The Science of Bonking: Carbohydrates, Glycogen and Endurance Fueling

Every cyclist has a “bonk” story – the moment when the legs are bare and the rider goes sideways. Mine came back in 2016 on a fast group tour, when I found out the hard way that fitness alone isn’t enough if you’re not fueling the work. In this month’s toolbox, I look at the real science behind crowing—from glycogen depletion to emerging research on blood glucose—and, more importantly, what that means for how you should be fueling your ride today.

~ I can still picture that ride like it was yesterday… It was the summer of 2016, not long after I started riding with my local club. Like many new riders, I kept things safe at first: sticking with a no-drop group, exploring local courses, and learning how to ride in a pack. But eventually, curiosity got the better of me. I wanted to see what the “fast” crowd was all about.

For the first few hours, it was felt Unbelievable! I was hooked, pumped and proud to actually contribute 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 hadn’t eaten anything the whole way! I vividly remember the group coming down the next hill from me, my legs feeling absolutely crushed. Over the next few minutes, I realized my mistake and settled into the long, lonely paddle back to the shop.

This ride was my first real lesson in just how important carbohydrates are to endurance performance. In this month’s Toolbox article, I want to look at why that is.

What exactly causes a bank?

For decades, “bonk” or “hitting the wall” has been explained as a simple story of glycogen depletion: the loss of stored carbohydrates, and performance collapses. But a recent review article has begun to challenge this view.

In a 2026 review, Knox argues that the practice is encouraged Hypoglycemia – A decrease in blood glucose levels – may be the main cause of fatigue during prolonged exercise, not a complete depletion of muscle glycogen. In this view, the brain plays a central role, reducing the production of electricity as blood glucose protects the body.

If this is true, it suggests something interesting from a practical standpoint: Even relatively small amounts of carbohydrates during exercise may be sufficient to maintain blood glucose, delay fatigue, and help improve performance.

That said, it certainly remains an area of ​​active research and debate. A large body of evidence still supports high-carbohydrate intake—especially for athletes whose goal is to maximize performance rather than prevent collapse. So while the mechanism can be debated, the transmission message continues: Fuel issue!

Why carbohydrates? The tank is smaller than you think

In endurance sports, performance is often limited not only by fitness, but by available carbohydrates. Even in well-trained athletes, glycogen stores are surprisingly limited. Muscle stores usually max out at around ~400 g, and the liver contributes another ~100 g. Together, it just represents 4% of your total energy reserves (Li et al., 2025).

It’s a small tank!

And yet, it is the fuel that powers your hard work. When the pace picks up—during hard intervals, during climbs, or during breaks—these carbs fuel the work. Fat can support low-intensity riding for hours, even for light cyclists, but it simply cannot deliver energy. Pretty quick When the intensity increases.

That’s why it’s possible to feel comfortable on a ride, only to completely fall apart afterward. It’s not always a lack of fitness – it’s often a lack of available carbohydrates.

Period-based instruction

If carbohydrates are the limiting fuel, the next question becomes: how much do we really need? One of the most widely used frameworks comes from Jeukendrup (2014), which breaks down fuel recommendations into three simple zones based on exercise duration:

• Less than 60 minutes: For short, intense efforts, carbohydrate consumption is less about fueling the muscles and more about signaling to the brain. even one Carbohydrate mouthwash Can improve performance by reducing attentional effort.
• 60-150 minutes: This is where buying is very important. around 40-60 grams of carbohydrates per hour Usually sufficient, and a single carbohydrate source (such as glucose or maltodextrin) works well.
• After 2-2.5 hours: Now fueling becomes important. Orders are increasing 60-90 grams per hourEspecially if performance is the goal.

I love the simplicity of this guide – it’s simple and practical. This directly ties your fueling strategy to the demands of the ride – the longer and harder your ride, the more important carb consumption becomes.

Why glucose + fructose changes the game

If you’ve tried eating too many carbs on the bike before and ended up with stomach issues, you’ve probably hit a physiological barrier. Unlike water, which moves easily through your bloodstream, carbohydrates need help—relying on special transport proteins to get out of your intestines and into the bloodstream:

• Glucose is absorbed via the SGLT1 transporter
• fructose uses another pathway, the GLUT5 transporter

Each of these has its own maximum capacity. When you rely on glucose alone, absorption reaches ~60 g per hour, which was previously considered the upper limit of carbohydrate consumption. It was thought that trying to push beyond that would result in unabsorbed carbs sitting in your gut and causing discomfort.

But when you combine glucose and fructose, you use it efficiently Two parallel paths.

This is why most modern fuel strategies – and sports nutrition products – use a glucose-to-fructose mixture (often around 2:1). This allows athletes to consume total carbohydrates above 100 g/hr without GI issues (Jeukendrup, 2014). This is not just a sports nutrition trend, but it is a reflection of our physiology. I will explore this topic further in next month’s Toolbox article.

fueling the work

That ride in 2016 felt like a fitness fail at the time, but now I look at it differently. The fitness was there, at least enough to hang with the group for a while. I simply didn’t have the fuel to keep it up. I was riding in a limited tank, with no plans to keep it up.

This season, I’ve been treating myself to a deliberate carb intake as part of the training process. For a long time, my default was to roll into a fasted morning ride—often with nothing more than a double shot of espresso. It feels simple, and in some ways even useful. On the other hand, doesn’t fasting promote fat burning?

But as I started to focus more on improving all aspects of my training – especially with the demands of Zwift racing – I started to change my approach. I am now much more intentional with my carb intake, even on endurance rides. This includes planning my snacks and bottles the night before, instead of having everything available 5 minutes before falling.

And while it’s a far from controlled experience, the early signs are hard for me to ignore. In endurance riding, improving my riding nutrition seems to help reduce heart rate. Day after day, I feel less tired of getting off my ride. Most importantly, my overall nutrition has improved. Instead of ending a long, fast session completely starved and reaching for whatever is nearby, I end the ride with a more moderate appetite – able to sit down and really focus on proper recovery food.

It’s a small change, but one that is already starting to change how I train and recover. Fueling isn’t just about avoiding bonk. It’s about making sure I stay fueled so I can really do it do the work.

In the next article, I’ll build on this by diving into the practical side – how to increase your carb intake during your ride, including the role of dual carbs and how to start training your gut.

That’s all for this month. Be safe, ride fast, and I’ll see you next time!

References

Li, X. et al. (2025). A review of carbohydrate supplementation approaches and strategies to optimize performance in elite long-distance endurance. Nutrients, 17(5), 918. https://doi.org/10.3390/nu17050918

Jeukendrup, AE (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/

Knox, TD (2026). Carbohydrate consumption in exercise metabolism and physical activity. Endocrine Reviews. https://doi.org/10.1210/endrev/bnaf038