Short-duration sprint events, and many team sports, require athletes to repeatedly exercise at a high-intensity for a short period. Many team sports are highly intermittent with bursts of high-intensity exercise interspersed with lower-intensity recovery periods. Repeated bouts of sprint activity present a unique bioenergetic requirement because this type of activity requires a significant amount of carbohydrate (CHO) oxidation for optimal performance (10). During repeated maximal exercise bouts, glucose and, especially, glycogen are catabolized at a high rate, largely through anaerobic glycolysis, particularly during shorter duration bouts. Oxidative metabolism becomes a more predominant source of ATP production as the duration of the exercise increases, and also during subsequent sprints, with a greater proportion of CHO being oxidized (10,26). Therefore, exercise performance in repeated-bout sprint events may be influenced by CHO ingestion either before or during the event (3). In fact, a number of studies on intermittent running exercise have demonstrated that with CHO feeding, before and during exercise, fatigue is delayed and time to exhaustion is increased (5,9,24,25,27).
Most studies examining whether CHO ingestion influences exercise performance have used endurance exercise rather than short-duration sprint exercise. Although some guidelines suggest that even for exercise as short as 30 minutes, small amounts of CHO (either ingested or through a mouth rinse) may be beneficial for performance (15); other general recommendations do not advocate CHO supplementation for exercise lasting less than 30 minutes (16) or between 30 and 45 minutes (4,35). There is some evidence that power output over short-duration exercise (5–15 minutes) may increase after CHO ingestion (4). However, the mechanism for performance improvements in shorter duration exercise bouts is unclear. Current evidence suggests that the activation of certain higher brain regions in response to oral receptors that detect CHO may play a role in the positive effects observed with CHO mouth rinsing (15,32), and it is possible that this may be true for short-duration, high-intensity exercise as well.
Carbohydrate intake before exercise will typically produce an insulin response and may reduce fat mobilization and utilization while simultaneously increasing CHO oxidation (35). Differences in factors including, but not limited to: hormonal responses, the degree of glycosylated hemoglobin formation, central drive responses to low blood glucose (BG), and changes in the rate of glycogenolysis may alter the individual response to CHO ingestion, and partially account for a discrepancy in findings between studies. For example, although CHO ingestion is believed to improve performance by delaying hypoglycemia, some studies indicate that pronounced hyperglycemia, which can acutely follow CHO ingestion, may impair oxygen delivery to the working muscles (2,6). Evidence regarding the effects of CHO ingestion on intermittent, repeated-bout exercise, is limited and conflicting (8,35). Importantly, those studies that have examined CHO ingestion during intermittent exercise have often used a design that includes CHO feeding either before or throughout exercise. Therefore, aside from the initial CHO bolus, the influence of latter CHO feedings is all influenced by previous intake and cannot be assessed strictly by themselves.
Therefore, the primary aims of this study were twofold: (a) to determine whether CHO feeding taken immediately before, early, or late during high-intensity cycling exercise affects time trial (TT) performance, and (b) to determine whether the BG response to CHO ingestion differed according to when the CHO was administered. We hypothesized that (a) CHO ingestion before, or during, a series of 3 repeated cycling TTs would improve performance in the subsequent bout and (b) that BG response to CHO ingestion would not differ according to when the CHO was administered.
