Carbohydrate & Cycling Performance
by Benjamin Bunting BA(Hons) PGCert
Written by Ben Bunting: BA(Hons), PGCert. Sport & Exercise Nutrition. L2 Strength & Conditioning Coach.
It is hypothosised that the ingestion of a carbohydrate sports drink will enhance fuel utilisation rates and performance during a 10k time trial compared to placebo in a recreational athlete.
This article is a hypothetical experiment and a review of the current available literature.
Carbohydrate and Sport
Athletes, whether recreational or elite will look to improve their performance. One of the two main sources of energy for the human body is carbohydrates (Jequier, 1994).
Various investigations within the scientific community from as early as the 1930s have indicated that increasing carbohydrate intake for activity increases energy stores which are used by the muscles to improve performance (Hohwu Christensen et al., 1939).
During digestion, carbohydrates and starches are broken down and converted into glucose which can be used immediately as fuel or is stored as glycogen.
Glycogen, can deplete rapidly as the body only stores relatively low amounts in skeletal muscle (300-900g) and liver (80-100g).
Carbohydrate consumption during activity is supported by the American College of Sports Medicine (ACSM) which recommends an intake of 30-90 grams of carbohydrate per hour of exercise. (Jeukendrup, 2014)
Glycogen provides up to 2000 to 3000 calories of energy which can fuel up to 120 minutes of high intensity exercise (Jeukendrup & Gleeson, 2004; Girard, 2000) Glycogen can be mobilized quickly and broken back down into glucose yielding high-energy ATP molecules through glycolysis to provide energy for muscle contraction (Berg et al., 2002)
The amount of glycogen stored is dependent on carbohydrate consumption, the body is always using (and replenishing) glycogen and blood glucose for exercise.
As the body requires less oxygen to use carbohydrates as fuel than fat it is regarded highly efficient being the only macronutrient that is rapidly broken down to fuel intense exercise (Helge, 2017).
Jeukendrup (2004) notes that single carbohydrates would saturate transporters, maximum benefit requires a combination of carbohydrates (eg. glucose and fructose) increasing oxidation by using different intestinal transporters for absorption.
Investigation has highlighted performance benefits including reduced fatigue when two carbohydrates are ingested compared to a single carbohydrate.
Endurance athletes (exercise in excess of 90 minutes) can suffer from glycogen depletion. Depleted glycogen means less or no carbohydrates being delivered to the muscles which can lead to fatigue and impair performance (Kanter, 2017; Havemann et al., 2006).
Once low on carbohydrate, the liver responds by producing lipid-derived molecules called ketone bodies. These ketone bodies become the alternative fuel source burning fat in the process (DelMedico & Lov 2020).
However, it is suggested that a state of ketosis does not hold the same level of oxidation efficiency compared to glucose (Prince et al., 2013).
The need for a high carbohydrate diet has been identified by a report published in 2008 that claims athletes do not consume enough carbohydrate to replenish glycogen (Cox et al., 2010).
Recommendations range from 5-7 gram per kilogram of bodyweight for recreational athletes up to 8-12 gram for elite athletes (Thomas et al., 2016).
The measurement of performance in an endurance event is the power or velocity generated and maintained from thirty minutes to four hours. However, performance can be limited by lactic acid and hyperthermia (Coyle, 1999).
Thus, performance is centred on three main factors which include lactate threshold, maximal oxygen consumption and efficiency (Joyner & Coyle, 2008).
Literature dictates that to determine performance VO2 there is an interaction between lactate threshold and VO2 max.
This is a point whereby oxygen consumption can be sustained for a certain period. Additionally, efficiency interacts with performance VO2 to establish the amount of power or speed that can be produced with the consumed oxygen. There is limited knowledge regarding efficiency in endurance sports when compared to that concerning VO2 max and lactate threshold.
For example, a road cyclists’ VO2 max would be 70-80 ml/kg/min. Yet, in terms of lactate threshold, a work rate of 50-60% of an athlete’s maximum oxygen intake does not increase muscle lactate as the appearance and subsequent disappearance of blood lactate is of equal measure (Ghosh, 2004).
