What is bonking in running?
Bonking isn't just "feeling tired." It's a metabolic crisis. Your brain runs almost exclusively on glucose, and when glycogen stores bottom out, your central nervous system starts rationing energy. The result is sudden, dramatic, and unmistakable.
Glycogen Depletion
Your muscles store glycogen — chains of glucose molecules — as their primary fuel source during moderate-to-high intensity exercise. You burn roughly 1 kcal per kg per km (Hall et al., 2004). Your stores hold ~1,500–2,000 kcal. Do the math.
Central Governor Shutdown
Your brain monitors fuel levels and starts throttling output before you're fully depleted — muscle biopsies show 15–25% of glycogen remains even at "exhaustion" (Noakes et al., 2005). Pace drops. Concentration fades. This is your body's emergency brake, not a lack of willpower.
The Wall
Full bonk. Legs feel like concrete. Vision narrows. Nausea, dizziness, emotional collapse. Your body has switched to fat oxidation, which can't sustain the pace you were running. You go from racing to surviving.
How it unfolds during a marathon
Glycogen stores are full. Effort feels easy. Most athletes make the mistake of banking time here, burning fuel faster than planned.
Glycogen is declining but still sufficient. If you've been fueling on schedule (every 20–30 min), you're replacing some of what you're burning. If you haven't started fueling, the clock is ticking.
Glycogen is getting low. Your body starts relying more on fat oxidation, which requires more oxygen per calorie. Pace feels harder even though HR is the same. The central governor is starting to pump the brakes.
Glycogen is depleted. The bonk hits. Every mile feels like three. This is where the race is won or lost, and it was decided by what you did in miles 1–15, not what you do now.
Marathon fueling strategy: 3 rules to never bonk
Carb-Load Right
The 36–48 hours before race day matter more than anything you eat during the race. Target 10–12g of carbs per kg of body weight per day (Burke et al., 2011). This isn't pasta night — it's a deliberate fueling protocol.
Fuel Early, Fuel Often
Start taking in carbs within the first 30 minutes and every 20–30 minutes after that. With a glucose-fructose mix and a trained gut, you can absorb up to 120g/hr using dual transport pathways (Jeukendrup, 2010; Hearris et al., 2022). By the time you feel like you need fuel, it's already too late.
Train Your Gut
Your gut is trainable. Practice your race nutrition on every long run. Research shows gut adaptations — improved tolerance and carb oxidation — occur within as little as 2 weeks of consistent practice (Cox et al., 2010; Miall et al., 2018). This is not optional.
Marathon Bonk Calculator
Estimate when you'll hit the wall based on your weight, pace, distance, and fueling plan. This is a simplified model — individual results vary.
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Fueling guides
Race-specific fueling breakdowns with the actual numbers — carb targets, dual-transport protocols, GI management, and what the research says versus what the forums say.
Fueling is just the start
Nutrition is one piece of the puzzle. But your bonk was decided in training — undertrained base, wrong intensity distribution, or a plan that doesn't know your data. DialedCoach checks your Strava activity, your local weather, and your race course before prescribing anything.
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