Experimental Approach to the Problem
A repeated-measures, randomized, double-blinded experimental design was implemented. Subjects arrived at the laboratory at least 6 hours postprandial before each testing session. Maximal oxygen consumption (V̇o2max) was determined for all participants using a previously described protocol established in our laboratory (7,30). Briefly, subjects cycled at a workload of 100 Watts (W), and the workload was increased stepwise by 25 W every minute until power output could no longer be sustained by the subject or if the subjects voluntarily terminated the test. Oxygen consumption (V̇o2) was determined by indirect calorimetry, and subjects were considered to have achieved V̇o2max if 2 of the following 3 criteria were met: (a) a heart rate ≥90% of the age-predicted maximal heart rate (220 participant's age); (b) a respiratory exchange ratio (RER) ≥1.10; and (c) evidence of a plateau in V̇o2 with an increase in exercise intensity, which is defined as ≤150 ml increase in V̇o2 with an increase in workload (30). During this test, subjects breathed room air into a thermoplastic face mask attached to a 2-way, nonrebreathing valve (Hans Rudolph, Kansas City, KS, USA). Inspiratory flow was measured with a pneumotachograph (Hans Rudolph), and respiratory gases were analyzed by 2 rapidly responding gas analyzers (Applied Electrochemistry, Pittsburgh, PA, USA) for O2 and CO2 fractions. Data were recorded using offline data acquisition software (DasyLab Measurement Computing, Norton, MA, USA) that allowed for calculation of V̇o2, V̇co2, V̇E, and RER.
Each subsequent visit consisted of 3 simulated 4-km TTs, separated by 15 minutes of active recovery, on an electromagnetically braked cycle ergometer interfaced with computer software (Velotron Racermate, Seattle, WA, USA). Visits were separated by 1 week and conducted on the same day of the week, and time of day. Visits included a familiarization session to reduce the effect of learning (34), followed by experimental sessions comprising a control (CTL) visit and 3 CHO ingestion visits. The order of the experimental visits was randomized using Latin squares. An illustration of the general study design is given in Figure 1.
Participants were 16 trained competitive male cyclists (age range 18–29 years) recruited from the university population and surrounding community who maintained their normal diet and regular training regimen throughout the duration of the study. All participants were free of a history of cardiopulmonary disease as assessed by questionnaire, participated in competitive cycling competitions within the last calendar year, and regularly engaged in a cycling interval training regimen, with a minimum experience of 3 months of 8 hours per week of training volume including interval training. Participants were all familiar with 4-km cycling TT competition. Subject characteristics are given in Table 1. Participants were instructed to refrain from strenuous exercise and caffeine and alcohol consumption for 24 hours before each testing session. All testing procedures and the informed consent statement were approved by the Indiana University Human Subjects Committee of the Institutional Review Board and conformed to the Declaration of Helsinki. Participants were informed of the benefits and risks of the investigation before signing the institutionally approved consent document to participate in the study. Written informed consent was obtained before participants were enrolled in the study.
Ingestion of a 16% CHO solution was given at 1 of 3 times: 15 minutes before the first TT (PRE1), 15 minutes before the second TT (PRE2), or 15 minutes before the third TT (PRE3). Sugar-free sweet placebo was given before trials in which CHO was not administered. The CHO beverage contained 80 g of sucrose dissolved in 500 ml of water. The sugar-free sweet placebo was also dissolved in 500 ml of water. Subjects were instructed to ingest the entire volume for each feeding within 5 minutes.
During the initial experimental trial, subjects were fitted to the same cycle ergometer according to their preferred seat height and preferred saddle and pedal choice. Seat height was recorded and duplicated in each subsequent experimental trial. The cycle ergometer was interfaced with computer software allowing for autonomous CTL of the workload through an electromagnetic brake applied to the flywheel and for generation of simulated TT courses that were displayed on a computer screen. Before each visit, participants completed a self-directed warm-up (15 minutes cycling at <150 W), and then began the series of three 4-km TTs. Active recovery between TTs was self-directed for 15 minutes, but participants were instructed to cycle at a power output that did not exceed 150 W. Participants were instructed to complete each TT in the shortest time possible and blinded to all feedback except a verbal warning at 1 km remaining during each TT. Mean power output (Pmean) and time to completion (TTC) were recorded for all trials.
Fingertip capillary blood samples (100 µl) were taken on each subjects' arrival to the laboratory to verify that their resting BG concentration did not differ between visits (Table 2). Samples were also collected immediately before each TT. Blood glucose concentration was determined in triplicate and averaged for each time point with an Analox P-GM7 Multi-Assayer.
