It is well established that carbohydrate ingestion during prolonged submaximal exercise increases plasma glucose availability during exercise, contributing to delayed fatigue (1) and enhanced performance (reviewed by ). Specific recommendations for carbohydrate ingestion will depend on the duration and intensity of the event. Current recommendations are to consume 30–60 g·h−1 carbohydrate for events lasting 1–2.5 h and up to 90 g·h−1 of multiple sources of carbohydrate for events over 2.5 h (3). For a rapidly absorbed single source of carbohydrate such as glucose, the maximal rate of exogenous carbohydrate oxidation is approximately 1.0–1.2 g·min−1 (4). Utilization of exogenous carbohydrate is primarily limited by intestinal absorption (5–7), but other limiting factors may include gastric emptying, hepatic glucose extraction, and/or uptake at the muscle (2).
Exogenous carbohydrate oxidation studies are often conducted using cycling as the mode of exercise. Many of these studies have adopted aggressive ingestion strategies to maximize carbohydrate delivery to the intestines through stimulation of gastric emptying (5–8). Typically, a large volume (400–600 mL) of fluid is provided immediately before exercise, with additional large boluses (100–200 mL) ingested at regular intervals (every 10–15 min) during exercise. This has resulted in maximal exogenous carbohydrate oxidation rates >1.0 g·min−1. The frequent ingestion of large volumes of carbohydrate-containing beverages has been well tolerated in laboratory-based cycling experiments. However, such ingestion strategies may not be feasible in the real world, especially in sports such as running where it is difficult to carry as much fluid as on the bike and where increased risk of gastrointestinal (GI) disturbances may be a consideration for fluid/carbohydrate intake.
In distance running, the mechanical motion has been linked to increased GI disturbances (9). As such, runners are unlikely to ingest a large prerace bolus and will typically consume a maximum of ~150–200 mL at drinks stations (positioned every 5 km: ~15–20 min). Recently, elite athletes in major marathon races and record attempts have experimented with ingesting drinks more frequently (every 1–2 km; ~2.5–6.0 min), allowing total delivery of a large amount of carbohydrate in relatively small volumes (~50–100 mL). However, the effect of this revised drinking strategy on exogenous carbohydrate oxidation has not been assessed in runners (or cyclists). Smaller, more frequent volumes of carbohydrate may minimize GI issues, but they also may reduce gastric emptying and limit exogenous carbohydrate oxidation, particularly in situations where a large “priming” bolus of drink is not ingested immediately before exercise. There are also practical challenges of collecting and then consuming fluid while running at high speeds. A large volume may be more difficult to ingest, but frequent consumption of smaller volumes may be inconvenient and make it more difficult to maintain race pace. Therefore, there remains a question surrounding the balance between optimal drink volume and frequency to maximize carbohydrate delivery and utilization, while minimizing GI disturbances.
The aim of this study was to investigate the effect of drink pattern (volume and frequency) on exogenous carbohydrate oxidation and markers of GI comfort. It was hypothesized that the ingestion of less frequent, larger volumes of carbohydrate would increase exogenous carbohydrate oxidation but also increase the prevalence of GI discomfort.
Twelve well-trained male runners and triathletes (27 ± 7 yr; 67.9 ± 6.7 kg; V˙O2peak, 68 ± 7 mL·kg−1·min−1) participated in this study. Subjects were provided with verbal and written information regarding the study before providing written informed consent and completing a health screen questionnaire. The study was approved by the Loughborough University Ethical Approvals (Human Participants) Sub-Committee. It was estimated that 12 subjects were required to detect a 10% difference in exogenous carbohydrate oxidation between trials based on a statistical power of 0.8 and an alpha of 0.05 (10).
Subjects visited the laboratory on four occasions: a preliminary peak oxygen uptake (V˙O2peak) test, a familiarization trial in which water was ingested at a rate of 200 mL every 20 min (this trial also provided a background correction for 13CO2 breath enrichment), and two experimental trials in which a 10% dextrose solution was ingested as either 200 mL every 20 min (CHO-20) or 50 mL every 5 min (CHO-5). The two carbohydrate trials were performed in a counterbalanced crossover design, separated by ≥7 d.
