What Is Bonking?
The Science of Hitting the Wall

Your body's fuel tank

Your muscles run on glycogen — long chains of glucose molecules packed into muscle and liver cells. Think of it as your body's high-octane fuel. You carry roughly 1,500 to 2,000 calories worth of it at any given time, depending on your size, fitness, and what you've eaten recently.[1]

That sounds like a lot, but it goes fast. During moderate-to-hard running — the kind you do in a marathon — your body burns a mix of carbohydrates and fat. The harder you run, the more it leans on glycogen rather than fat. At roughly 75% of your VO2max (a pace where you can talk in short sentences but not hold a conversation), about 80% of your energy is coming from carbohydrates.[2]

Fat is a nearly unlimited fuel source — even a lean runner has 30,000+ calories of stored fat. But fat is slow. It requires more oxygen to convert into energy per calorie. Your body can only oxidize fat at a rate that supports relatively easy running. The moment you push into race pace, glycogen is doing the heavy lifting.

The math of the bonk

Running burns roughly 1 kcal per kilogram of body weight per kilometer.[3] For a 70 kg (154 lb) runner, that's about 70 kcal per kilometer, or roughly 113 kcal per mile. Over a marathon's 42.2 km, that's around 2,950 kcal of total energy expenditure.

If ~80% of that comes from carbohydrates at race pace, you're burning about 2,360 kcal of glycogen. But you only started with 1,500–2,000 kcal. Even with carb loading, you're short by anywhere from 400 to 900 kcal.

The math doesn't work. Without taking in carbohydrates during the race, your glycogen tank runs dry somewhere between kilometer 28 and 35 — mile 17 to 22. That's not a theory. That's arithmetic.

Parameter	Value
Glycogen stores (carb-loaded)	~2,000 kcal
Total marathon energy cost (70 kg runner)	~2,950 kcal
Carbohydrate share at race pace	~80%
Glycogen demand for full marathon	~2,360 kcal
Glycogen deficit without fueling	360–860 kcal
Predicted bonk point (no fueling)	Km 28–35 / Mile 17–22

What it actually feels like

Runners describe it the same way almost universally: your legs turn to concrete. Pace drops off a cliff. You can't think straight. Some people cry. Some feel nauseous. Some experience tunnel vision or intense chills on a warm day. It's not a gradual fade — it arrives with surprising speed.

This isn't just your muscles running out of fuel. Your brain runs almost exclusively on glucose. When blood sugar drops and glycogen is depleted, your central nervous system starts rationing energy. Decision-making deteriorates. Perceived effort skyrockets. You feel like you're sprinting at what was an easy pace 10 minutes ago.

About 40% of recreational marathon runners experience significant late-race pacing collapse consistent with hitting the wall.[4] A survey of 315 marathon finishers found that 43% reported hitting the wall, most commonly between miles 18 and 22.[5]

Why mile 20?

There's nothing magical about mile 20. It's simply where the math tends to converge for a typical runner at a typical pace with typical glycogen stores. Rapoport's biophysical model of marathon performance predicts the wall at approximately 32 km (mile 20) for a runner at 75–85% VO2max with ~2,000 kcal of starting glycogen.[6]

But your bonk point is not fixed. It moves based on three variables:

Starting glycogen — how well you carb-loaded. A runner who loaded properly (10–12 g of carbs per kg of body weight for 24–48 hours) starts with substantially more fuel than someone who just had pasta the night before.[7][8]

Burn rate — how fast you're going. This is the crossover concept at work. Going out even 5–10% too fast dramatically increases your glycogen burn rate. The relationship isn't linear — it's exponential at higher intensities.

Fueling during the race — how many carbs you're taking in. This is the biggest lever. With modern sports nutrition, you can replace a significant portion of what you're burning.[9]

Your brain vs. your legs

There's a long-running debate in exercise science about what actually causes the bonk: is it your muscles running out of fuel, or your brain pulling the handbrake?