A study into the comparison of black Eritrean distance runners with Caucasian Spanish runners concluded that the Eritrean’s have a better running economy (lower VO2 cost), and their success is not based on enhanced aerobic capacity (Lucia et al., 2006).
Understanding these factors can help towards determining performance levels, as hypothesised by M J Joyner, albeit the fastest time to be predicted based on what is theoretically possible has yet to be achieved by an athlete, it could be argued that there are limitations (Joyner, 1991).
Jeukendrup (2010) identifies carbohydrate feeding to be an ergogenic aid but carbohydrate consumption guidelines for intensive exercise can be difficult to achieve.
As such there are products available that can include sources of carbohydrate to replenish glycogen stores conveniently and to minimise gastric distress (Prado de Oliveira & Burini, 2014).
The most popular ‘functional’ products are sports drinks, these can help improve performance by containing carbohydrates, electrolytes and nutrients whilst also hydrating the athlete (Evans et al., 2017). Other may help increase strength, muscle mass or maintain general health and prevent interruptions in training from injury or competition schedules (Maughan, 1999).
A study measuring nutritional and ergogenic aid strategies in ultraendurance mountain events demonstrated those who performed better ingested the most nutritional aids (Belinchon-Demiguel & Clemente-Suarez, 2018).
Therefore, based on the available literature it is hypothesised that the carbohydrate drink will demonstrate an improvement of performance compared to the placebo.
The participant is a female recreational cyclist, 23 years of age, 160 cm tall, and 51 kg body mass. Weight and height were measured by portable scales (SECA) and a stadiometer (SECA) respectively.
Blood pressure (Omron Digital Blood Pressure Monitor HEM-907), heart rate (HR) (Polar Heart Rate monitor), and weight (SECA scales) were monitored at the start of each visit.
The participants performed the trials at the same time of the day (~ 11 am) having had a standardised breakfast around 8 am.
Visit 1: Familiarisation trial
In addition to screening the tests and testing procedures were fully explained to the participant. Subsequently, the participant cycled for 30 min on a Lode-Excalibur bike at an estimated intensity of ~60% of their maximum oxygen uptake (VO2 max).
Visit 2: Baseline trial (VO2max)
Peak power output (Wpeak) and VO2 max were measured using a Lode-Excalibur bike and a Cortex MetaLyzer 3B-R3.
The participant performed a 2 min self-paced warm up before commencement of the test at an initial workload of 60 watts, which was then increased by 25 watts every 2 minutes.
The test stopped when the participant reached exhaustion as determined by maximal exertion using the rating of perceived exertion (RPE) scale by Borg.
Visits 3 and 4: Experimental trials (Carbohydrate or Placebo)
This was a double-blind randomised placebo-controlled trial whereby the participant ingested either a placebo or a CHO-based drink 30 min before completing an exercise trial, which included a 30 min steady state at 60% of the participant’s VO2 max and a 10 km time trial (TT).
The carbohydrate drink (CHO) was made using 480ml water, 60g of unflavoured dextrose powder, strawberry flavoured drops.
The placebo drink (PLA) was made with 480ml water and strawberry flavoured drops. Urine osmolality was measured at the start of the experimental trials by a portable analyser (Osmocheck).
RPE and heart rate (HR) were monitored every 5 minutes during and at the end of the steady trial. Upon completion of the steady state, the participant switched immediately to a Wattbike Pro Cycle Ergometer to undertake a 10km trial.
Blood glucose and lactate concentrations were measured at the start of the trials, 30 min post-drink ingestion and immediately after the end of the time trials by collecting fingertip blood samples. The analysis was completed by a YSI 2300 STST Plus.
Urine osmolality was within the normal range as was resting heart rate and blood pressure.
The CHO TT result was 18 minutes 10 seconds. The PLA TT duration was 18 minutes 20 seconds.
Comparison of blood lactate levels before, after consumption of drinks and post TT.
Blood lactate levels were significantly higher after the CHO trial compared to the PLA by 13%. (Figure 1)
Comparison of blood glucose levels before, after consumption of drinks and post TT.