After measuring height and nude body mass (AFW-120 K; Adam Equipment Co., UK), subjects performed a V˙O2peak test on a motorized treadmill (mercury h/p/cosmos, Nussdorf, Germany). Subjects completed 4-min stages at progressive speeds until a heart rate >160 bpm was elicited (measured using Polar M400, Polar, Kemple, Finland). Expired gas was collected into Douglas bags during the final minute of each stage and analyzed for oxygen and carbon dioxide concentrations (Servomex 1400 Oxygen and Carbon Dioxide Gas Analyzer; Servomex, Crowborough, UK). Gas volume (Harvard dry gas meter; Harvard Apparatus Ltd., Edenbridge, UK) and temperature (RS Pro digital thermometer; RS Components, Corby, UK) were measured and corrected to standard temperature and pressure, dry. To correct V˙O2 and V˙CO2 values, ambient air was collected simultaneously with expired gas samples (11). After a short period of rest (~10–15 min), subjects ran to volitional exhaustion using a ramped protocol. Set at the speed of the final submaximal stage, incline was increased by 1% each min until volitional exhaustion. An expired gas sample was collected during the final minute of exercise.
Subjects were asked to record physical activity and dietary intake in the 24 h before the water trial and were then asked to replicate this before each of the carbohydrate trials. They were also asked to refrain from strenuous exercise in the 24 h before all trials. In addition, subjects were asked to follow a specific exercise and diet regime designed to reduce the background shift in 13CO2 (12). At least 2–4 d prior to each experimental trial, subjects completed an exhaustive bout of exercise (replicated at the same time prior to subsequent trials). This was to oxidize 13C-enriched glycogen. After this, subjects were asked to refrain from eating foods that had a naturally high abundance of 13C, including commercially available sports drinks and carbohydrate derived from C4 plants (e.g., cane sugar and maize). A detailed list of foods to avoid was provided to each subject. Subjects confirmed adherence to all pretrial standardization requirements before commencing each trial.
For each trial, subjects arrived at the laboratory at the same time of day (standardized for each subject between 0600 and 1000 h) after an overnight fast and 90 min after ingesting 500 mL water.
On arrival, subjects provided a urine sample and had nude body mass measured. A 20-gauge Teflon cannula was then inserted into an antecubital vein of one arm to allow repeated blood sampling during the trial. After 20 min sitting, expired breath samples were collected into 12 mL evacuated glass tubes (Exetainers; Labco, Ceredigion, UK) for determination of 13CO2 enrichment, followed by a 5-min collection of expired gas (analyzed for oxygen and carbon dioxide content and gas volume and temperature as described for the preliminary testing). A 5.5-mL venous blood sample was then collected and heart rate was recorded. Subjects were asked to rate feelings of thirst, GI comfort, and stomach bloatedness using scales (0 = no feeling, 10 = extreme feeling). As used in similar studies (13), responses ≥5 were classed as severe, with values <5 classed as nonsevere.
Immediately before the start of exercise, subjects consumed the first drink. They then completed 100 min running at 70% V˙O2peak (13.1 ± 1.0 km·h−1). A sample period occurred every 20 min: at 18 min, expired breath samples were collected into exetainers, a 60-s expired gas collection occurred at 19 min, with heart rate and rating of perceived exertion (RPE) recorded during this collection. At 20 min, subjects straddled the treadmill for 90 s, and a 5.5-mL blood sample was collected. Once running had recommenced, the next drink was ingested. This was repeated every 20 min. Additional drinks were ingested in CHO-5 at 5-min intervals, with final drinks consumed at 80 min (water and CHO-20) and 95 min (CHO-5). Heart rate, RPE, and subjective scales of thirst, GI comfort, and stomach bloatedness were measured every 10 min, with all scales measured on completion of exercise. Airflow provided by a 0.5-m diameter fan was placed on the floor and angled at the chest to provide similar airflow in both trials (~3.0–3.5 m·s−1). After the completion of exercise, subjects towel dried to remove unevaporated sweat and had nude body mass measured.