Statistical analyses were conducted using SPSS 23.0 (IBM Corporation, Chicago, IL, USA) statistical software. Mean power, TTC, and BG for each trial condition were analyzed for differences using a 2-way (condition × time [TT number]) repeated-measures analysis of variance (ANOVA). Data are expressed as mean ± SD. Sphericity of data was assessed using Mauchly's test. When sphericity was violated, the departure from sphericity (ε) was calculated. When ε < 0.75, the Greenhouse-Geisser correction was applied, and when ε > 0.75, the Huynh-Feldt correction factor was applied (11). The Bonferroni adjustment for multiple comparisons was applied when the significant main effects were observed in the ANOVA. Significance was accepted at p ≤ 0.05. Based on previously published findings (23,28), an a priori power analysis (G*Power 3.1, Franz Faul, Germany) at a power of 0.8 (d = 0.9) determined that a minimum of 15 subjects would be required to observe significant differences in TTC, Pmean, and BG concentration.
Physiological and performance data across the complete study for every condition are reported in Table 3. There was neither significant interaction between condition and time for Pmean or TTC nor were there significant differences among conditions (Table 3). However, a significant main effect for time was found for Pmean and TTC. Regardless of condition, performance worsened after TT1. Mean power for TT1 was significantly higher than Pmean for TT2 (p = 0.001) and TT3 (p = 0.004), whereas Pmean for TT2 was not significantly different than Pmean for TT3 (p = 0.69). In addition, a significant effect for time was found for TTC, with TT1 being completed significantly faster than TT2 (p = 0.01).
Blood Glucose Concentration
There were significant interaction effects for BG (p = 0.001). Blood glucose concentration data are presented in Table 3. When CHO was consumed before exercise, BG was unaffected relative to CTL; however, when glucose was consumed between bouts, BG was elevated for the subsequent trials. In the PRE2 condition, BG concentrations were significantly greater for TT2 compared with TT1 (p = 0.006), TT3 compared with TT1 (p = 0.001), and TT3 compared with TT2 (p = 0.01). During the PRE3 condition, glucose concentrations were significantly greater for TT3 compared with TT1 (p = 0.001) and for TT3 compared with TT2 (p = 0.001). Blood glucose was significantly greater immediately before TT1 in the PRE1 condition compared with the PRE3 condition. Finally, BG concentrations were significantly greater for TT3 in the PRE2 condition compared with the CTL condition (p = 0.001) and in the PRE3 condition compared with the CTL condition (p = 0.05).
The main aim of our study was to evaluate whether CHO feeding before, early, or late in a series of repeated high-intensity cycling exercise TTs influenced BG concentration and performance outcomes. Overall, the data indicate that altering the timing of CHO ingestion influenced BG concentration but did not influence performance in repeated high-intensity cycling TTs.
On average, Pmean was significantly lower in TT2 and TT3 comparedwith TT1, and TTC was significantly higher in TT2 compared with TT1; however, we observed no difference between treatment conditions in exercise performance (Pmean and TTC). Despite treatment with CHO before exercise, or between exercise bouts, Pmean and TTC did not improve compared with CTL (no CHO ingestion). Therefore, there does not appear to be an ergogenic effect of CHO ingestion on repeated, high-intensity cycling exercise of ∼18–21 minutes duration (6–7 minutes per bout).
When CHO is ingested during, but not before, prolonged endurance exercise, plasma glucose concentration is maintained, and performance has been shown to improve (8,37). This is likely due to an increase in muscle glucose uptake, or a delay in hypoglycemia (21). However, these performance gains were observed either during or after endurance exercise lasting upward of 2 hours or more. When applied to high-intensity, intermittent exercise, changes in energetic demands, substrate use, and duration of exercise limit the relevance of these findings.