Blood glucose levels were elevated 30 minutes after CHO drink ingestion compared to PLA by a significant margin of 23.2% (Figure 2)
Post TT the blood glucose levels were still higher for the CHO versus PLA by 12%.
Blood glucose levels for CHO post consumption and after TT reduced by 4.2%.
The glucose levels increased by 4.9% after the TT compared to 30 minutes after consumption of the PLA drink.
Analysis of substrate oxidation
Both the carbohydrate and fat oxidation rates from gas exchange elements were calculated using the Modified Stochiometry Equations (Jeukendrup & Wallis, 2004).
Mean carbohydrate oxidation rates of 5-minute intervals during a 10km cycle TT comparing a carbohydrate drink versus placebo.
Figure 3 illustrates significant differences for oxidation rates of CHO and PLA over a 30-minute period during the 10km TT.
Illustrating a 24% difference between peak mean averages of both drinks. This correlates with a significant difference in trends for both sets of data.
Mean oxidation rate for CHO peaked sharply between 5 and 10 minutes by 14% reducing rapidly from 10 to 15 minutes by 11%. The oxidation rate plateaued with a 4% decline in value from 15 to 25 minutes rising again 1.25% from 25 to 30 minutes.
Oxidation rate of carbohydrate for PLA presents a near identical sequence of peaks and troughs every 5 minutes of 8%.
Mean fat oxidation rates of 5-minute intervals during a 10km cycle TT comparing a carbohydrate drink versus placebo.
Mean fat oxidation is higher for PLA during the TT versus CHO.
Data in figure 4 illustrates that the CHO trial oxidised less fat than PLA with a peak mean oxidation difference of 111%.
The trend for the CHO fat oxidation rate is almost opposite for the carbohydrate oxidation. The mean data drops sharply during the initial 5 minutes to rise between 10 and 15 minutes with a plateau showing only slight variations on oxidation rates.
PLA fat oxidation shows a similar trend as the CHO trial displaying oxidation peaks and troughs with 26.6% difference in values. The initial 5 minutes of the TT shows no increase of oxidation.
Carbohydrate & Cycling Performance Discussion
The purpose of this report was to demonstrate whether the ergogenic effects of a carbohydrate drink would increase the performance of a recreational cyclist during a 10km TT when compared to a placebo.
The main finding is that the duration of the CHO TT was quicker than PLA, but did not yield a significant difference.
However, there was a statistically significant level of carbohydrate oxidation after CHO was consumed versus PLA supported by higher blood lactate volumes.
Carbohydrate & Cycling Performance Interpretation of Results
The TT was less than 90 minutes and did not lead to a significant difference in results. This is a contrast to some findings which appears to yield an increase in performance (Mcmurray et al., 1982) with the common acceptance that muscle, and liver glycogen depletion often results in fatigue during endurance events (Hawley, 2001; Coyle & Coggan, 1984; Yaspelkis et al., 1985; Hulston & Jeukendrup, 2009) particularly since a large performance improvement has been identified when 60g of carbohydrate has been ingested (Smith et al., 2010).
Figure 1 illustrates raised blood lactate levels normally associated with physical exertion. The increase in work rate (watts) and increase of blood lactate correlates with intense exertion as previously reported (Goodwin et al., 2007).
Data in figure 2 shows raised blood glucose levels for CHO which is expected post consumption and the TT.
Literature dictates that higher blood lactate accumulation as seen in figure 1 results in increased carbohydrate oxidation (figure 3) with reduced fat oxidation (figure 4) (San-Millan & Brooks, 2017).
This reduction may be the result of a decreased free fatty acid transport into the mitochondria (Ormsbee et al., 2014).
Pre-TT feeding of carbohydrate would have provided glucose availability, the results of figure 3 illustrate increased glucose oxidation correlating with literature (McGarry et al., 2014). This increase of CHO oxidation correlates with literature outlining supplementation (Costill et al., 1977).
Studies associate an increase of carbohydrate oxidation with increases in mean power output with time improvements during TT tests due to the additional carbohydrate being able to maintain plasma glucose levels (Stellingwerff et al., 2007).
