). Ingestion of exogenous carbohydrate at a rate greater than 1.1 g·min−1 provides no further benefit, and can become detrimental in some cases because of gastrointestinal distress. The present study limited carbohydrate ingestion to 1.0 g·min−1 for any given period throughout the 2-h cycling bout, allowing ingested carbohydrate to be absorbed. Therefore, the carbohydrate ingested in the current study late in exercise should have been available for oxidation by the exercising muscle during the 10-km time trial. In the present study, participants received a total of 60 g of carbohydrate. This amount was chosen so that regardless of the timing of ingestion, all carbohydrate could be absorbed. Additionally, this amount of carbohydrate may limit the gastrointestinal distress associated with consuming highly concentrated carbohydrate beverages during exercise. The overall rate of carbohydrate ingestion in the current study for the 3 trials in which carbohydrate was given, 0.5 g·min−1, has previously been shown to be sufficient to induce performance benefits (Murray et al. 1991; Mitchell et al. 1989), and is confirmed by the present study. It should also be noted that differences in the outcomes of the 2 studies may be due to limited statistical power. While there were not significant differences in performance between trial CP and any other trials, it is possible that with additional subjects, statistical power would have been increased, and significant differences in performance between trial CP and trials PP, PC, and CC could have been observed. It has been previously suggested that maintenance of blood glucose levels late in exercise is 1 mechanism of increased performance with carbohydrate ingestion throughout exercise (Coggan and Coyle 1987; Coyle et al. 1986). The data from the present study supports this assertion. In the 2 trials (CC and PC) where serum glucose concentrations were significantly higher than the control trial (PP) at the start of the 10-km time trial, performance was also significantly better than in the control trial. In the trial (CP) where serum glucose concentrations were not different than the control trial at the start of the 10-km time trial, performance in the 10-km time trial was also not significantly different. Participants were not hypoglycemic in any of the trials. It has been previously stated by Coyle (2004) that carbohydrate feeding should not cease once it has been started during exercise. The findings of this study support this viewpoint, as during trial CP, when carbohydrate ingestion was stopped after the first hour of exercise, blood glucose levels fell during the second hour of exercise, and the 10-km time trial performance was not significantly different than a placebo trial. There were no differences in serum insulin concentrations between the trials. This finding is likely due to the relatively small amount of carbohydrate ingested (never more than 1 g·min−1) not being sufficient to elicit an insulin response, as well as the normally observed blunting of insulin response during exercise. Serum FFA levels were higher in trial PC than in trial PP at rest; however, this observation did not persist throughout exercise. In fact, FFA concentrations were significantly lower in trial PC than PP at 120 min and post-time trial. FFA concentrations were ele-
Appl. Physiol. Nutr. Metab. Vol. 39, 2014
