How to Avoid Bonking When Running?

What “bonking” means and why it happens?

Bonking is a sudden drop in energy caused by glycogen depletion, when your body runs out of readily available carbohydrates.

In running, it typically shows up as:

• Heavy legs
• Slowing pace despite effort
• Dizziness or mental fog
• Inability to maintain intensity

It’s not just fatigue. It’s a fuel failure. For endurance runners, avoiding bonking is critical for maintaining pace, especially in long runs, half marathons, and marathons.

Why runners bonk?

Bonking is rarely caused by one factor. It’s usually a combination of pacing, fueling, and preparation errors.

Running too hard early

Starting faster than your sustainable pace increases carbohydrate usage rapidly.

This leads to:

• Faster glycogen depletion
• Earlier onset of fatigue

Even small pacing mistakes early can trigger a major slowdown later.

Poor fueling strategy

If you don’t consume enough carbohydrates before or during your run, your body runs out of fuel.

This is especially common in:

• Long runs over 75 to 90 minutes
• Races where fueling is neglected

Inadequate pre-run nutrition

Starting a run under-fueled is one of the fastest ways to bonk.

Common issues:

• Skipping meals
• Not eating enough carbohydrates beforehand
• Running fasted when intensity is high

Lack of training adaptation

Your body can be trained to:

• Store more glycogen
• Use fat more efficiently

Without consistent endurance training, your ability to sustain energy is limited.

How to pace your runs to avoid bonking?

Pacing is the first line of defense against energy collapse.

Start controlled

Your early pace should feel comfortable, not forced.

Focus on:

• Relaxed breathing
• Smooth stride
• Effort that feels sustainable

If the first part of your run feels easy, you’re pacing correctly.

Avoid early surges

Sudden increases in pace, especially on hills, accelerate glycogen usage.

Instead:

• Keep effort consistent
• Let pace adjust naturally to terrain

Run within your capacity

Trying to hold a pace beyond your current fitness leads to early burnout.

A well-paced run allows you to:

• Maintain effort
• Finish strong or steady

For pacing strategies, checkout our guide on how to improve running pace and why it is important.

During the run

For runs beyond moderate duration, start fueling early.

Key principles:

• Begin fueling before you feel tired
• Take in small, regular amounts
• Use easily digestible carbohydrates (gels, drinks, chews)

Waiting until you feel low on energy is too late.

Hydration matters

Dehydration accelerates fatigue and contributes to bonking.

Focus on:

• Drinking consistently
• Adjusting intake based on heat and effort

Even mild dehydration can impact performance.

Training your body to resist bonking

You can improve your ability to sustain energy through structured training.

Long runs

Long runs increase:

• Glycogen storage capacity
• Fat utilization efficiency

They are essential for endurance development.

Progressive runs

Running with increasing effort toward the end teaches your body to:

• Maintain performance under fatigue
• Manage energy distribution

Practicing fueling in training

Never try new fueling strategies on race day.

Use training runs to:

• Test different products
• Find what your stomach tolerates
• Dial in timing and frequency

Recognising early signs of bonking

Catching it early allows you to respond before it worsens.

Physical signs

• Sudden drop in pace
• Heavy or unresponsive legs
• Increased perceived effort

Mental signs

• Loss of focus
• Irritability
• Difficulty maintaining rhythm

What to do if you start to bonk?

If you feel energy dropping mid-run:

• Slow your pace immediately
• Take in quick carbohydrates
• Focus on controlled breathing
• Allow your body time to recover

You may not fully recover, but you can limit the damage.

Common mistakes that lead to bonking

• Starting too fast
• Skipping pre-run nutrition
• Not fueling during long runs
• Ignoring hydration
• Trying to “push through” early fatigue
• Not practicing fueling in training

Practical checklist to avoid bonking

• Eat properly before longer runs
• Start at a controlled, sustainable pace
• Fuel early and regularly during runs
• Stay hydrated throughout
• Include long runs in your training
• Practice your race-day fueling strategy
• Avoid sudden pace surges

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