Could Carbohydrate ‘Unloading’ Help Improve Cycling Performance?
Carbohydrate loading is a concept most cyclists will be familiar with and is common-practice before many cycling events. However, there’s new evidence to suggest that some cycling disciplines might actually benefit from ‘carbohydrate unloading’ - i.e. the intentional restriction of carbohydrate intake in order to reduce muscle glycogen levels before competition. In this article, we’ll take a look at this theory, and how it might translate into real-world best practices.
What is carbohydrate loading?
Carbohydrates are a primary source of fuel when cycling, particularly at high intensities. The body stores carbohydrates in the muscles and liver, in the form of ‘glycogen’, and in the blood as glucose. A typical endurance cyclist following a moderately high carbohydrate diet will store enough muscle glycogen to fuel an all-out effort of roughly 90-minutes. Running out of the body’s internal carbohydrate stores is the main cause of fatigue in endurance events, and it’s usually what makes us ‘bonk’ or ‘hit the wall’.
Carbohydrate loading is the idea that, by following a very high-carbohydrate diet (typically 10-12g/kg body weight per day) over the ~48-hours before a race, muscle glycogen stores can be increased, which helps to stave off fatigue, and improve performance in longer races.
There’s some very robust scientific evidence that supports this practice when races last longer than roughly 90-mins (e.g. Burke et al., 2011; Thomas et al., 2016). Indeed, carbohydrate loading has been shown to extend time to exhaustion at a fixed workload by as much as 20%, and to increase power output during a time trial by around 2-3% (Hawley et al., 1997).
For races that are shorter than this, there’s a little less benefit to carbohydrate loading, because they are so short that they don’t challenge the body’s normal carbohydrate reserves. Therefore, historically, advice for events lasting less than 90-mins has been to consume a moderately high carbohydrate intake (e.g. 7-12g/kg body weight per day) leading into an event, and to ‘top up’ muscle glycogen levels before a race, by eating a high-carbohydrate breakfast 3-4 hours before racing. This is intended to bring muscle glycogen levels to a ‘normal’ level.
In either case, sports nutrition advice has always focussed on ensuring a relatively high availability of carbohydrates, irrespective of the event or discipline.
Is carbohydrate always needed?
While it’s super important to have enough muscle glycogen to fuel a race, the problem with glycogen is that each gram of glycogen is bound with 3-4g of water. A typical 70kg endurance athlete can retain as much as 3-4kg of water due to stored glycogen.
Since events like short hill climbs or time trials only use a relatively small amount of the body’s usual muscle glycogen stores, we believe it could be advantageous to purposefully reduce stored muscle glycogen in the lead-up to key events, in order to reduce body weight, and thus improve Watts/kg.
In the hill climbing scene, people will pay large amounts just to save a few hundred grams on their bike, so a simple nutritional strategy that could shed even a kilogram is very intriguing!
We wrote about this theory in our Hill Climb Handbook, but at the time, there was only very minimal evidence to help corroborate our theory, and to provide evidence-based guidance on how exactly carbohydrate ‘unloading’ can be achieved.
Recently, however, a scientific paper was published looking at this exact idea!
The Study
Lowered muscle glycogen reduces body mass with no effect on short-term exercise performance in men
This 2023 study looked specifically at (i) how much weight can be lost by following a glycogen reduction strategy and (ii) whether power output over 1-min, 15-min and an all-out sprint is impacted by this strategy.
The study involved 22 recreationally active males.
These participants initially came to the lab for some baseline testing to determine VO2max and the power at VO2max achieved in a ramp test (aka ‘maximal aerobic power’). They also had the opportunity to practice and become familiar with the various exercise protocols and tests involved in the study.
The participants then followed the 4-day protocol, as shown below.
Diagram showing carbohydrate unloading protocol used in Schytz et al., 2023. Figure taken directly from Schytz et al., 2023.
Day -4 was a standardisation day, where the participants all followed a medium carbohydrate diet (M-CHO, 4g/kg body weight of carbs). This was intended to help standardise muscle glycogen levels at the start of the trial.
Day -3 was a glycogen depletion day, where participants performed vigorous cycling and arm exercise, to deplete muscle glycogen in both the lower and upper body.
The exercise comprised of:
On the bike:
10-mins at 100W
3x 6-sec sprints
2x 30-mins at 75% maximal aerobic power (i.e. roughly FTP), with 5-mins recovery between
10-mins easy
3x 1-min all-out efforts with 5-mins rest
On the arm crank:
3x 5-mins arm cranking at 1W/kg body weight, followed by 5-min rest
3x 1-min all-out efforts followed by 5 min rest
Back on the bike:
2-mins at 100W followed by 3x 6-sec sprints
After this exercise, the participants either followed a high-carbohydrate diet (H-CHO, 10g/kg body weight of carbs per day), or a medium-carbohydrate diet for the remaining days, along with one more bout of exercise on Day -1, which comprised of:
10-mins at 100W
4x 2-mins at maximal aerobic power, with 2-mins recovery
Finally, on Day 0, the participants performed a performance test, involving either a 1-min maximal test, or a 15-min maximal test, each followed by 3x 6-sec all-out sprints. 10 participants were allocated to the 1-min test, and 12 participants were allocated to the 15-minute test.
