Every cyclist has a “bonk” story — that moment when the legs go empty and the ride goes sideways. Mine came on a fast group ride back in 2016, when I discovered the hard way that fitness alone isn’t enough if you don’t fuel the work. In this month’s Toolbox, I dig into the real science behind bonking — from glycogen depletion to emerging research on blood glucose — and, more importantly, what it means for how you should be fueling your rides today.
~ I can still picture that ride like it was yesterday… It was summer 2016, not long after I’d started riding with my local club. Like most new riders, I kept things safe at first: sticking with the no-drop group, exploring the local courses, & learning how to ride in a pack. But eventually, curiosity got the better of me. I wanted to see what the “fast” group was all about.
For the first couple of hours, it felt incredible! I was hanging in, taking pulls & proud to be actually contributing to the group… I was on top of the world!
And then, just past the two-hour mark, about 30 minutes from the shop, I completely fell apart – I had not eaten a thing all ride! I vividly recall watching the group roll away from me up over the next hill, my legs feeling completely sapped. Over the next few minutes, I realized my mistake and settled into the long, lonely pedal back to the shop.
That ride was my first real lesson in just how important carbohydrates are for endurance performance. In this month’s Toolbox article, I want to dig into why that is.
What Really Causes the Bonk?
For decades, the “bonk” or “hitting the wall” has been explained as a simple story of glycogen depletion: run out of stored carbohydrate, and performance collapses. But a recent review article has started to challenge that idea.
In a 2026 review, Noakes argues that exercise-induced hypoglycaemia – a drop in blood glucose levels – may be the primary driver of fatigue during prolonged exercise, rather than complete depletion of muscle glycogen. In this view, the brain plays a central role, reducing power output as blood glucose falls to protect the body.
If that’s true, it suggests something interesting from a practical standpoint: even relatively modest carbohydrate intake during exercise may be enough to maintain blood glucose, delay fatigue, and help boost performance.
That said, this certainly remains an area of active research and debate. The broader body of evidence still supports higher carbohydrate intakes – particularly for athletes aiming to maximize performance rather than simply avoid collapse. So while the mechanism may be debated, the takeaway message remains consistent: Fueling matters!
Why Carbohydrate? The Tank Is Smaller Than You Think
In endurance sport, performance is often limited not only by fitness, but by available carbs. Even in well-trained athletes, glycogen storage is surprisingly limited. Muscle stores typically max out around ~400 g, with the liver contributing another ~100 g. Together, that represents a mere 4% of your total energy reserves (Li et al., 2025).
It’s a tiny tank!
And yet, it’s the fuel that powers your hardest efforts. When the pace lifts – during hard intervals, on climbs, or in breakaways – it’s carbohydrates fueling the work. Fat can support lower-intensity riding for hours, even for the leanest cyclists, but it simply can’t deliver energy fast enough when intensity rises.
That’s why it’s possible to feel comfortable early in a ride, only to completely fall apart later. It’s not always a lack of fitness – it’s often a lack of available carbohydrates.
Duration-Based Guidelines
If carbohydrate is the limiting fuel, the next question becomes: how much do we actually need? One of the most widely used frameworks comes from Jeukendrup (2014), who breaks fueling recommendations into three simple zones based on exercise duration:
- Under 60 minutes: For short, intense efforts, carbohydrate intake is less about fueling the muscles and more about signaling the brain. Even a carbohydrate mouth rinse can improve performance by reducing perceived effort.
- 60-150 minutes: This is where intake starts to matter more. Around 40-60 g of carbohydrate per hour is typically sufficient, and a single carbohydrate source (like glucose or maltodextrin) works well.
- Beyond 2-2.5 hours: Now fueling becomes critical. Recommendations increase to 60-90 g per hour, especially if performance is the goal.
I like the simplicity of this guide – it’s simple & practical. It directly connects your fueling strategy to the demands of the ride – the longer and harder your ride will be, the more important carb intake becomes.
Why Glucose + Fructose Changes the Game
If you’ve previously tried to eat more carbs on the bike and ended up with stomach issues, you’ve probably hit a physiological bottleneck. Unlike water, which moves easily into your bloodstream, carbohydrates need help – relying on specific transport proteins to get from your gut into circulation:
- Glucose is absorbed via the SGLT1 transporter
- Fructose uses a different pathway, the GLUT5 transporter
Each of these has its own maximum capacity. When you rely on glucose alone, absorption tends to cap out around ~60 g per hour, which was previously believed to be the upper limit of carb intake. It was believed that trying to push beyond that would result in unabsorbed carbohydrate sitting in your gut & leading to discomfort.
But when you combine glucose and fructose, you effectively use two parallel pathways.
This is why many modern fueling strategies – and sports nutrition products – use a glucose-to-fructose blend (often around 2:1). It allows athletes to increase total carbohydrate intake well above 100 g/hr without any GI issues (Jeukendrup, 2014). This isn’t just a trendy sports nutrition fad, but it’s a reflection of our physiology. I’ll explore this topic further in next month’s Toolbox article.
Fueling the Work
That ride back in 2016 felt like a fitness failure at the time, but now I see it differently. The fitness was there, at least enough to hang in with the group for a while. I simply didn’t have the fuel to sustain it. I was riding on a limited tank, without any plan to keep it topped up.
This season, I’m intentionally treating carbohydrate intake as part of the training process itself. For a long time, my default was to roll out for morning rides fasted—often with nothing more than a double shot of espresso. It felt simple, and in some ways even productive. After all, doesn’t fasted riding improve fat-burning?
But as I’ve started to focus more on optimizing all aspects of my training—especially with the demands of Zwift racing—I’ve begun to shift my approach. I’m now far more intentional with my carbohydrate intake, even on endurance rides. That includes planning my snacks and bottles the night before, rather than just grabbing whatever’s available 5 min before rolling out.
And while this is far from a controlled experiment, the early signs for myself have been hard to ignore. On endurance rides, improving my in-ride nutrition seems to help limit heart rate drift. Day-to-day, I have been feeling less fatigued coming out of my rides. More importantly, my overall nutrition has improved. Instead of finishing a long, fasted session completely famished and reaching for whatever is closest, I’m finishing rides with a more moderate appetite – able to sit down and actually focus on a proper recovery meal.
It’s a small shift, but one that’s already starting to change how I train and recover. Fueling isn’t just about avoiding the bonk. It’s about making sure I stay fueled so I can actually do the work.
In the next article, I’ll build on this by diving into the practical side – how to increase carbohydrate intake during your rides, including the role of dual-source carbohydrates and how to start training your gut.
That’s all for this month. Stay safe, ride fast, and I’ll see 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
The post Bonking Explained: Why You Run Out of Energy on the Bike (and How to Fix It) appeared first on PezCycling News.
