Is it good to carbo load before a run? The expert guide

December 6, 2025 •

4 min read

Carbohydrate loading can boost performance by up to 3% and endurance by 15-25% in events lasting longer than 90 minutes. This strategy, known as carbo loading, is highly effective for endurance athletes but can be detrimental for shorter distances if not performed correctly.

Quick Summary

Carb loading is a proven nutritional strategy for endurance events over 90 minutes. The article details the scientific basis, proper timing, specific food choices, and common mistakes to help runners maximize glycogen stores and enhance performance on race day.

Key Points

Endurance is Key: Carbo loading is primarily beneficial for endurance events over 90 minutes, such as marathons, by maximizing glycogen fuel stores.
Timing Matters: Begin increasing your carbohydrate intake 2-3 days before your race, not with one large meal the night before, to effectively fill your muscle glycogen.
Choose Low-Fiber Carbs: Focus on easily digestible carbohydrates like white rice, pasta, and bananas, and minimize high-fiber and high-fat foods to avoid gastrointestinal issues on race day.
Don't Forget Hydration: Increase your fluid intake during the loading phase, as each gram of stored glycogen binds with extra water, which helps with hydration during the race.
Practice and Personalize: Always test your carb-loading strategy during your long training runs to see how your body responds and adjust your plan accordingly before race day.
Balance Your Macronutrients: During the loading period, increase carbohydrates by reducing your protein and fat intake to avoid overeating and feeling sluggish.

The Science Behind Carbo Loading

Carbohydrates are converted into glucose by the body and stored as glycogen in muscles and the liver, providing the primary energy source for moderate to high-intensity exercise. For runs lasting over 90 minutes, these glycogen stores can become depleted, leading to fatigue. Carbo loading maximizes these reserves, ensuring a full fuel tank to delay fatigue and improve endurance performance. Relying on fat for energy when glycogen is low is less efficient, forcing a slower pace.

Why Do Runners Need Glycogen?

Sufficient glycogen is crucial because it provides readily available energy. Without it, the body uses fat, which is a slower energy conversion process, impacting speed and endurance. Adequate glycogen allows muscles to maintain intensity during longer races.

Who Should Carbo Load Before a Run?

The need for carbo loading depends on the length of the run.

Long-Distance Runners (over 90 minutes): This includes marathon and ultramarathon runners for whom maximizing glycogen stores is vital.
Short-Distance Runners (under 90 minutes): For 5K and 10K races, normal glycogen levels are sufficient. Carbo loading is unnecessary and could add unwanted water weight.

How to Properly Carbo Load

A modern and effective approach involves increasing carbohydrate intake for two to three days alongside reduced training.

The 2–3 Day Protocol

Timing: Begin 36-48 hours before the race.
Dosage: Target 10-12 grams of carbohydrates per kilogram of body weight daily. For a 70kg runner, this is 700-840 grams of carbs daily. Distribute this across multiple smaller meals and snacks.
Food Choices: Prioritize carbohydrates while lowering fat and protein to avoid feeling too full. Meals should be carb-heavy.
Hydration: Increase fluid intake as glycogen stores bind with water. Incorporating fruit juice or sports drinks can help meet carb goals.

Common Mistakes to Avoid

Loading Only the Night Before: A single large meal isn't enough; loading takes several days. It can also lead to digestive issues on race day.
Eating Too Much Fiber or Fat: High-fiber and high-fat foods can cause stomach problems during a run. Choose low-fiber options like white rice or plain pasta in the final 24-48 hours.
Skipping the Taper: Reducing training volume is essential so muscles can store the extra glycogen.