vated in trials PP and CP compared with trials CC and PC at the start of the 10-km time trial, which may have resulted in an inhibition of glucose uptake into the exercising muscles. Elevated FFA levels have been shown to inhibit glucose uptake into muscle by 33% during 1 h of moderate-intensity dynamic leg extension exercise (Hargreaves et al. 1991). However, ingestion of medium-chain tryglycerol during a 2-h cycling bout at 63% of maximal V˙O2 increased serum FFA concentration, but did not alter performance trial results or substrate use (Goedecke et al. 1999). Although the current investigation did not measure FFA inhibition of glucose transport, this mechanism may be supported by the lower performance results in the trials where FFA was elevated at the start of the 10-km performance trial. Serum lactate concentration did not differ between trials, but did increase at each time point during the 2-h cycling bout, regardless of trial. Therefore, lactate concentration cannot explain the observed differences in time trial performance. During the 2-h cycling bout preceding the time trial, there were no differences between trials in V˙O2, HR, carbohydrate oxidation, fat oxidation, or RPE. However, V˙O2, HR, fat oxidation, and RPE all drifted upwards during the second hour of exercise, as commonly observed during prolonged exercise. Total carbohydrate oxidation was not affected by carbohydrate ingestion. The carbohydrate oxidation findings in this study are in agreement with the findings of others. The amount of carbohydrate and fat oxidation is most dependent on exercise intensity and the training status of the participant (Coggan et al. 1992; Deuster et al. 1992). While exogenous carbohydrate oxidation can make up a significant portion of metabolism in carbohydrate supplemented exercising subjects, total carbohydrate oxidation does not increase (Hawley et al. 1992; Jeukendrup et al. 1997; Jeukendrup and Jentjens 2000; Massicotte et al. 1990; Moodley et al. 1992; Saris et al. 1993; Wagenmakers et al. 1993). Since oxidized carbohydrate may come from exogenous carbohydrate, blood glucose, liver glycogen, or muscle glycogen, these findings contribute to the proposed mechanism that ingestion of carbohydrate during exercise may spare endogenous carbohydrate sources. Some studies have found no evidence of muscle glycogen sparing when exogenous carbohydrate is ingested during exercise (Coyle et al. 1986; Hargreaves et al. 1984), while others have reported its occurrence (Stellingwerff et al. 2007; Tsintzas et al. 1996; Tsintzas and Williams 1998). Sparing of liver glycogen, on the other hand, has been clearly demonstrated, and offers a more concrete explanation of the differences in endogenous carbohydrate oxidation when carbohydrate is ingested during exercise (Jeukendrup et al. 1999). The present study did not measure endogenous glycogen stores, although sparing of muscle glycogen may have occurred with carbohydrate feedings. In conclusion, the ingestion of carbohydrate either throughout or late during prolonged exercise improves 10-km time trial performance compared with a placebo following a 2-h cycling bout. However, early carbohydrate ingestion without later carbohydrate ingestion does not improve performance when compared with a placebo. Observed performance benefits coincided with elevated serum glucose levels and lower serum FFA concentrations at the start of the 10-km time trial. These findings demonstrate that carbohydrate ingestion either throughout or late during prolonged cycling can improve time trial performance, possibly through alteration of blood glucose and FFA levels.
References Borg, G.A. 1973. Perceived exertion: a note on history and methods. Med. Sci. Sports 5(2): 90–93. Carter, J.M., Jeukendrup, A.E., and Jones, D.A. 2004. The effect of carbohydrate mouth rinse on 1-h cycle time trial performance. Med. Sci. Sports Exerc. 36(12): 2107–2111. doi:10.1249/01.MSS.0000147585.65709.6F. PMID:15570147. Chambers, E.S., Bridge, M.W., and Jones, D.A. 2009. Carbohydrate sensing in the human mouth: effects on exercise performance and brain activity. J. Physiol. 587(Pt 8): 1779–1794. doi:10.1113/jphysiol.2008.164285. Coggan, A.R., and Coyle, E.F. 1987. Reversal of fatigue during prolonged exercise Published by NRC Research Press
Appl. Physiol. Nutr. Metab. Downloaded from www.nrcresearchpress.com by UNIV OF SOUTHERN CALIFORNIA on 04/05/14 For personal use only.
Heesch et al.
