Every cyclist has encountered the infamous “bonk” — that unfortunate moment when energy levels plummet and the ride takes a turn for the worse. My own experience of this came during a rapid group ride back in 2016, a harsh reminder that fitness alone is insufficient without proper nutrition. In this month’s Toolbox, we will explore the science behind bonking, from glycogen depletion to the latest findings on blood glucose levels, and, more importantly, how you can optimize your nutrition for better rides.
~ I can vividly recall that ride as if it were yesterday… It was summer 2016, shortly after I had joined my local cycling club. Initially, I kept things comfortable by sticking to the no-drop group, familiarizing myself with local roads, and learning how to ride efficiently in a pack. However, my curiosity soon got the best of me, and I felt compelled to experience the thrill of the “fast” group.
For the first couple of hours, it was amazing! I felt fantastic, contributing to the group by taking pulls, and relishing the feeling of being part of something exhilarating. I was riding high!
But just past the two-hour mark, about 30 minutes from our starting point, everything unraveled — I had neglected to eat anything during the ride! I can still visualize the group pulling away from me as I struggled up the next hill, my legs utterly drained. In those shortly ensuing moments, I came to understand my error and began the long, solitary ride back to the shop.
This experience taught me a crucial lesson about the importance of carbohydrates for endurance sports. In this month’s Toolbox article, we’ll delve deeper into why carbohydrates are essential.
What Really Causes the Bonk?
For years, the concept of the “bonk,” or “hitting the wall,” has been simplified to glycogen depletion: run out of stored carbohydrates, and performance deteriorates. However, a recent review challenges this notion.
In a 2026 review, Noakes posits that exercise-induced hypoglycaemia — a decline in blood glucose levels — could be the principal cause of fatigue during prolonged exercise, overshadowing total muscle glycogen depletion. According to this perspective, the brain plays a pivotal role by decreasing power output as blood glucose dwindles, thus safeguarding the body.
If this theory holds, it presents a particularly interesting insight: even moderate carbohydrate intake during exercise could be sufficient to maintain blood glucose levels, stave off fatigue, and enhance performance.
Nonetheless, this topic continues to spark research and debate. The majority of evidence still indicates that higher carbohydrate intake is crucial, particularly for athletes looking to optimize their performance rather than merely avoid collapse. Hence, while the underlying mechanisms remain under discussion, the key takeaway remains clear: Fueling is essential!
Why Carbohydrates? The Tank Is Smaller Than You Think
In endurance events, performance isn’t just restricted by fitness; available carbohydrates also play a significant role. Notably, even well-trained athletes have limited glycogen stores. Typically, muscle stores max out around ~400 g, while the liver contributes an additional ~100 g. Combined, this amounts to a mere 4% of your total energy reserves (Li et al., 2025).
That’s quite a small tank!
Yet, this limited fuel powers your most demanding efforts. When the intensity increases – whether during challenging intervals, climbs, or breakaways – it’s carbohydrates that provide the necessary energy. While fat can fuel lower-intensity rides for extended periods, it simply cannot supply energy quickly enough when demand rises.
This explains why it’s common to feel fine early in a ride, only to falter later. It’s not merely a lack of fitness; often, it’s a shortage of available carbohydrates.
Duration-Based Guidelines
With carbohydrates as the limiting fuel, an important question arises: how much do we truly need? One widely accepted framework comes from Jeukendrup (2014), who categorizes fueling recommendations into three straightforward zones based on exercise duration:
- Under 60 minutes: In short, intense efforts, carbohydrate intake primarily influences the brain rather than fueling the muscles. Even a carbohydrate mouth rinse can enhance performance by lessening perceived exertion.
- 60-150 minutes: During this timeframe, intake becomes increasingly significant. Consuming approximately 40-60 g of carbohydrates per hour is generally sufficient, often using a single carbohydrate source such as glucose or maltodextrin.
