Shei, R., Paris, H. L., Beck, C. P., Chapman, R. F., & Mickleborough, T. D. (2017). Repeated high-intensity cycling performance is unaffected by timing of carbohydrate ingestion. Journal of Strength and Conditioning Research, 1. doi:10.1519/jsc.0000000000002226
Introduction: Many team sports require short bursts of intense movement interspersed with lower intensity periods. Repeat bouts of sprints provide a unique energy requirement, as this type of movement requires the use of carbohydrate as the main source of energy. Anaerobic glycolysis is the main energy system used during high intensity repeat sprints. The activation of this energy system results in high burn rates of glycogen, especially during shorter and higher intensity bouts. Thus, exercise and sporting performance in repeat sprints may be influenced by pre or intra carbohydrate feeding. This feeding would theoretically result in a greater availability of carbohydrate and result in delayed fatigue and time to exhaustion. Most of the research concerning pre-trial carbohydrate has been undertaken in the endurance model. Preliminary research into high-intensity repeat bouts suggests even small amounts of carbohydrates or rinsing may be beneficial to performance, in particular, increasing power output over a short duration. The mechanism for this potential improvement remains unclear. One proposed mechanism is the activation of brain regions in response to oral carbohydrate receptors. This would explain the effects of carbohydrate rinsing. Carbohydrate intake pre-exercise results in a release of insulin, which inhibits or reduces fat mobilization while increasing carbohydrate oxidation. In addition, the mixed results of previous data may be secondary to differences in response to low blood glucose. Carbohydrate ingestion is believed to delay hypoglycemia, resulting in improved performance. It has also been hypothesized large intakes of carbohydrates may produce hyperglycemia, which could reduce the oxygen delivery to working muscles. Previous research has focused solely on ingestion either prior to, or throughout exercise. Therefore, the purpose of this research study was to determine whether carbohydrate (CHO) feeding taken immediately before, early, or late during high intensity cycling affects time-time performance. A second purpose was to decipher whether blood glucose response to CHO differed in reference to the time of ingestion.
The authors thoroughly explained the background and theory behind carbohydrate timing and performance. The first question which comes to mind is, are glycogen stores the main limiting factor to anaerobic repeat performance? If the answer is yes to this question then I could see ingesting carbohydrates as being beneficial. If fatigue occurs secondary to another mechanism, then carbohydrate ingestion may not beneficial. This study is similar to the previous article I wrote about regarding carbohydrate loading in soccer players. In those athletes, carbohydrate loading a few days pre-match increased performance. The difference with this study is these subjects are less likely to be in a state of glycogen depletion.
Subject Description: The subjects were 16 trained competitive male cyclists from a university population and surrounding community. All subjects were considered healthy and regularly engaged in a cycling interval training program. The minimum experience required was 3 months and subjects were required to be engaged in a least 8 hours of training per week. All subjects were familiar with a 4 km time trial. The demographics were reported in a table format.
The table format made it easy to decipher the training status of the individuals. It was clear these individuals were experienced cyclists, with the average training experience of nearly 3 years (34 months) and average age of 21.1 y. The authors also justified the sample size. They noted in order to achieve a power of 0.8 with p<0.05, they would need a minimum number of 15 subjects.
Methods: This study used a repeated-measures, randomized, double-blinded experimental design. Subjects were required to arrive at the lab 6 hours post-prandial to each testing session. Maximal oxygen consumption (VO2max) was determined at baseline on a cycle ergometer using a workload of 100 W which increased stepwise 25 W each minute until the power output could no longer be sustained. Successful achievement of VO2max was considered if heart rate ≥ 90% of the age-predicted maximal heart rate (220- 85 participant’s age), a respiratory exchange ratio (RER) ≥ 1.10, and evidence of a plateau in V̇O2 with an increase in exercise intensity. Each subsequent visit consisted of three 4 km time trials. Each trial was separated by a 15 minute active recovery period. Sessions included a familiarization session to reduce the learning effect. Following this session, the experimental sessions included a control and three different CHO protocols. The order of these session was randomized.
