Essay: Physiological responses to a moderate intensity cycle while consuming carbohydrate drinks

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  • Physiological responses to a moderate intensity cycle while consuming carbohydrate drinks
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The ingestion of carbohydrate (CHO) has been proven to improve cycling endurance capacity and performance during prolonged exercise. Research in this area didn’t start till around the 20th century; the importance of CHO feeding was first recognised by Krogh and Lindhard (1920). They found that athlete’s found exercise easier when they had consumed a high-CHO diet to when they consumed a high-fat diet. Triglycerides need to be hydrolysed to fatty acids and glycerol to release energy, however to produce the same amount of ATP as CHO, the oxidation of fatty acids requires more oxygen. To experience elevated plasma glucose, it has been found that progressive consumptions are more beneficial, as after 60 minutes, the plasma glucose levels are the same as before consuming the CHO drink (Blacker et al., 2011). Coyle et al (1986) investigated the physiological changes when consuming a high-carbohydrate meal 4 hours before exercise. It was discovered that this caused an increase of 45% in carbohydrate oxidation, and a 42% elevation of muscle glycogen, which ultimately spared the blood glucose levels, resulting in an improvement in endurance performance. In contrast, it has been found that an athlete with higher endurance in cycling, had a higher percentage of Type 1 muscle fibres (Coyle et al., 1991), with a correlation coefficient of r=0.75, indicating a very good correlation between the two. A depletion in muscle glycogen stores can be seen to cause fatigue due to the decreased release Ca2+ (Duhamel, Perco and Green 2006). Therefore, to prevent this it is vital to spare the muscle glycogen stores by increasing either the liver glycogen stores or blood glucose. Recent research investigated the consumption of a glucose-fructose solution compared to varying levels of glucose solutions and a placebo, while cycling at a moderate intensity (Baur et al., 2014). Blood glucose levels were recorded as “very likely” to increase compared to the placebo trial but it was “unclear” whether moderate glucose levels had a significant difference during the late-exercise stages. During this investigation, the statistical power and validity of the experiment were notably reduced. The participants that volunteered were all male, resulting in a lack of ecological validity, only eight complete datasets were recorded, decreasing statistical power, and internal validity was also decreased as the age of the participants were highly distributed, shown by the high standard deviation.

Therefore, the aims of this study were to investigate the physiological responses to a moderate intensity cycle for an hour while consuming a carbohydrate drink. Due to the previous research, it was hypothesised that ingesting a carbohydrate progressively, resulting in the participants blood glucose increasing, and therefore increasing carbohydrate utilisation. This would spare muscle glycogen and delay the fatigue response, making the athlete perceive the exercise as easier than in the placebo trial.


Participants. A total of fifteen second year Sport Science students, five females and ten males, volunteered to participate in the study; their physiological characteristics are shown in Table 1. They were all studying at Nottingham Trent University. Each individual had the procedures and associated risks explained prior to providing written informed consent to participate in the study, a health screen was signed, and approved by the Nottingham Trent Sport Science team.

Table 1. Participants physiological characteristics, displayed as mean  SD.
Physiological Characteristic
Gender Male (n=10)
Female (n=5)
Height (cm) 172  6
Body Mass (kg) 67.2 11.3

