How to Avoid Bonking in a Race

Most runners know the feeling. You’re cruising, splits ticking along nicely, then suddenly it’s as if someone quietly pulled the plug. Legs heavy, mind foggy, pace slipping. This is what runners call “hitting the wall” or “bonking”, and while it feels dramatic, it’s actually a very predictable physiological event. The good news? Once you understand it, you can train and fuel to delay it or even avoid it altogether.

Bonking is a state of acute fatigue caused primarily by the depletion of glycogen, the body’s most accessible form of stored carbohydrate. Glycogen is stored in your muscles and liver and acts as your preferred fuel during moderate to high intensity exercise like distance running.

When you’re running at race pace, your body relies heavily on glycogen because it can be broken down quickly to produce energy. Fat, on the other hand, is a much larger energy reserve but it’s slower to convert into usable fuel. It’s like having a massive slow-burning log versus a small pile of kindling that catches instantly. Most runners store roughly 1800 to 2000 calories of glycogen. Given that running burns approximately 90 to 110 calories per 1.6 km depending on body size and efficiency, these stores typically begin to run low somewhere around the 28 to 32 km mark in a marathon.

As glycogen becomes depleted, your body is forced to rely more heavily on fat metabolism. The issue is not that fat can’t fuel running, it absolutely can. The issue is that it cannot support the same intensity. This is why pace drops, coordination feels off and even your mood can dip. The brain, in particular, is highly sensitive to low carbohydrate availability, which is why bonking feels just as mental as it does physical.

1. Train your metabolic flexibility

Long runs are not just about distance, they are metabolic training sessions. Over time, consistent long runs improve your ability to oxidise fat at higher intensities and spare glycogen. This does not mean you are “turning fat into glycogen”, but rather that you become better at using fat alongside carbohydrates. The result is slower glycogen depletion and more stable energy late in a run.

Aim for regular long runs in your program, and occasionally include sections at marathon pace to teach your body to manage fuel under stress.

2. Build overall fitness

As your aerobic fitness improves, your running economy improves. This means you use less energy at a given pace. A fitter runner burns a lower percentage of glycogen at submaximal speeds and can maintain pace longer before depletion becomes critical. This is why two runners can run the same race, but only one hits the wall.

3. Get carbohydrate intake right before racing

Carbohydrate loading is about maximising glycogen stores, not overeating. In the 24 to 48 hours before a race, focus on increasing carbohydrate intake while slightly reducing training load. This allows your muscles to store more glycogen than usual. Simple, familiar, low fibre carbohydrate sources tend to work best. The goal is topped-up stores, not digestive stress.

4. Fuel early and consistently during your run

One of the most effective ways to prevent bonking is to take in carbohydrates during exercise. The key mistake many runners make is waiting until they feel tired. By then, glycogen levels are already dropping. Instead:

This helps maintain blood glucose levels and reduces the rate at which glycogen is used.

5. Hydration supports performance more than you think

While dehydration does not directly cause glycogen depletion, it significantly impacts performance and perceived effort. Fluid loss reduces blood volume, making it harder to deliver oxygen and fuel to working muscles. It also increases cardiovascular strain and can accelerate fatigue. Including electrolytes, especially in warm conditions, helps maintain fluid balance and supports muscle function.

6. Protect your running economy under fatigue

As fatigue builds, biomechanics often deteriorate. You might overstride, collapse through the hips or lose cadence. This increases energy cost right when fuel is already limited. Simple cues can help:

Efficiency is your best friend late in a race.

7. Use mental strategies strategically

There is a real neurological component to fatigue. The brain constantly regulates effort based on perceived energy availability. Shifting your focus can help override some of that protective signalling:

It is not about ignoring fatigue, but about not amplifying it.

8. Respect pacing

Going out too hard is one of the fastest ways to guarantee a bonk. Higher intensity running increases carbohydrate reliance, meaning glycogen is burned more quickly. Even a slightly aggressive early pace can have a large impact later. Even pacing, or slightly negative splitting, helps preserve glycogen and maintain performance deeper into the race.

Bonking might feel like a dramatic collapse, but it is really just physiology playing out in real time. With the right training, fuelling and pacing, you can push that wall further and further away until it becomes less of a crash and more of a gentle fade. And when you do get it right, those final kilometres feel less like survival and more like control.