By James Frost, founder of Flaming Phoenix. I make dense, real-food fuel for endurance athletes, and a large part of my job is helping people understand the difference between running out of fuel and running out of fitness. I am not a coach or a dietitian, and a plan built for your body and event will beat any general guide, but this is the practical version. Last reviewed June 2026.

Hitting the wall, also called bonking, the knock, or a hunger flat, is the sudden, brutal fatigue that arrives when your body runs low on its stored carbohydrate. It is not weakness and it is not bad luck. It is arithmetic. You carry only a limited tank of glycogen, around 2,000 calories in a trained athlete, enough for roughly 90 minutes of hard effort or 18 to 20 miles of running, and when that tank empties faster than you refill it, your performance falls off a cliff. The good news is that because the wall is predictable, it is largely preventable. You have three levers: start with a full tank, empty it more slowly, and top it up as you go. This guide explains what is actually happening when you hit the wall, the three ways to push it back, how to tell a true bonk from dehydration or a turned stomach, and what to do if it catches you anyway.

What hitting the wall actually is

Your body runs hard efforts mainly on carbohydrate, stored as glycogen in your muscles (around 300 to 400 grams) and your liver (around 75 to 100 grams). Carbohydrate is the fuel that runs out first, well before you exhaust fat or protein, and a trained athlete holds only about 2,000 calories of it, which covers roughly 90 minutes of hard running or 18 to 20 miles before the tank runs low. That is why marathon runners so reliably blow up between miles 18 and 22: it is the point where the maths catches up with them.

It helps to know that two related things are happening, because they feel slightly different. As your muscle glycogen drains, your legs turn to lead and you cannot hold pace. That is the wall in the literal sense. Separately, as your liver glycogen falls, your blood sugar drops, and because your brain runs on glucose, you get the mental side: dizziness, fogginess, irritability, nausea, and the dark, catastrophic thoughts that make you certain you cannot go on. That blood-sugar crash is the bonk proper. Usually they arrive together. Once glycogen is gone, your body is forced to rely on fat, which burns far less efficiently and simply cannot support a fast pace, which is what turns the closing miles of an under-fuelled race into a shuffle. Performance is already impaired once you drop below about half your stores and badly compromised by the time you are down to a quarter, so the goal is never to get close to empty in the first place.

The three ways to push the wall back

There are exactly three levers, and it is worth pulling all of them.

The first is to start with a full tank. In the days before a long effort, eat plenty of carbohydrate so your glycogen stores are topped right up, and have a carbohydrate-rich meal on the morning itself, which is what the high-calorie breakfast guide is for. Starting full simply buys you more miles before the tank runs low, and carb loading and training can raise your reserves substantially.

The second is to empty the tank more slowly, and the biggest free lever here is pacing. Going out too hard burns glycogen disproportionately fast and drags the wall forward, so an even, controlled effort is one of the most effective things you can do to delay it. Training matters too: a strong aerobic base, and some thoughtfully placed easier sessions, can nudge your body toward burning more fat and sparing glycogen. That approach is debated, is best done carefully rather than on your hard sessions, and is never a reason to under-eat overall, since chronic under-fuelling causes far more problems than it solves.

The third is to top the tank up as you go, and the cardinal rule is to start before you feel low, because once you are deep in a hole it is very hard to climb out. Aim for roughly 30 to 60 grams of carbohydrate an hour for efforts of one to two and a half hours, rising toward 90 grams an hour for longer ones, taken in small regular amounts rather than occasional large hits. For short, high-intensity efforts, simple fast-absorbing carbohydrate reaches the muscle quickest. Over longer, steadier efforts, real food provides more durable energy and spares your gut the relentless sugar hit that causes its own problems, a case made in full on the energy gel alternatives guide and the ultra-high-carb solid foods guide. This is where a Phoenix Bar fits: each 120g bar is up to 557 calories of dense real food, a substantial and steady contribution to your hourly target that sits well over long efforts and comes in six flavours so it does not become the thing you dread, eaten cold on the move or stirred into a warm porridge. Being honest about it, for the very fastest top-up at threshold a simple sugar acts quicker, so a bar earns its place on longer, lower-intensity efforts and as part of your pre-effort loading rather than as a last-second rescue. The wider principle of getting a lot of energy into a small package is on the calorie-dense foods guide.