For each carbohydrate trial, 137.5 g of glucose (D-glucose monohydrate; MyProtein, Northwich, UK) was measured and then made up to a volume of 1250 mL to produce a 10% carbohydrate solution. To account for the different molecular masses of glucose and dextrose monohydrate, the amount of glucose was multiplied by 1.1. The glucose had a low natural abundance of 13C (−27.22 δ‰ vs Pee Dee Bellemnitella [PDB]); therefore, drinks were enriched with uniformly labeled 13C glucose (~0.100 g [U-13C] glucose·L−1; Cambridge Isotopes Laboratory Inc., Andover, MA). A total of 1.0 L of each drink, with 13C mean glucose enrichment of 62.5 ± 3.7 δ‰ vs PDB, was ingested during each trial to provide an average ingestion rate of 1.0 g·min−1.
For each 5.5 mL of venous blood, 1.0 mL was aliquoted into a tube containing anticoagulant for analysis of hemoglobin (cyanmethemoglobin method; Sigma, St. Louis, MO) and hematocrit (micro-centrifugation; Hawksley, Worthing, UK). Plasma volume changes were calculated from hemoglobin and hematocrit values (14). The remaining blood was allowed to clot at room temperature before centrifugation at 2200g for 15 min at 4°C. The serum was removed and frozen at −20°C until later analysis of glucose (spectrophotometry, ABX Pentra 400; Horiba Medical, Northampton, UK) and insulin concentration (enzyme immunoassay; DRG Instruments GmbH, Marburg, Germany). Urine was immediately analyzed for osmolality (refractometry, Osmocheck; Vitech Scientific, Horsham, UK).
Breath (gas chromatography isotope ratio mass spectrometry, Hydra 20–20 IRMS; Europa Scientific, Crewe, UK) and drink (elemental analyzer isotope ratio mass spectrometry, 20–20 IRMS; Europa Scientific, Crewe, UK) samples were analyzed for 13C/12C ratio (both Iso-Analytical Ltd., Crewe, UK).
Stoichiometric equations were used to calculate rates of total carbohydrate and fat oxidation (15), with the assumption that oxidation of protein was negligible:
For each expired breath sample, the isotopic enrichment was expressed as δ per milliliter difference between 13C/12C ratio of the sample and a known laboratory reference standard. The formula (16) used was as follows:
Following this, δ13C was then related to the international standard Vienna PDB.
Exogenous carbohydrate oxidation rates were calculated as follows:
where δExp is the 13C expired breath enrichment, δIng is the 13C enrichment of the ingested beverage, δExpbkg is the 13C expired breath enrichment of the water trial (background), and k is the CO2 volume that is produced by oxidation of 1 g of glucose (k = 0.7467 L).
When calculating exogenous carbohydrate oxidation rates using 13CO2 from expired gas, a methodological consideration is the 13CO2 trapped in the bicarbonate pool. During the onset of exercise, some CO2 arising from the oxidation of glucose will be retained (17); however, V˙CO2 will increase during exercise until a condition of physiological steady state occurs, resulting in equilibration between the 13CO2 in expired gas and the CO2/HCO3 pool. The recovery of 13CO2 from oxidation of enriched glucose will approach 100% after approximately 60 min (18). Therefore, there is likely to be some underestimation of exogenous carbohydrate oxidation rates during the first 60 min. Data should be interpreted as the minimum estimates.
Data were checked for normality of distribution using the Shapiro–Wilk test. All data with one factor (urine osmolality, sweat losses, body mass) were normally distributed and analyzed using a one-way ANOVA. All data with two factors except thirst and GI measures were normally distributed. Data with two factors (variables measured over time in each trial) were measured using a two-way repeated-measures ANOVA. When a significant two-way ANOVA was observed, a paired samples t-test with Holm–Bonferroni correction was performed to identify where the difference occurred. Statistical significance was accepted when P ≤ 0.05. Data were presented as mean ± SD. All statistical analysis was conducted using Statistical Package for the Social Sciences for Windows (version 23.0; SPSS, Chicago, IL).