The peripheral fatigue model says it's straightforward: muscles run out of glycogen, can't contract as forcefully, performance drops. There's solid evidence for this — Amann and Dempsey showed in 2008 that blocking sensory feedback from fatigued muscles (using epidural anesthesia) caused athletes to dramatically increase their power output, proving that signals from the muscles directly constrain effort.[10]

The central governor model, championed by Tim Noakes, argues that your brain anticipatorily reduces motor output before you're truly empty. The evidence: even at "complete exhaustion," muscle biopsies show 15–25% of glycogen remains. You never actually hit zero.[11] Your brain won't let you — it's protecting you from metabolic catastrophe.

Marcora's research added another layer: mental fatigue alone — from a demanding cognitive task before exercise — reduced endurance by 15%, without any change in cardiovascular or metabolic markers.[12] The effort felt harder, so athletes stopped sooner. Perception is reality.

The pacing trap

Here's something most bonking articles don't cover: pacing is a fueling problem.

When you go out too fast in a marathon, you're not just "using up your legs." You're accelerating glycogen depletion. The crossover concept means that at 65% VO2max, roughly half your energy comes from fat. At 85% VO2max, it could be 80% or more from carbohydrate.[2] Going out 10% too fast doesn't cost you 10% more glycogen — it costs you substantially more, because you've shifted the fuel mix.

An analysis of 91,929 marathon performances found that the degree of late-race slowing was strongly predicted by first-half/second-half pace ratio.[13] A study of 137,000 results showed that slower runners — who tend to go out harder relative to their ability — show much greater late-race collapse than faster runners.[14]

How to prevent it

Before the race: carb loading

Carb loading works. It reliably increases muscle glycogen and improves performance in events lasting more than 90 minutes by roughly 2–3%.[7] But most people do it wrong.

The old protocol — a multi-day depletion phase followed by carb loading — is unnecessary. Research from Bussau et al. showed that a single day of 10 g/kg body weight in carbohydrates plus rest achieves full glycogen supercompensation.[8] No depletion ride. No miserable low-carb days. For a 70 kg runner, that's about 700 g of carbs — roughly 7 cups of cooked rice or 14 bagels spread across the day. It's a lot, but it doesn't require suffering.

During the race: the new 80-120 gram window

The science here has moved faster than most running websites have kept up with. Here's the current state:

Your gut can absorb glucose through a transporter called SGLT1, which maxes out at about 60 g per hour. For years, that was the ceiling. Then Jeukendrup's lab showed that adding fructose — which uses a different transporter called GLUT5 — opens a second absorption channel. Together, they allow up to 90 g/hr of carbohydrate to reach your bloodstream.[9][15]

Recent research has pushed the boundary further. Hearris et al. demonstrated in 2022 that trained athletes can oxidize exogenous carbohydrate at rates consistent with 120 g/hr intake, and that it works comparably across delivery formats — gels, drink mix, chews, or a combination.[16] Podlogar and Wallis's 2022 review in Sports Medicine updated the practical recommendations to 80–120 g/hr for well-trained athletes with adapted guts.[17]

What to look for: products that use a glucose-fructose blend (sometimes listed as maltodextrin + fructose). Popular options include Maurten, SiS Beta Fuel, and Neversecond C30 — all designed around dual-transport absorption. The optimal ratio is approximately 2:1 glucose-to-fructose at moderate intakes, with newer research suggesting closer to 1:0.8 at very high intakes (above 90 g/hr).[17]

Training your gut

Here's the catch: you can't just show up on race day and slam 90 g of carbs per hour if you've never practiced it. Your gut will rebel — nausea, cramping, the kind of GI distress that sends you hunting for porta-potties.

But the gut is trainable. Cox et al. showed that 28 days of high-carb feeding during training significantly increased exogenous carbohydrate oxidation rates — your intestinal transporters literally upregulate in response to repeated demand.[18] More remarkably, Miall et al. demonstrated that just two weeks of gut training reduced GI symptoms and carbohydrate malabsorption during exercise.[19]

The protocol is simple: practice your race nutrition on your long runs. Start at a tolerable level — maybe 40–50 g/hr — and build up over 2–4 weeks to your race target. Use the same products you'll use on race day. If you're targeting 80+ g/hr, make sure your products contain both glucose and fructose to take advantage of dual transport.

This isn't optional. In ultramarathons, GI distress affects up to 96% of participants and is the single strongest predictor of failing to finish.[20][21] At the marathon distance, the stakes are lower but the principle is the same: an untrained gut is a liability.

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