by carbohydrate infusion or ingestion. J. Appl. Physiol. 63(6): 2388–2395. PMID:3325488. Coggan, A.R., and Coyle, E.F. 1989. Metabolism and performance following carbohydrate ingestion late in exercise. Med. Sci. Sports Exerc. 21(1): 59–65. doi:10.1249/00005768-198902000-00011. PMID:2927302. Coggan, A.R., Kohrt, W.M., Spina, R.J., Kirwan, J.P., Bier, D.M., and Holloszy, J.O. 1992. Plasma glucose kinetics during exercise in subjects with high and low lactate thresholds. J. Appl. Physiol. 73(5): 1873–1880. PMID:1474063. Coyle, E.F. 2004. Fluid and fuel intake during exercise. J. Sports Sci. 22(1): 39–55 Coyle, E.F., Coggan, A.R., Hemmert, M.K., and Ivy, J.L. 1986. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. J. Appl. Physiol. 61(1): 165–172. PMID:3525502. Davis, J.M., Bailey, S.P., Woods, J.A., Galiano, F.J., Hamilton, M.T., and Bartoli, W.P. 1992. Effects of carbohydrate feedings on plasma free tryptophan and branched-chain amino acids during prolonged cycling. Eur. J. Appl. Physiol. Occup. Physiol. 65(6): 513–519. doi:10.1007/BF00602357. PMID: 1483439. Deuster, P.A., Singh, A., Hofmann, A., Moses, F.M., and Chrousos, G.C. 1992. Hormonal responses to ingesting water or a carbohydrate beverage during a 2 h run. Med. Sci. Sports Exerc. 24(1): 72–79. doi:10.1249/00005768-19920100000013. PMID:1549000. Febbraio, M.A., Chiu, A., Angus, D.J., Arkinstall, M.J., and Hawley, J.A. 2000. Effects of carbohydrate ingestion before and during exercise on glucose kinetics and performance. J. Appl. Physiol. 89(6): 2220–2226. PMID: 11090571. Goedecke, J.H., Elmer-English, R., Dennis, S.C., Schloss, I., Noakes, T.D., and Lambert, E.V. 1999. Effects of medium-chain triaclyglycerol ingested with carbohydrate on metabolism and exercise performance. Int. J. Sport Nutr. 9(1): 35–47. PMID:10036340. Hargreaves, M., Costill, D.L., Coggan, A., Fink, W.J., and Nishibata, I. 1984. Effect of carbohydrate feedings on muscle glycogen utilization and exercise performance. Med. Sci. Sports Exerc. 16(3): 219–222. doi:10.1249/00005768198406000-00004. PMID:6748917. Hargreaves, M., Kiens, B., and Richter, E.A. 1991. Effect of increased plasma free fatty acid concentrations on muscle metabolism in exercising men. J. Appl. Physiol. 70(1): 194–201. PMID:2010376. Hawley, J.A., Dennis, S.C., and Noakes, T.D. 1992. Oxidation of carbohydrate ingested during prolonged endurance exercise. Sports Med. 14(1): 27–42. doi: 10.2165/00007256-199214010-00003. PMID:1641541. Hayter, A.J. 1986. The maximum familywise error rate of Fisher’s least significant difference test. J. Am. Stat. Assoc. 81: 1001–1004. doi:10.2307/2289074. Jeukendrup, A.E., and Jentjens, R. 2000. Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research. Sports Med. 29(6): 407–424. doi:10.2165/00007256-20002906000004. PMID:10870867. Jeukendrup, A.E., and Wallis, G.A. 2005. Measurement of substrate oxidation during exercise by means of gas exchange measurements. Int. J. Sports Med. 26(Suppl. 1): S28–S37. doi:10.1055/s-2004-830512. Jeukendrup, A.E., Mensink, M., Saris, W.H., and Wagenmakers, A.J. 1997. Exogenous glucose oxidation during exercise in endurance-trained and untrained subjects. J. Appl. Physiol. 82(3): 835–840. doi:10.1097/00005768-19960500100576. PMID:9074971. Jeukendrup, A.E., Wagenmakers, A.J., Stegen, J.H., Gijsen, A.P., Brouns, F., and Saris, W.H. 1999. Carbohydrate ingestion can completely suppress endogenous glucose production during exercise. Am. J. Physiol. 276(4 Pt 1): E672–E683.