Baseline measures and pretrial standardization
Body mass (CHO-20, 67.5 ± 6.5; CHO-5, 67.4 ± 6.5 kg; P = 0.384) and urine osmolality (CHO-20, 319 ± 193; CHO-5, 398 ± 265 mOsmol·kg−1 H2O; P = 0.076) were not different between trials, providing a good indication that subjects arrived at the laboratory in a similar state of hydration.
Resting values of 13C breath enrichment were not different between water and CHO trials (WAT, −27.14 ± 0.32 δ‰ vs PDB; CHO-20, −27.27 ± 0.38 δ‰ vs PDB; CHO-5, −27.18 ± 0.34 δ‰ vs PDB; P = 0.635).
Mean V˙O2, V˙CO2, and RER during exercise were not different between trials (Table 1). Exogenous carbohydrate oxidation rates were greater in CHO-20 than CHO-5. This effect was observed both early (20–60 min) and late (60–100 min) in exercise, with average exogenous carbohydrate oxidation rates across the whole exercise period being 23% higher in CHO-20 than CHO-5 (Table 1). There was also a time–trial interaction effect for exogenous carbohydrate oxidation and post hoc analysis detected this at the 100-min time point (CHO-20, 0.68 ± 0.14 g·min−1; CHO-5, 0.61 ± 0.14 g·min−1; Fig. 1A). These values represent the peak exogenous carbohydrate oxidation rates for each trial. Exogenous carbohydrate oxidation efficiency was a minimum estimate of 38% in CHO-20 compared with 31% in CHO-5. Total carbohydrate oxidation and average endogenous carbohydrate oxidation rates during exercise were not different between trials (Table 1). There was a significant time–trial interaction effect (P = 0.041) for endogenous carbohydrate oxidation rates, but after correction for multiple comparisons, no post hoc differences were detected. During exercise, mean fat oxidation was not different between trials (Table 1).
Venous blood data were collected in 11 out of 12 subjects because of sampling problems in one trial and therefore presented accordingly. In both carbohydrate trials, serum glucose concentrations increased from baseline after drink ingestion (time effect, P < 0.0001; Fig. 2A). There was no trial (P = 0.938) or time–trial interaction effect (P = 0.095). Serum insulin concentrations (Fig. 2B) were not different between trials (mean value during exercise; CHO-20, 42.9 ± 21.5 pmol·L−1; CHO-5, 38.2 ± 16.2 pmol·L−1; P = 0.373).
After the onset of exercise, plasma volume decreased from resting values in both trials (approximately −10.0% to −12.5% decrease from baseline; time effect, P = 0.020; Fig. 3). The pattern of drink ingestion did not influence changes in plasma volume (time–trial interaction effect, P = 0.425).
Heart rate increased progressively throughout exercise and was not different between trials, with mean exercise values of 155 ± 13 and 157 ± 13 bpm for CHO-20 and CHO-5 trials, respectively (P = 0.310). Mean RPE values were not different between trials (CHO-20, 11 ± 2; CHO-5, 11 ± 2; P = 0.723). Sweat losses during (CHO-20, 2.04 ± 0.34 L; CHO-5, 2.09 ± 0.40 L; P = 0.416) and percentage body mass loss at the end (CHO-20, 1.52% ± 0.40%; CHO-5, 1.59% ± 0.44%; P = 0.385) of exercise were not different between trials. Mean subjective feelings for thirst (CHO-20, 1 ± 1; CHO-5, 1 ± 1; trial effect, P = 0.836), GI comfort (CHO-20, 2 ± 1; CHO-5, 2 ± 1; trial effect, P = 0.830), and stomach bloatedness (CHO-20, 2 ± 1; CHO-5, 2 ± 1; trial effect, P = 0.880) were not different between trials. There were no time–trial interaction effects (thirst, P = 0.682; GI comfort, P = 0.313; stomach bloatedness, P = 0.126). Subjective feelings of GI discomfort and stomach bloatedness gradually increased over the course of the exercise period (time effect, P < 0.0001, Fig. 4). For GI comfort, four subjects reported a score of 5 in both trials, with a further subject reporting a score of 5 in the CHO-20 trial. Two subjects reported a score of 6 at single time points in CHO-5. For reported measures of stomach bloatedness, three subjects reported a score of 5 in both trials, with a further subject reporting a score of 5 in CHO-5.
