Is it okay to not eat before a marathon? The Expert Take on Fueling

October 18, 2025 •

2 min read

The average adult body stores only enough glycogen to fuel approximately 90 minutes of continuous, high-intensity exercise. Given this fact, it is emphatically not okay to not eat before a marathon, a demanding endurance event lasting much longer for nearly every participant.

Quick Summary

Skipping your pre-marathon meal leaves your body without a crucial energy source, leading to a phenomenon known as 'bonking.' Strategic fueling is essential for maximizing performance, avoiding severe fatigue, and ensuring a safer race experience.

Key Points

Do Not Run on Empty: Running a marathon without eating is dangerous and will lead to severe performance issues and extreme fatigue.
Prevent Bonking: The primary risk of inadequate fueling is hitting "the wall" or "bonking," a complete crash caused by glycogen depletion.
Strategically Carb-Load: Start increasing your carbohydrate intake 2-3 days before the marathon to maximize glycogen stores in your muscles and liver.
Time Your Breakfast: Eat a high-carb, low-fiber meal 2-4 hours before the race to allow for proper digestion and energy availability.
Practice During Training: Use your long training runs to experiment with and finalize your race-day fueling plan. Never try new foods on race day.
Be Mindful of Risks: Inadequate fueling increases the risk of injury, muscle loss, and a compromised immune system.

The Science Behind Pre-Marathon Fueling

A marathon is a significant test of endurance requiring careful nutritional preparation. Your body primarily uses carbohydrates, stored as glycogen, for fuel during endurance activities. Without sufficient glycogen, your performance will decline.

Why Carbohydrates are Key

Carbohydrates provide the most efficient energy source for high-intensity exercise. While fat also provides energy, it is slower to access. Maintaining adequate glycogen levels through proper fueling before and during the race is crucial.

Understanding the Bonk

"Bonking" or "hitting the wall" occurs when muscle glycogen stores are depleted, forcing the body to rely heavily on fat for energy. This results in a sudden loss of pace, extreme fatigue, and heavy legs. Symptoms can include overwhelming fatigue, dizziness, nausea, and muscle weakness. Inadequate fueling makes bonking highly likely in a marathon.

The Risks of Running on Empty

Beyond bonking, running a marathon without proper fuel carries several risks:

Muscle Breakdown: The body may break down muscle protein for energy when glycogen is gone.
Weakened Immune System: Depleted glycogen can suppress immune function after intense exercise.
Poor Performance and Injury: Fatigue and lack of coordination increase injury risk.
Physical Danger: Severe low blood sugar can lead to confusion.

How to Fuel for Your Marathon

A solid fueling plan, practiced during training, is key.

Carb-Loading

Carb-loading maximizes glycogen stores. Start increasing carb intake 2-3 days pre-race, focusing on easy-to-digest options like pasta and rice. Avoid high-fiber, high-fat foods.

Race-Day Breakfast

Eat 2-4 hours before the start to allow digestion. Stick to familiar, high-carb, low-fiber foods like oatmeal or a bagel. Don't overeat.

Train Your Gut

Practice your race-day fueling plan during long training runs to ensure your stomach can handle the foods and drinks you'll use.

Pre-Marathon Fueling: Right vs. Wrong


Metric	Fasted (Wrong)	Fueled (Right)
Primary Energy Source	Primarily fat (slower)	Glycogen (carbohydrates) (faster)
Marathon Performance	Severely compromised; high risk of bonking	Optimized; sustained energy and pace
Injury Risk	Higher due to fatigue and muscle breakdown	Lower due to sustained muscle strength and coordination
Fatigue Level	Acute, rapid onset, and debilitating	Gradual and manageable with proper fueling
Recovery	Longer recovery time due to muscle damage	Faster recovery due to less stress on the body

Conclusion: Fuel Your Body, Respect the Distance

Skipping your pre-marathon meal is detrimental to performance and safety. A strategic nutrition plan, including carb-loading and a well-timed breakfast, is essential for a successful marathon. Fuel adequately, practice your plan, and enjoy a stronger race.

For more detailed guidance on marathon nutrition from a leading resource, refer to this guide from Marathon Handbook.

Frequently Asked Questions

If you don't eat anything before a marathon, your body will deplete its stored glycogen reserves, causing you to 'bonk' or 'hit the wall,' resulting in overwhelming fatigue and a dramatic slowdown in performance.

No, fasted running is not a recommended strategy for race day. While some athletes use fasted training runs sparingly to improve fat-burning efficiency, it is not suitable for a high-intensity, long-duration event like a marathon.

The best foods are high-carbohydrate, low-fiber, and easy to digest. Examples include a bagel with peanut butter, oatmeal, bananas, and a sports drink. The key is to stick with foods you have tested during training.

You should have your pre-marathon breakfast 2 to 4 hours before the race starts. This timing allows for digestion so the energy is readily available in your bloodstream at the start line.

Carb-loading is important because it maximizes the glycogen stored in your muscles, providing a sustained energy source for the marathon. You should begin a strategic carb-load 2 to 3 days before the race.

Hitting the wall feels like a sudden and profound onset of fatigue. Your legs feel heavy, your pace plummets, and you may experience dizziness, nausea, or confusion, indicating that your glycogen stores are empty.

No, you cannot rely solely on energy gels. While gels are crucial for mid-race fueling, they are meant to supplement, not replace, a proper pre-race meal. Starting on an empty tank puts you at a significant disadvantage.