Why Runners Bonk After 25K

The physiological reasons behind the dreaded bonk at 25 kilometers and how proper fueling planning prevents it.

The 25-kilometer mark is where marathons truly begin. It’s also where most runners hit the wall — that sudden, devastating loss of energy and focus.

The Glycogen Depletion Curve

Your body stores roughly 2,000–2,500 calories of glycogen. At marathon pace, you burn approximately 600–700 calories per hour. Simple math: by 2.5–3 hours (around 25–30km), you’re running on empty.

The Gut Shutdown Problem

As intensity persists, blood flow diverts from the digestive system to working muscles. By 90 minutes, absorption efficiency drops. If you haven’t fueled adequately in the first hour, it’s too late by 25km.

Sodium’s Hidden Role

Bonking isn’t always about carbs. Low sodium impairs nerve transmission and muscle contraction, accelerating perceived fatigue. Many runners who think they “bonked” actually crashed from electrolyte imbalance.

Prevention Through Planning

Pre-load: Start fueling at minute zero, not minute sixty
Double-source: Use multiple carb types for better absorption
Sodium discipline: 500–800mg/hr for most conditions
Know your course: Aid station spacing determines refill feasibility

Nakamuve helps you plan these variables before you’re 25km deep with no strategy.