Understanding Glycogen and Bonking

Glycogen is stored carbohydrate in your muscles and liver—your body's primary fuel for running. When it runs out, you "bonk" or "hit the wall."

What Bonking Feels Like

• Sudden, severe fatigue
• Legs feel like lead
• Pace drops dramatically
• Difficulty concentrating
• Sometimes dizziness or confusion

The wall: In marathons, "the wall" typically hits around miles 18-22 when glycogen depletes.

Your Glycogen Stores

Average runner stores:

• Muscle glycogen: ~1,400-1,800 calories
• Liver glycogen: ~300-400 calories
• Total: ~1,700-2,200 calories

With carb loading:

• Can increase muscle glycogen by 20-40%
• Total may reach 2,500-3,000 calories

How Running Burns Glycogen

Your body burns a mix of fat and carbohydrates. The harder you run, the more carbs you burn.

Intensity	Carb % of Energy	Approximate Burn Rate
Easy	50-60%	200-400 cal/hour
Moderate	65-75%	400-600 cal/hour
Hard	80-90%	600-900 cal/hour
Sprint	90%+	1000+ cal/hour

Key insight: Running easier burns more fat and spares glycogen, extending your range.

Fueling Strategies

Pre-Run Fueling

2-3 hours before:

• 50-100g carbohydrates
• Low fat, low fiber
• Familiar foods

This tops off liver glycogen depleted overnight.

During-Run Fueling

Start at 45-60 minutes for runs over 90 minutes.

Duration	Carbs/Hour	Strategy
60-90 min	0-30g	Optional
90-120 min	30-45g	1 gel every 45 min
2-3 hours	45-60g	Gel every 30-40 min
3+ hours	60-90g	Multiple sources

Carb Loading

For races over 90 minutes:

• Increase carbs to 8-10g per kg body weight
• 2-3 days before the race
• Can increase glycogen by 20-40%

Preventing the Bonk

Training adaptations:

• More long runs → better fat utilization
• Fueling practice → trained gut
• Consistent training → efficient metabolism

Race day tactics:

• Start fueling early (don't wait until you're depleted)
• Pace conservatively early
• Practice everything in training

Fat as Fuel

Your body has essentially unlimited fat stores—even lean runners have 30,000+ calories of fat.

The problem: Fat can only fuel slower running. As intensity increases, your body must rely more on glycogen.

The solution: Train your body to burn more fat at faster paces through:

• Consistent easy running
• Long runs
• Some fasted easy runs (occasionally)

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Understanding your glycogen needs helps you fuel appropriately. The calculator estimates when you might deplete—but individual variation is significant. Practice in training to learn your body's signals.

Plan your race nutrition with our Race Nutrition Plan Template.