Design. Each subject completed two exercise trials in a double-blind, placebo controlled, randomised cross-over study design, separated by three weeks. The trials consisted of 60 minutes of cycling while either ingesting 300ml of sugar free orange squash or 6.4% CHO drink, 300ml of sugar free orange squash mixed and 19.2g of dextrose. The drinks were made up by the technicians before each experimental trial. The laboratory temperature was maintained constant throughout the trials. After the consent forms had been approved, body mass (BWB0800, Allied Weighing, Hoogoorddreef 56e, 1101BE Amsterdam, Netherlands) and stature was measured, and a heart rate monitor (FS1, Polar, Kempele, Finland) was fitted. The participants were asked to keep the average power output the same between the two trials.
Experimental Trials. A blood sample was taken and heart rate (HR) was recorded before the participant started cycling. The blood sample was taken and analysed through Biosen (EFK, Penarth, Cardiff, Wales). A 3-5 warm-up was completed on a Wattbike (Wattbike, West Bridgford, England) to adjust the workload to elicit a heart rate response of 135 bpm. Once the warm-up was completed, the participant was asked to stop and made sure that they knew what was required of them. During the experimental trial, the participant was required to drink the solution after 5, 20 and 35 minutes of the 60-minute cycle. Participants rated their level of perceived exertion (RPE) on a 15-point Borg Scale (Borg 2006) every 15 minutes, as well as power output and volume of O2 and CO2 expired. Blood samples were taken at minute 16, 31, 46 and 60. Servomex (Mimi HF 5200, Servomex, Crowborough, East Sussex, England) was used to analyse the Douglas bags, and a dry gas meter (Harvard Apparatus, Cambourne, England) was used to measure the volume of the bags.

Calculations. CHO and fat oxidations rates (g.min-1) and Respiratory Exchange Ratio (RER) were calculated from VO2 and VCO2 (L.min-1), with the assumption that protein oxidation during exercise was negligible.

CHO oxidation=4.586〖VCO〗_2-3.226〖VO〗_2
Fat oxidation=1.695〖VO〗_2-1.701〖VCO〗_2

Statistical Analysis. Paired t-tests were used when two means were compared. Two-way ANOVA were used to compare differences in substrate utilisation and blood metabolites between trials, where contrasts were calculated. Statistical analyses were performed using SPSS (IBM SPSS Statistics 24, Chicago, IL).


T-test analysis indicate that the RER response was higher in the carbohydrate trial compared to the placebo trial (t=-3.67; p=0.04). RER was significantly difference between trials (F(1,14)=44.63; p=0.00) and a significant main effect of time was found (F(1.78,24.94)=5.35; p=0.01). Contrasts revealed that RER was higher at each timepoint when compared with the first 15 minutes (p<0.00). There was a significant interaction effect between the trial completed and the time the variable was measured (F(3.42)=8.96; p=0.00), and the contrasts revealed that the significant difference occurred between the first and the last 15 minute interval (F(1,14)=14.01; p<0.00).

Figure 1. RER responses during cycling. The RER decreases during the placebo trial, whereas it is maintained during the carbohydrate trial.

T-test analysis indicate that the blood glucose levels were higher in the carbohydrate trial compared to the placebo trial (t=-5.2; p=0.01). There was a significant main effect between the trials (F(1,14)=18.57; p<0.00), and a significant main effect of time was found (F(4,56)=57.90; p=0.00). Contrasts revealed that blood glucose levels were significantly different at each timepoint when compared to pre-exercise (p<0.00). Figure 2. Interaction of Blood Glucose levels between trial 1 (Placebo) and trial 2 (Carbohydrate).

T-test analysis indicate that the blood lactate levels had no significant differences between trials (t=-1.78; p=0.173). There was a significant effect of time on the blood lactate levels (F(2.46,34.42)=37.87; p<0.00); contrasts revealed that these all occurred when comparing the timepoint to the pre-exercise level. No significant main effect of trial was found.

Repeated Measures ANOVA showed that there was a significant difference in RPE between trials (F(1,14)=51.725; p<0.00) and there was no significant main effect on time (F(1.86,26.03)=0.07; p=0.92).

Figure 3. Interaction in RPE between trial 1 (Placebo) and trial 2 (Carbohydrate).

There was a significant main effect of time (F(1.43,19.97)=8.121; p=0.01) as well as trials (F(1,14)=6.081; p=0.03). Contrasts revealed that when compared the first 15 minutes to another, there was only a significant difference between the first and the last 15 minutes (F(1,14)=6.16; p=0.03). There was a significant main effect between trials (F(1,14)=6.08; p=0.03). There was a significant interaction effect between the trial completed and the time the variable was measured (F(1.03,14.39)=6.16; p=0.03).