Carbohydrate ingestion consisted of a 16% CHO solution administered at one of three times: 15 minutes pre-trial (PRE1), 15 minutes before the second trial (PRE2), or 15 minutes before the third trial (PRE3). Sugar free sweet placebo was given during the control. The solution consisted 80g sucrose dissolved in 500 ml of water. During time trials, subjects were placed in the same cycle ergometer with a preferred seat height along with saddle and pedal choice. This protocol was repeated on subsequent tests. Prior to the testing, subjects completed a 15 min warm-up. Active rest periods were self-paced but were instructed to remain under 150 W. Subjects were instructed to complete the time trial in as short as time as possible, with a verbal warning with 1 km left. Mean power output and time to completion (TTC) were recorded for all trials.
Capillary blood samples were obtained via the fingertip and taken upon arrival at the laboratory. The sample was used to ensure blood glucose levels were the same between visits. Additional samples were taken before and after each time trial.
The methods to this study were pretty straightforward. They authors provided a placebo, and then explained the procedures which would allow future replication. One detail I wish the authors would have covered a little more was the decision to choose sucrose as the source of carbohydrate. A reasoning behind choosing this over say glucose or fructose would have been informative.
Results: There were no significant interactions between conditions and time for Pmean or TTC. There was also no significant difference among conditions. A significant main effect for time was found for Pmean and TTC. All performances decreased following TT1. Pmean was significantly higher for TT1 than TT2 (p=0.001) and TT3 (p=0.004). Pmean for TT2 was not significantly different for TT3 (p=0.69). TTC1 was completed significantly faster than TT2 (p=0.01).
Significant interaction effects were found for blood glucose (p=0.001). PRE ingestion did not have an impact on blood glucose relative to control. Inter-trial ingestion of carbodyrates resulted in blood glucose elevation during the subsequent trial. PRE2 blood glucose was significantly greater for TT2 than TT1 (p=0.006), TT3 compared to TT1 (p=0.001), and TT3 compared to TT2 (p=0.01). PRE3 blood glucose was significantly elevated for TT3 compared to TT1(p=0.001) and TT2 (p=0.001). Blood glucose for PRE1 was significantly greater prior to TT1 compared to PRE3. Lastly, blood glucose for TT3 in PRE2 and PRE3 were significantly greater than the control (p=0.001)
Similar to the methods, the results in this study were pretty straightforward. There wasn’t a whole lot of variables measured and thus the reporting was short. It was presented in a logical manner with accompanying tables and figures making it easier to get a feel for the actual data.
Discussion: The main finding of this study was CHO feeding, regardless of timing, failed to produce performance benefits in trained cyclists. The data did demonstrate various timing of CHO ingestion produced rises in blood glucose during subsequent time trials. This rise in blood glucose failed to play a role in improving performance. The fact Pmean decreased following TT1 indicate fatigue was not attenuated via CHO ingestion. The reasoning for this failure may be due to the intensity of exercise. In this study, the high intensity nature of the time trial may have caused a preferential shift to use muscle glycogen rather than blood glucose as a fuel source. The inter-trial ingestion of carbohydrates may have been effected by a blunted insulin response, resulting in a failure to shuttle CHO into the muscles. Future work should include insulin levels to reaffirm this theory. The authors acknowledged most of the work done on CHO has used glucose as the main source. The authors used sucrose because of its ability to upregulate CHO transport from the gut. They also acknowledged the potential role of a particular CHO may influence performance. A limitation to this study was the subjects’ diet up to the trial and so-forth were unregulated. The authors chose 80 g of sucrose to provide ~ 1.2 g/kg, which has been shown to replenish CHO.
Conclusion: In conclusion, it appears carbohydrate ingestion, regardless of timing does not appear to influence anaerobic repeat performance in trained cyclists. Future work should focus on determining the impact of different CHO types, exercise modes, and diet.
My Thoughts: This study was simple, yet practical. The authors came up with a study which can be applied to many different settings. It doesn’t appear acute ingestion of CHO impacts anaerobic performance. It will be interesting if future research is in agreement with the authors hypothesis that muscle glycogen is most likely the preferred energy source vs. blood glucose. This would be in agreement with the previous soccer research study I mentioned, which performed carbohydrate loading over the days prior to competition. Soccer players are probably more likely to be in a reduced muscle glycogen state (due to repetitive practice), making the loading probably more effective. Also, it appeared the CHO was difficult for the body to digest during the exercise. This was shown as the blood glucose remained sustained for periods longer than anticipated. This is probably because the decreased rate of digestion, thus greater sustained release of CHO. For now, it appears if an athlete is at risk for being in a state of reduced muscle glycogen, loading may be effective (via supercompensation). If they are not, then carbohydrate timing and loading will probably be ineffective.
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