Each participant performed both the high-carb and the medium-carb protocol, in a randomised order, and performed the same performance test after each protocol.
Body weight was monitored regularly throughout, as shown in the figure above.
Food during the study period was supplied by the lab, and closely controlled throughout the investigation. All participants received the same amount of fat, protein and fibre, with the additional carbohydrate in the high-carb diet being made up of easily-digestible, low-fibre carbohydrates, such as sweets and sugary drink.
What did the study find?
The researchers had a number of key findings…
Both protocols resulted in reduction in body weight. However, the medium carbohydrate diet resulted in the greatest body weight loss (2% body weight vs 1.2%), and this was determined to be statistically significant.
Power output over 1-min and over 15-mins was not impacted by the intervention. In other words, following the medium carbohydrate diet did not negatively impact power output, even though muscle glycogen levels were lower.
There was a tendency for W/kg to be higher in the medium-carbohydrate trial, but there were insufficient participant numbers to detect this reliably.
Study limitations
When reviewing scientific literature, it’s always important to consider whether the study has any limitations or shortcomings.
One key limitation of the study is that overall energy intake was not controlled between the high-carbohydrate and the moderate-carbohydrate groups. The high-carbohydrate group ate more kcals than the moderate carbohydrate group. Therefore, it’s unclear whether and to what extent the reduction in body weight in the moderate carbohydrate trial may have been a result of body fat loss, rather than glycogen reduction. That said, it would be difficult to lose notable body fat over just a 4-day period, so it’s likely a significant portion of weight loss was attributable to glycogen reduction.
It also appears that all participants were in a calorie deficit when following the plan (i.e. they were eating fewer calories than they were burning), and it’s not clear whether this is an essential component of a successful carbohydrate unloading plan.
Another limitation is that the exercise performed on Day -3 was very challenging, involving 1H near threshold, and a total of 6x all-out sprints, and 6x 1-min maximal efforts. This is probably not compatible with tapering for, and being in peak shape on the day of an event. Thus the protocol presented in the present study may not be completely translatable into practice.
Finally, we know from this study, that ‘carbohydrate unloading’ doesn’t negatively impact power output for durations up to 15-mins. But we don’t yet know the tipping point where carbohydrate unloading becomes detrimental to performance. It’s not clear, for example, whether carbohydrate unloading would be a help or hindrance in a hilly 10-mile time-trial. Our sense from the theory and experience is that events lasting up to roughly 30-mins may benefit from carbohydrate unloading. However, if you’re competing over ~15-30 minutes, then you should definitely test this nutritional strategy out yourself in training and lower priority races to see how you fare, before using in a high-priority race.
Nutritional recommendations for short cycling events
So what should you be doing in the lead-up to a short race like a hill climb?
Given the ‘aggressiveness’ of the exercise program used in the study, we’ve tried to come up with an adapted protocol that’s better suited to a traditional taper period, allowing an athlete to be in peak shape on race day.
Evidence from the scientific literature (e.g. Tønnessen et al., 2014) and our own experience does suggest that it’s beneficial to include some intensive training roughly 4-days out from competition, which could be used for glycogen depletion. This would usually be a slightly abbreviated interval session, so would not be as ‘aggressive’ as the session used on Day-3, and probably wouldn’t deplete muscle glycogen to such a large extent.
That being the case, it might be helpful to extend the period of time over which a lower carbohydrate intake is followed, meaning other training sessions that form part of the taper can also contribute to reducing muscle glycogen levels.
A good taper that helps to reduce muscle glycogen levels might look something like:
On each of these days, a reduced carbohydrate intake would be followed, aiming for roughly 4g/kg body weight per day.
Of course, this particular protocol is not yet supported by science, but our own personal experimentation suggests a protocol like this can be effective in reducing body weight by a meaningful amount without compromising power output.
If you’d like to learn more about nutritional strategies to enhance performance or improve training adaptations across a range of cycling disciplines, then check out our new guide: Optimal Nutrition in Cycling.
References
Burke, L. M., Hawley, J. A., Wong, S. H., & Jeukendrup, A. E. (2011). Carbohydrates for training and competition. Journal of sports sciences, 29(sup1), S17-S27.
Hawley, J. A., Schabort, E. J., Noakes, T. D., & Dennis, S. C. (1997). Carbohydrate-loading and exercise performance: an update. Sports medicine, 24, 73-81.
Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). Nutrition and athletic performance. Med Sci Sports Exerc, 48(3), 543-568.
Tønnessen, E., Sylta, Ø., Haugen, T. A., Hem, E., Svendsen, I. S., & Seiler, S. (2014). The road to gold: training and peaking characteristics in the year prior to a gold medal endurance performance. PloS one, 9(7),e101796.
Schytz, C. T., Ørtenblad, N., Birkholm, T. A., Plomgaard, P., Nybo, L., Kolnes, K. J., ... & Gejl, K. D. (2023). Lowered muscle glycogen reduces body mass with no effect on short‐term exercise performance in men. Scandinavian journal of medicine & science in sports.