The aim of the study was to examine the effect of different drinking patterns (frequency and volume) on exogenous carbohydrate oxidation and GI comfort during prolonged submaximal running. The main finding was that consuming larger volumes at less frequent intervals (200 mL every 20 min) increased exogenous carbohydrate oxidation compared with consuming smaller volumes at more frequent intervals (50 mL every 5 min). In addition, both drinking strategies were well tolerated, and there was no difference in the reported level of GI comfort between trials.
Average exogenous carbohydrate oxidation during exercise was 23% higher in CHO-20 than in CHO-5, which, given the same amount of carbohydrate was ingested, is a possibly important result that may have beneficial implications in a performance situation. Over the course of the trials, mean fluid delivery was the same (200 mL per 20 min), but the larger boluses provided in CHO-20 at the start and throughout exercise likely increased gastric pressure, resulting in increased gastric emptying (19,20). As fluid enters the stomach, there is increased pressure on the antral region, which will contribute to an increased emptying rate (21). Increased gastric contents and pressure will stimulate stretch receptors in the gastric mucosa, therefore increasing the rate of gastric emptying (22,23). Costill and Saltin (20) examined progressively larger volumes of fluid than the initial boluses in the present study and found increased gastric emptying up to 600 mL. It was therefore likely that neither of the initial boluses in the present study were large enough to maximize carbohydrate delivery to the intestines, providing a partial explanation for the relatively low exogenous oxidation rates as well as the failure to observe a plateau. The findings by Costill and Saltin (20) were conducted at rest, and whilst they provide an indication of the role of drink volume in gastric emptying, there must also be consideration for other factors, such as exercise intensity, carbohydrate concentration, and dehydration.
Peak rates of exogenous carbohydrate oxidation were comparable with studies providing similar amounts of carbohydrate during exercise that either included (24) or did not include (10,25–27) a bolus of carbohydrate before exercise. Although exogenous oxidation rates were similar in Hulston et al. (24), there was a marked difference in oxidation efficiency (63%) compared with the present study (minimum estimate of at least 38% in CHO-20 and 31% in CHO-5). Despite the delivery of slightly less carbohydrate during exercise (0.8 g·min−1) in Hulston et al. (24), this increased efficency is likely driven by the provision of a large, 600 mL bolus of carbohydrate (6% glucose solution) prior to exercise. It also appears, from other studies, that in order to achieve maximal exogenous oxidation rates of 1.0–1.2 g·min−1, both a preexercise bolus of carbohydrate is required as well as a much larger amount of carbohydrate ingestion during exercise (5–8); however, particuarly in runners, this combination may exacerbate GI distress. As exogenous oxidation rates were still increasing at 100 min, there are potential improvements to both strategies used in this study, but this must be balanced against the practicalities of the exercise. Whilst mode of exercise has shown similar exogenous carbohydrate oxidation rates between running and cycling (28), exogenous carbohydrate oxidation rates in the present study may have been affected by using runners with limited experience of carbohydrate ingestion during exercise (four subjects had completed at least one marathon and had used carbohydrate in the buildup and during the race but, along with the other subjects were not regular users of carbohydrate during typical training). Running has been shown to decrease gastric emptying (19), but training with carbohydrate ingestion (i.e., increased exposure to carbohydrate ingestion) has been shown to enhance exogenous oxidation rates (29).