An increased carbohydrate oxidation rate (illustrated in figure 3 that is higher than the placebo) is considered to be essential for the increase of endurance performance (Correia-Oliveira et al., 2013).
Simultaneously this effect would also reduce fat oxidation (Newell et al., 2018) which is illustrated by the lower fat oxidation rates for the carbohydrate drink TT (figure 4).
Carbohydrate & Cycling Performance Implications
The experiment utilises the time trial test as it offers a resemblance to actual competition and can predict real performance, the variation in changes are lower than that of a time to exhaustion test although it could lead to pacing issues for the cyclist.
The steady state trial is set at 60% of VO2 max which can allow for a sustained effort over the 30 minutes and (George et al., 2009) fatty acid substrates are identified as being important for moderate intensity activity whereas carbohydrates will be the preferred fuel source for the TT as lipolysis is supressed (van Loon et al 2003).
The statistical data supports the hypothesis that carbohydrate supplementation prior to the TT would enhance performance, yet the timed result does not follow the statistical trends.
An aspect to consider is the timing of carbohydrate consumption. It has been identified for trained male cyclists that there is a potential ergogenic benefit of carbohydrate supplementation for exercise periods in excess of 90 minutes with (Pochmuller et al., 2016) studies suggesting TT improvements are seen when carbohydrates are ingested during to maintain plasma glucose levels. A mouth rinse has been identified as beneficial (Correia-Oliveira et al, 2013).
Data has shown mixed results based on the timing of carbohydrate consumption before exercise particularly when consumed 30 minutes prior as with the TT (Ormsbee et al., 2014). Figures have reported an enhanced performance rate with pre-loaded carbohydrate for a longer duration TT (Wright et al., 1991) whereas studies of durations less than 2 hours have reported either no change (Sparks et al., 1991) or very few reports of enhanced performance (Sherman et al., 1991).
These results do not track improved endurance performance seen for a 1 hour TT with pre-exercise carbohydrate consumption (Coyle et al., 1985).
The reliability of this data is confounded by some factors. Only 1 trial each of CHO and PLA was performed. Additional TT’s would provide more data to identify potential trends or inconsistences.
The recovery time between the TT’s was unknown. Sufficient muscle recovery is considered to take from 48 to 72 hours. The second trial was potentially performed prior to 48 hours (ACSM, 2019).
The TT test has environmental limitations, particularly knowing the distance to finish. Using a recreational cyclist rather than an elite cyclist can potentially yield unreliable pacing and lack of experience can impact consistency.
Urine osmolality fell within the normal range at rest prior to both trials which would suggest they are hydrated, drinking the 480ml of fluid could lead to a fluid/electrolyte imbalance and overhydration which could impair performance (Kenny, ACoE).
It could be recommended that multiple trials are completed with more participants including strict recovery protocols more than 72 hours to ensure full body recovery between CHO and PLA trials.
To reduce pacing variables elite cyclists should be used, if recreational athletes are the only option 3 familiarisations trials would help toward limiting variables.
Carbohydrate & Cycling Performance Conclusion
The physiological data suggests that there would be a significant difference between the two TT results. Increased blood glucose, lactate volumes along with higher rates of CHO oxidation point towards reduced fatigue and greater performance indicators.
The results did not reflect these values. Yet there were limitations as part of the study that could have been compounding factors towards the statistics. Supporting literature also has reported inconsistent results regarding similar CHO consumption timings prior to exercise and the TT duration being less than 2 hours.
Therefore, in terms of a time trial that is less than two hours, while there is a slight performance improvement of drinking a carbohydrate drink it did not yield a significant increase of performance than the water in the amateur cyclist.
This result is also smilar to some other studies. Carbohydrate feeding appears to be more beneficial for exercise over 90 minutes. Note that in this hypotherical experiment there was still an improvement, just not to the degree as perhaps expected.
Of course, if this slight improvement was replicated in a professional cyclist it could potentially be the difference of a podium finish ir not.
It is worth noting that any athlete would benefit by following the Position Stand for Nutrition and Athletic Performance by the ACSM whereby CHO consumption is outlined to acheive optimal results in their discipline.
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