- Beyond 2-2.5 hours: At this point, proper fueling is critical. Recommendations increase to 60-90 g per hour, particularly if performance optimization is the goal.
This guide’s simplicity appeals to me — it connects your fueling strategy to the demands of the ride directly. The longer and more intense your ride is, the more significant carbohydrate intake becomes.
Why Glucose + Fructose Changes the Game
If you’ve attempted to increase your carbohydrate intake while cycling only to be met with stomach issues, you may have encountered a physiological bottleneck. Unlike water, which enters your bloodstream effortlessly, carbohydrates rely on specific transport proteins to move from your gut into circulation:
- Glucose is absorbed via the SGLT1 transporter.
- Fructose uses a different pathway, the GLUT5 transporter.
Each of these transporters has its own maximum absorption capacity. When consuming glucose alone, absorption tends to plateau around ~60 g per hour. It was previously thought that trying to surpass this threshold would result in unabsorbed carbohydrates lingering in the gut, leading to discomfort.
However, when you combine glucose with fructose, you effectively utilize two parallel pathways.
This is why many contemporary fueling strategies — including various sports nutrition products — incorporate a blend of glucose and fructose (commonly in a 2:1 ratio). This combination allows athletes to raise their total carbohydrate intake beyond 100 g/hr without experiencing gastrointestinal issues (Jeukendrup, 2014). This approach is not merely a trend in sports nutrition but a reflection of our physiological capabilities. I will delve deeper into this topic in next month’s Toolbox article.
Fueling the Work
That 2016 ride felt like a significant fitness failure at the time, but I’ve come to see it in a different light. My fitness was adequate — at least enough to keep pace with the group initially. The real issue was that I simply didn’t have the nutritional support to sustain it. I was riding on a deficient fuel tank and had no plan to replenish it.
This season, I’m consciously incorporating carbohydrate intake as part of my training regimen. For too long, my routine involved embarking on morning rides fasted, often fueled by little more than a double shot of espresso. I felt that this approach was simplistic and, in some respects, effective. After all, doesn’t fasting enhance fat oxidation?
However, as I’ve begun focusing on refining all aspects of my training — particularly with the demands of Zwift racing — I’ve transformed my strategy. I’m now more deliberate about my carbohydrate consumption, even on endurance rides. This includes planning my snacks and drinks the night before instead of hastily grabbing whatever is available five minutes before departure.
Although this isn’t a controlled study, the early outcomes for me have been noteworthy. On endurance rides, improved in-ride nutrition appears to assist in limiting heart rate fluctuations. Day to day, I’ve noticed a decrease in post-ride fatigue. More significantly, my overall nutritional habits have improved. Instead of completing a lengthy, fasted ride completely ravenous and reaching for the nearest snack, I’m now finishing rides with a more balanced appetite, allowing me to focus on preparing a proper recovery meal.
This subtle shift is already beginning to transform how I train and recover. Fueling is not just about dodging the bonk; it’s about ensuring I’m sufficiently energized to perform the work.
In the next article, I’ll build on this topic by tackling the practical aspects of increasing carbohydrate intake during your rides, including the benefits of dual-source carbohydrates and how to condition your gut.
That’s it for this month. Stay safe, ride hard, and I look forward to seeing you next time!
References
Li, X., et al. (2025). A Review of Carbohydrate Supplementation Approaches and Strategies for Optimizing Performance in Elite Long-Distance Endurance. Nutrients, 17(5), 918. https://doi.org/10.3390/nu17050918
Jeukendrup, A. E. (2014). A Step Towards Personalized Sports Nutrition: Carbohydrate Intake During Exercise. Sports Medicine, 44(S1), S25–S33. https://pmc.ncbi.nlm.nih.gov/articles/PMC4008807/
Noakes, T. D. (2026). Carbohydrate Ingestion on Exercise Metabolism and Physical Performance. Endocrine Reviews. https://doi.org/10.1210/endrev/bnaf038
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