Markers of GI discomfort progressively increased throughout exercise in both conditions; however, there was no difference between drinking pattern, and only 3% of time points could be considered as severe (≥5 out of 10). GI issues can be increased by a greater volume and/or an increase in the carbohydrate concentration/osmolality of the beverage (30). When large volumes of carbohydrate drinks have been ingested less frequently, an increase in GI discomfort has been reported (31), however, in the low frequency group, drink ingestion was every 60 min in a far greater volume (~690 mL) than the current study. The high-frequency, small volume pattern used in the study by Stocks et al. (31) was more closely related to the CHO-20 condition with similar GI responses reported.
The intensity used in the present study was 70% V˙O2peak, which is comparable with several other studies (5,6,24,32–35). At this intensity, there is a high reliance on carbohydrate oxidation (36), and depending on the ingestion strategy, it is possible to achieve peak oxidation rates of ~0.9–1.0 g·min−1 (5,32). It is also an intensity that can be sustained for exercise durations/race distances over which carbohydrate ingestion is known to be beneficial to performance. Whilst elite marathon runners will race at even higher intensities (~75%–85% V˙O2max ), this may not be achievable for subelite and recreational runners, especially when performing on a treadmill. Therefore, we selected an exercise intensity and duration that was both achievable for our subjects and that could provide translational results for competitive running races. There does, however, need to be further investigation into the effect of drinking pattern at an increased intensity, as more severe GI symptoms may be reported. The potential increases in symptoms may be exacerbated by decreased gastric emptying rates above 75% V˙O2max (38) and/or the increased mechanical stress placed on the GI system when running at these increased velocities (39). In addition, at an increased pace, the process of drinking a large volume may be hindered due to increased speed and breathing frequency.
Further work is needed to investigate the impact of drinking pattern on performance. Whilst this study was able to demonstrate a metabolic advantage with CHO-20, there is no guarantee that this will directly translate to improved performance, as there must also be consideration for factors such as the nonoxidative roles of carbohydrate. The effect of carbohydrate sensing in the mouth on performance should also be considered (3) as more frequent sensations of carbohydrate by the central nervous system may be more advantageous. In addition, from a practical perspective, there should be consideration of the opportunity to drink during a race setting as well as the time saved when drinks are consumed less frequently. Whilst a treadmill setting allows greater measurements to improve mechanistic understanding, there is reduced ecological validity in terms of recreating race practicalities. The design of the current study restricts comparison with one of two distinct drinking strategies, whereas in practice athletes are free to consume different volumes as they see fit. It may be that the preferential strategy is to combine both approaches by ingesting a larger volume at the start and during the early part of the race, thereby priming the system, before switching to a smaller volume as the race progresses.
A limitation of the study was that the glycogen depleting exercise was not supervised, whilst it is also preferential to provide a standardized dietary intake (40) rather than just providing guidance and ask subjects to repeat their intake. However, the use of well-motivated subjects, pretrial interviews confirming diet and depletion exercise, and the minimal shift in expired 13C during the water trial were indicative of good adherence to pretrial standardization procedures.
In conclusion, drinking a larger volume of a single source of carbohydrate (glucose) every 20 min increased exogenous carbohydrate oxidation compared with the same volume of fluid ingested in smaller volumes every 5 min. At this intensity, there was no difference in reported GI issues, and neither pattern induced prolonged severe symptoms. However, further work is warranted to investigate the effect of increasing intensity and also how drinking pattern may affect performance.
The authors acknowledge the work of Marianna Apicella and Kelsie Johnson in assisting with data collection.
No external funding was obtained for the current study. The results of the present study do not constitute endorsement by the American College of Sports Medicine. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. L. J. J. has previously received funding from PepsiCo Inc. and Volac International, performed consultancy for PepsiCo Inc. and Lucozade Ribena Suntory, and received conference fees from PepsiCo Inc. and Danone Nutricia. In all cases, monies have always been paid to L. J. J.’s institution and not directly to L. J. J.
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