Comparison of Carb-Loading Strategies


Feature	Long Distance (Marathon+)	Short Distance (5K/10K)
Need for Carb Load	Necessary and highly beneficial	Not necessary, may be counterproductive
Timing of Carb Increase	Start 2-3 days before the event	No specific loading period; focus on a normal, healthy diet
Carbohydrate Goal	10-12 g/kg of body weight per day	Normal daily carbohydrate intake for training
Focus Foods	Low-fiber, easily digestible carbs (white rice, white pasta, bananas, juice)	Balanced, regular diet (including whole grains, fruits, vegetables)
Tapering	Essential to maximize glycogen storage	Standard taper for recovery, not for specific glycogen storage
Associated Effects	Temporary water weight gain (1-2kg), full feeling	No significant changes in weight or digestion

The Race Day Breakfast

Race morning breakfast replenishes liver glycogen and should be eaten 2-4 hours before the start to allow for digestion and stable blood sugar. Choose a familiar, high-carb, low-fiber meal like a bagel with jam or plain oats. A small snack 30-60 minutes before the race can provide a final energy boost.

Conclusion: Practice Makes Perfect

Carbo loading is beneficial for runs over 90 minutes but not necessary for shorter distances. The best approach involves increasing low-fiber carbohydrates over 2-3 days while tapering training and staying hydrated. Practicing your fueling strategy during training is vital to determine what works best for your body and avoid race day issues. A well-executed carb load ensures you are properly fueled for peak performance. {Link: Johns Hopkins Medicine https://www.hopkinsmedicine.org/health/wellness-and-prevention/runners-diet} provides further information on fueling.

What to eat for carb loading before a run (example meals)

Examples include white rice porridge, white rice with lean protein, pasta, baked potato (without skin), plain bagels with jam, plain oats, and white bread sandwiches. These meals focus on easily digestible, low-fiber, high-carbohydrate options to maximize glycogen storage and minimize stomach issues on race day. Practice these meals during training.

The Role of Fluids in Carbo Loading

Increased fluid intake is crucial as glycogen binds with water (3-4g water per gram of carb). Hydration is key for race day. Drink fluids regularly in the days before, avoiding excessive intake. Liquid carbs like juice or sports drinks can help reach carb goals without excessive fullness. Adding salt can aid fluid retention.

Conclusion

Carbo loading is a valuable strategy for endurance events over 90 minutes. It requires increasing low-fiber, high-carb foods for 2-3 days while tapering and hydrating. Practice is crucial for personalization and avoiding issues. This preparation helps runners perform their best.

Key Learnings

Benefits: Carbo loading improves performance in events over 90 minutes.
Distance Matters: Not needed for 5K or 10K; normal diet suffices.
Timing: 2-3 days before, not just the night before.
Food Focus: Prioritize low-fiber, high-carb foods; reduce fats/high-fiber.
Hydration is Key: Increase fluids during loading as glycogen stores water.
Practice is Crucial: Test strategy during training runs.
Avoid Overeating: Spread carbs across smaller meals/snacks.
Race Day Meal: Carb-rich breakfast 2-4 hours before to top liver glycogen.

Frequently Asked Questions

No, it is not necessary to carbo load for a 5K race. The distance is short enough that your body’s normal glycogen stores are sufficient for performance. A full carb load for a short race may only lead to unnecessary water weight gain.

Instead of one big meal the night before, true carbo loading happens over 2-3 days. A good meal the night before is a familiar, high-carb, low-fiber choice like white pasta with plain marinara sauce. Avoid heavy, fatty sauces or unfamiliar foods that might cause digestive issues.

Yes, it can cause a temporary weight gain of 1-2 kg. This is not fat but rather the extra water your body stores along with the glycogen. This is a positive sign that your fuel stores are topped up.

For endurance events, aim for 10-12 grams of carbohydrates per kilogram of body weight per day for two days leading up to the race. A 70kg person, for example, would need 700-840 grams of carbs daily.

Modern research shows that a depletion phase (exercising with a low-carb diet) is not necessary and can be detrimental. Simply tapering your training while increasing carbs for 2-3 days is sufficient to maximize glycogen stores.

Eat a light, easily digestible, high-carb breakfast 2-4 hours before the race. Examples include a bagel with jam, plain oats with a banana, or white toast. This tops off your liver glycogen without causing stomach upset.

During the final days of loading, it's best to reduce high-fiber foods. While healthy normally, too much fiber before a race can cause gastrointestinal distress. Stick to low-fiber options like white pasta and white rice.