There was no significant main effect of trial on the Power Output (F(1,14)=3.447; p=0.09), no significant main effect of time on the power output (F(1.04,14.52)=3.64; p=0.08) and no interaction effect between the trial completed and the time the variable was measured (F(1.04,14.51)=104.33; p=0.55).


In this study, we investigated the CHO oxidation and blood glucose levels during cycling compared to the placebo trial. The primary findings of this study was that blood glucose levels increase when ingesting exogenous CHO, and it increases carbohydrate utilisation. This can be shown by the RER value, or the two equations stated in the methods section.

Previous research investigating carbohydrate ingestion on blood glucose and lactate concentration during a soccer match, and Russel, Benton and Kingsley (2014), found that the “supplementation influenced the blood glucose response to exercise”, and although soccer is a high-intensity intermittent sport, the similar results in the current study provide evidence that the previous CHO ingestion findings can be extrapolated to moderate intensity, continuous exercise. During this research it was also found that blood lactate concentrations increased during the first 15 minutes, but the supplementation did not influence this pattern. There is evidence of this during our findings, where no significant main effect of trial was found.

The interaction between time and trial of blood glucose is shown in figure 2. During the placebo trial, the levels of blood glucose decreased, whereas during the carbohydrate trial the blood became more saturated with glucose. As carbohydrates are broken down to glucose and other monosaccharides in the small intestine, the blood is absorbed into the bloodstream via the villi by active transport. The glucose then travels in the bloodstream and taken up into the cells for glycolysis to produce ATP. As carbohydrates are being ingested, this then in turn increases blood glucose.

When ingesting CHO, RER was found to increase, as carbohydrate oxidation had a greater predominance, and therefore producing a greater volume of CO2. By using the equations, it was found that during the CHO trial, the participants used 91% of the time compared to the placebo trial where they used 77% carbohydrates. This is due to glucose being more readily available, and easier to breakdown to ATP, instead of oxidising fats. This ultimately spares muscle glycogen and then delays fatigue shown by Duhamel, Perco and Green (2006). Although this can be largely affected by exercise intensity, the participants were told to exercise at the same power output across the trials and this was shown in the results where there was no significant difference between the trials, time or time and trial interaction.
The rate of perceived exertion interaction can be seen in figure 3. Up until timepoint 3, 45 minutes, the ratings decreased in the carbohydrate trail, while the ratings increased during the placebo trial. When glucose is broken down, the maximum rate of energy production is greater and therefore more ATP can be produced. The participants then perceive that the exercise is of less intensity, as there is more ATP available for muscular contraction. The last CHO drink was consumed at 35 minutes and so at the 60 minute timepoint, the participants had not consumed CHO for 25 minutes. There would be less glucose as readily available as there was at 45 minutes, therefore less ATP would be available for muscle contractions, making muscle glycogen be used, which speeds up fatigue.

Flynn et al. (1987) found that the glucose drink did increase the blood glucose levels at 60 minutes, also evident in our research, but they did not find significant difference in the rate of perceived effort or the respiratory exchange ratio, however Coyle et al. (1986) reported that the blood glucose performed similarly between the carbohydrate and placebo trial, which contradicts our research and the work of Flynn et al. (1987). Although in the work of Coyle et al. (1986), the participants were asked to cycle to exhaustion and therefore different workloads would have been recorded, making the comparison difficult. Coyle et al. (1986), investigated the muscle glycogen used, where we only looked at the blood glucose levels, creating a limitation. To improve this next time we could look at muscle biopsies to see if using blood glucose does spare muscle glycogen. If the research was to be done again, longer exercise would be beneficial to investigate the effects of glucose drinks on prolonged exercise.

Consuming a glucose drink is beneficial when exercising; this is dependent that moderate intensity is achieved, when cycling at lower intensity, the CHO drink may not have the benefits.

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