A recent investigation (33) examined CHO ingestion before, or during, 3 high-intensity running bouts lasting ∼2.5 minutes, with a 25-minute recovery period between each sprint. The authors concluded that when sprints were performed with active recovery, the repeated sprint bout performances, including the initial sprint, were not affected by CHO supplementation; this conclusion supports the findings of this study. One possible explanation for the lack of improvement in TTC may be the high-intensity nature of the exercise. Although BG uptake by the skeletal muscle contributes appreciably to overall CHO oxidation in more prolonged exercise of lower intensity, at higher intensities, the contribution of muscle glycogen to CHO oxidation is far greater than that of BG (28). A blunted insulin response when CHO was consumed between exercise bouts may have limited the shuttling of CHO into skeletal muscle in the PRE2 and PRE3 conditions, which may explain, in part, the observed BG response in these conditions and a lack of improvement in performance. However, blood insulin concentration was not measured in this study, and therefore, this line of reasoning remains speculative. The lack of improvement in TTC with CHO ingestion was contrary to our hypothesis, which was in part based on CHO mouth rinse data demonstrating performance improvement. Mechanisms behind the ergogenic properties of CHO mouth rinsing strongly suggest an increase in central motor drive to exercising muscle and a lowering of the perception of effort. Previously, Jeffers et al. (18) investigated the influence of a CHO mouth rinse on a 15-minute TT and found that mouth rinsing attenuated fatigue but did not influence TT performance. These performance results are in agreement with this study and may suggest a time threshold, below which mouth rinsing is ineffective.
During a single, 4-km TT cycling bout without CHO ingestion, BG would be expected to remain fairly constant because of the relatively brief duration of the bout, with the bulk of oxidized CHO being supplied by muscle glycogen stores (3,12). Despite our use of repeated 4-km TTs in this study, our data are consistent with previous studies that have shown stable BG concentrations when CHO was not ingested during high-intensity cycling exercise (i.e., the CTL condition).
Pre-exercise CHO ingestion has been shown to elevate BG in the minutes before exercise, but then decreases toward fasting values at the onset of exercise (8). Our study confirmed this observation, as evidenced by the BG concentration response for TT1 in PRE1 vs. PRE3. However, it should be noted that no statistical difference was observed for BG concentration in TT1 between the PRE1, PRE2, and CTL conditions. Our observation that CHO supplementation did not affect pre-exercise BG in PRE1 relative to CTL was surprising but may be attributed, in part, to a decrease in BG concentration stimulated by increased insulin production, or the larger variability in BG concentration observed in PRE1. In addition, this study showed that an elevation in BG concentration also occurs when CHO is ingested after an intense bout of brief exercise, i.e., CHO ingestion immediately postexercise raised BG in a similar manner as CHO ingestion pre-exercise. This postexercise rise in BG after CHO ingestion is also in agreement with previous findings (13).
An interesting observation in our study was that when CHO was ingested after an initial bout of exercise (PRE2), BG concentration continued to be elevated approximately 40 minutes later, after another intense bout of exercise. Although a rise in BG during TT2 was anticipated, it was expected that a decrease in BG during TT3 (much like the progressive decline in glucose concentration observed in PRE1) would follow. However, BG moved in the opposite direction, being elevated still further beyond TT2 levels. Although this BG response was not what was expected, it is reasonable to speculate that the exercise bout just before the CHO ingestion may have delayed absorption and lessened uptake by the exercising muscle, such that not only did BG remain elevated, but peak concentrations were delayed as well. For example, the previous exercise bout may have upregulated the release of vasoactive hormones, which could impair gastric emptying (23). In addition, the high-intensity nature of the exercise may have reduced blood flow to the gut (29), and coupled with the impaired gastric emptying, may have delayed CHO absorption. In agreement with this, Leiper et al. (19) have previously demonstrated a delay in gastric emptying when performing high-intensity intermittent exercise. Of particular interest in this study was the fact that gastric emptying was delayed, even when the overall exercise intensity was less than 70% of V̇o2max.
