Bonking is one of the most brutal feelings you can experience in endurance racing. One moment you feel reasonably strong, and the next your body simply refuses to cooperate anymore. Power disappears. Pace collapses. Every single endurance athlete, or basically anyone who has pushed themselves into unknown territory (athletically speaking), is almost guaranteed to run into the good old bonk zone. Are you really an athlete if you never quite bonked?

For full transparancy, I present you below my Garmin output, from my very first Marathon. While now ages ago, I recall having trained for it well enough (or so I thought), and was aiming for at that time a 6 min/km pace. Yes, nothing heroic, but we all start somewhere, and that’s alright. Aim was, finish without too much suffering. What I did not know at that time, as obvious as it may seem, is that I had to fuel properly during the race. What I had done was eat a big bowl of spaghetti with brown sugar the night before (can not recommend) and during the marathon I only drank the water at the aid stations, nothing else. Just thinking about that today makes me shiver, I had absolutely no clue what I was doing 🙂 Anyway, enough teasing, see here the visual representation, Garmin stats don’t lie, of me bonking. You can clearly see me hitting the wall at around the 3 hour mark, I never recovered, I did make it in time for the cutoff:

In simple terms, bonking is what happens when your body runs critically low on available carbohydrates during prolonged exercise. More specifically, it is heavily linked to depleted glycogen stores, which are the stored form of carbohydrates inside your muscles and liver. This may sound obvious, but the first time I hit that dreaded wall, I had no clue why this was happening. I had trained for months! Yet here I was, exactly like everyone had mentioned in conversations leading up to the race “hopefully you don’t hit the wall too early”(too early? I thought.. how about never?).

Your body can burn both fat and carbohydrates for fuel, but carbohydrates are the fast-access premium fuel source. Once intensity rises beyond very easy aerobic effort, your dependence on carbohydrates increases significantly. The harder you go, the more you rely on them. This is why you carb load days (yes, days, not day, not evening, dayS) before your marathon or endurance event. Load up on fuel you can burn during the race.

Even well-fueled athletes only store enough glycogen for a few hours of moderate-to-hard exercise. If you burn through it faster than you replace it, the body starts protecting itself by dramatically reducing performance. This is why bonking often feels sudden and catastrophic rather than gradual. Bonking is not just “being tired.” You can be perfectly fit and still hit the wall, still bonk!

Also a small note here, but if you check out interviews with modern endurance athletes (marathon, ironman, triathletes, ultra runners etc) you will see that one of their key focus areas nowadays is to find ways to keep pushing higher and higher carb intake. This takes training and the ceiling keeps getting pushed and broken. Where you and I may be happy with a 60carb load per hour in a certain event, they are hitting almost tripple that nowadays. Wild stuff.

Watt I learned

The most common cause for bonking, the most common cause for hitting the wall, is simply underfueling.

A huge number of amateur athletes massively underestimate how many carbohydrates endurance exercise actually burns. You see this constantly in long rides, marathons, and Ironman training. People fuel reactively instead of proactively. They wait until they feel hungry, weak, or mentally foggy before eating, but by then the process is already underway. Also, I always like to lose a bit of weight here and there, so I figured if I didn’t fuel during a long run, surely that was a good thing, right? Wrong!

Now, next to that, another major cause is clearly pacing mistakes.This can be both starting too slow, but of course most often, starting too fast.

Adrenaline is high, everybody around you feels fast, and suddenly you are riding or running harder than planned in the opening phase. The problem is that higher intensities burn carbohydrates at a much faster rate. You may feel amazing for an hour or two while quietly emptying the tank at unsustainable speed.

Keep in mind heat also makes things worse. Higher temperatures increase physiological stress, elevate heart rate, and can accelerate carbohydrate usage. Dehydration compounds the problem further because cardiovascular strain rises as fluid loss increases.

So how do you avoid the good old bonk? Here’s Watt have I learned?

The first rule is brutally simple: eat earlier than you think you need to.

In endurance racing, fueling is proactive, not reactive. Waiting until you feel terrible is already too late. Most modern endurance fueling guidelines recommend regular carbohydrate intake during long sessions, often somewhere around 60–90 grams per hour for prolonged events, depending on duration, intensity, and gut tolerance. Please do check with your own coach / doctor / etc! We are all unique 🙂

The second lesson is pacing discipline.

Most spectacular bonks are not random accidents. They are pacing decisions that simply took a while to collect payment. Riding slightly easier early in a race often feels frustrating in the moment but pays back massively later. If you bonk, go back to your data after the race (always track your data, always) and try and see if you messed up pacing, chances are that you did.

The third is practicing nutrition during training, not experimenting during races. Not only will this increase your tolerance, but also, my friends, believe me here, it will mitigate potentially very embarassing moments during the race!

And finally, understand that bonking is not a sign of weakness or lack of toughness.

In endurance sports, physiology eventually wins every argument. You cannot mentally override depleted glycogen forever. The body always collects the bill eventually. You and the World Champion are exactly the same in this respect, you have limited “energy” storage and it will run out eventually.

Below some good sources and further reading which I thought were a good read:

• https://journals.lww.com/acsm-msse/fulltext/2016/03000/nutrition_and_athletic_performance.25.aspx
• https://pmc.ncbi.nlm.nih.gov/articles/PMC8133477/

Dear fellow Watters, a friendly Request: If you have any more up to date papers or research findings you know of, please don’t hesitate to share in the comments. I am always fascinated to read more about this particular topic.