63
Karelis, A.D., Smith, J.W., Passe, D.H., and Péronnet, F. 2010. Carbohydrate administration and exercise performance: What are the potential mechanisms involved? Sports Med. 40(9): 747–763. doi:10.2165/11533080-000000000-00000. PMID:20726621. Massicotte, D., Péronnet, F., Brisson, G., Boivin, L., and Hillaire-Marcel, C. 1990. Oxidation of exogenous carbohydrate during prolonged exercise in fed and fasted conditions. Int. J. Sports Med. 11(4): 253–258. doi:10.1055/s-20071024802. PMID:2228353. McConell, G., Kloot, K., and Hargreaves, M. 1996. Effect of timing of carbohydrate ingestion on endurance exercise performance. Med. Sci. Sports Exerc. 28(10): 1300–1304. doi:10.1097/00005768-199610000-00014. PMID:8897388. Mitchell, J.B., Costill, D.L., Houmard, J.A., Fink, W.J., Pascoe, D.D., and Pearson, D.R. 1989. Influence of carbohydrate dosage on exercise performance and glycogen metabolism. J. Appl. Physiol. 67(5): 1843–1849. PMID: 2600017. Moodley, D., Noakes, T.D., Bosch, A.N., Hawley, J.A., Schall, R., and Dennis, S.C. 1992. Oxidation of exogenous carbohydrate during prolonged exercise: the effects of the carbohydrate type and its concentration. Eur. J. Appl. Physiol. Occup. Physiol. 64(4): 328–334. doi:10.1007/BF00636220. PMID:1592058. Murray, R., Paul, G.L., Seifert, J.G., and Eddy, D.E. 1991. Responses to varying rates of carbohydrate ingestion during exercise. Med. Sci. Sports Exerc. 23(6): 713– 718. doi:10.1249/00005768-199106000-00010. PMID:1886479. Pallikarakis, N., Jandrain, B., Pirnay, F., Mosora, F., Lacroix, M., Luyckx, A.S., and Lefebvre, P.J. 1986. Remarkable metabolic availability of oral glucose during long-run duration exercise in humans. J. Appl. Physiol. 60(3): 1035–1042. PMID:3514570. Quanjer, Ph.H., Tammeling, G.J., Cotes, J.E., Pedersen, O.F., Peslin, R., and Yernault, J.-C. 1993. Lung volumes and forced ventilatory flows. Eur. Respir. J. 6(Suppl. 16): 5–40. PMID:8499054. Saris, W.H., Goodpaster, B.H., Jeukendrup, A.E., Brouns, F., Halliday, D., and Wagenmakers, A.J. 1993. Exogenous carbohydrate oxidation from different carbohydrate sources during exercise. J. Appl. Physiol. 75(5): 2168–2172. PMID:8307875. Seaman, M.A., Levin, J.R., and Serlin, R.C. 1991. New developments in pairwise multiple comparisons some powerful and practical procedures. Psychol. Bull. 110: 557–586. doi:10.1037//0033-2909.110.3.577. Siri, W.E. 1993. Body composition from fluid spaces and density: analysis of methods. 1961. Nutrition, 9(5): 480–491; discussion 480, 492. PMID:8286893. Stellingwerff, T., Boon, H., Gijsen, A.P., Stegen, J.H., Kuipers, H., and van Loon, L.J. 2007. Carbohydrate supplementation during prolonged cycling exercise spares muscle glycogen but does not affect intramyocellular lipid use. Pflugers Arch. 454(4): 635–647. [Epub 2007 Feb 27.] doi:10.1007/s00424007-0236-0. PMID:17333244. Tsintzas, K., and Williams, C. 1998. Human muscle glycogen metabolism during exercise. Effect of carbohydrate supplementation. Sports Med.25(1): 7–23. doi:10.2165/00007256-199825010-00002. PMID:9458524. Tsintzas, O.K., Williams, C., Wilson, W., and Burrin, J. 1996. Influence of carbohydrate supplementation early in exercise on endurance running capacity. Med. Sci. Sports Exerc. 28(11): 1373–1379. doi:10.1097/00005768-19961100000005. PMID:8933487. Wagenmakers, A.J., Brouns, F., Saris, W.H., and Halliday, D. 1993. Oxidation rates of orally ingested carbohydrates during prolonged exercise in men. J. Appl. Physiol. 75(6): 2774–2780. PMID:8125902.
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