In this study, the CHO supplement was derived from a sucrose powder. Previous studies, that are similar in nature to the present one, have used either a glucose polymer or an unspecified CHO. We note the importance of considering the type of CHO used in our study because the ingestion of multiple monosaccharides (as is found in sucrose) is known to upregulate CHO transport from the gut (31). In this study, participants consumed the same CHO source during each visit, but the type of CHO may need to be considered when drawing comparisons with other findings. This is particularly important, given that multiple transporter CHOs (e.g., glucose + fructose) have been shown to increase CHO oxidation rates compared with single transporter CHOs (e.g., glucose only) (10). Given that sucrose, a disaccharide consistence of glucose and fructose, was given as the CHO supplement in this study, both the rate and extent of uptake through GLUT4 and GLUT5 may impact the overall kinetics of uptake. In addition, although all subjects arrived in the postabsorptive state, the composition of diets leading up to the TTs neither was monitored nor was pre-exercise glycogen levels measured. The possibility exists; then glycogen values varied from one condition to the next. However, fasting glucose concentrations taken on arrival to the laboratory did confirm that resting, fasted BG concentrations were consistent between visits. In addition, because of the random ordering of conditions, the likelihood that one condition consistently saw higher or lower glycogen values, or that diet favored one condition over another is likely not a factor. Regardless, we acknowledge that a lack of dietary CTL is a limitation to this study.
Finally, the data from this study were based on a CHO load of 80 g in a 16% solution. Previous research has shown that ingesting CHO at a rate >30–60 g·h−1 offers little in the way of performance improvement. Furthermore, high CHO loads can actually be counterproductive as gastric emptying may be delayed, and gastric distress may ensue (4,36). Indeed, exercising in the heat CHO concentration should be kept minimal (∼8%) to enhance gastric emptying and fluid absorption (22). However, this study took place in a thermoneutral laboratory setting, and we acknowledge a limitation in this study that no measures of gastrointestinal distress were recorded. This study sought to optimize CHO availability and provide approximately 1.2 g·kg−1 of CHO. These rates are in line with recommendations for CHO replenishments (1), although emphasis should be placed on the fact that the 1.2 g·kg−1 recommendations are meant to span the hour immediately postexercise, and thus the high rate of intake likely saturated the capacity for absorption and exogenous CHO availability. A similar concentration has been used previously (14), and some studies have used even greater concentrations of CHO (17). Still, we recognize that the concentration was above the ∼6–8% concentrations more commonly used, and note that this CHO load is above recommended concentrations for maximal absorption of fluids and the minimal CHO load necessary to see improvements in performance (17,20).
In summary, we have demonstrated that the timing of CHO ingestion during a series of repeated, high-intensity cycling bouts did not influence performance, but did influence BG concentration. When CHO was ingested before the start of the exercise session, BG concentration did not change over the course of 3 high-intensity cycling bouts, whereas when CHO was ingested between exercise bouts, BG concentration rose in subsequent exercise bouts. Our findings suggest that CHO ingestion is not necessary for performance optimization before or during repeated high-intensity 4-km cycling exercise.
Our data indicate that competitive cyclists do not benefit from ingesting CHO before or during a series of repeated, high-intensity 4-km cycling bouts. However, the timing of CHO ingestion did influence BG concentration. Furthermore, given that the ingestion of exogenous fluids and CHO may influence gut activity and discomfort, it may be in the best interest of cyclists to abstain from ingesting CHO before or during intermittent, high-intensity cycling bouts (e.g., high-intensity interval training or repeated heats in events such as track cycling).
The authors thank the participants of this study. The authors have no funding or conflicts of interest to disclose. Research was conducted in the Human Performance Laboratory in the Department of Kinesiology at Indiana University.
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