The Ultimate Guide to Carb Loading for Ultra Running

How to Start Carb Loading Before a Race: Fuel for Ultra Marathon Endurance 

Ultra running, a demanding endurance event, pushes the limits of an athlete’s endurance, grit, and strategic nutritional planning. A pivotal element in gearing up for such a strenuous challenge is carb loading, a well-established technique embraced by endurance athletes, particularly marathon and ultra marathon runners. As the race day approaches, understanding and implementing effective carbohydrate loading becomes crucial for fueling the intense physical exertion these races demand.

In this guide, we explore the intricacies of carb loading, a method that significantly boosts muscle glycogen stores, providing the much-needed fuel source for prolonged high-intensity activities. Whether you’re a veteran marathoner or gearing up for your first ultra, mastering carb loading can be the difference between hitting the wall and powering through to the finish line.

What is Carb Loading?

Carb loading, or carbohydrate loading, is a nutritional strategy widely used by endurance athletes, including marathon runners and ultra marathon participants. It’s a process of increasing carbohydrate intake in the days leading up to an endurance event like a marathon or an ultra marathon. This technique is aimed at boosting the amount of fuel stored in the muscles, essential for long-distance events.

How Does Carbohydrate Loading Work?

Maximising Glycogen Storage

Carb loading enhances glycogen storage in muscles. Glycogen is the stored form of glucose and serves as a readily available energy source for endurance sports. Normally, the body can store glycogen in muscles and the liver, but these stores are finite. By increasing carb intake, you can significantly expand these glycogen reserves.

Energy Boost for Endurance Performance

The fundamental purpose of carb loading is to ensure that your fuel tank – the glycogen stores in your muscles – is as full as possible come race day. This is crucial for endurance sports, where energy demand is high and sustained over a long period. Proper carb loading can help avoid the dreaded scenario of ‘hitting the wall’, where runners experience a significant drop in energy due to depleted glycogen stores.

Taper and Carb Load

A typical strategy involves a taper in training load while simultaneously increasing carb intake. This combination allows the body to store more glycogen than usual. The process often starts about 3-4 days before the race and includes a tapering period where the training intensity decreases, allowing the body to rest and maximise glycogen storage.

The Science Behind Carb Loading Before a Race

Glycogen Storage

  • Muscle Glycogen: The primary focus of carb loading is to increase muscle glycogen stores.
  • Liver Glycogen: Carb loading also affects liver glycogen, which helps maintain blood glucose levels, crucial for brain function and overall energy balance.

Enhanced Performance

  • Endurance Benefits: Increased glycogen stores translate to improved endurance performance. This means a runner can maintain a desired pace for longer without running out of energy.
  • Impact on Marathon Time: For many athletes, effective carb loading can make the difference between achieving a personal best or struggling in the later stages of a race.

The Importance of Carbohydrates for Ultra Runners

Carbohydrates are the cornerstone of an endurance athlete’s diet, especially for events like marathons or ultra marathons.

Carbs as an Energy Source

Quick Energy

  • Immediate Fuel: Carbohydrates provide quick and efficient energy, which is vital for the high-intensity demands of ultra running.
  • Muscle Glycogen Stores: Consuming carbs before and during a race replenishes muscle glycogen, ensuring a steady energy supply.

Sustained Performance

  • Long-Distance Fuel: Adequate carb intake is essential for maintaining energy levels throughout the race. This is particularly important in events like half marathons, marathons, or ultra marathons, where energy demand is continuous and intense.

Carbs vs. Other Energy Sources

Fats and Proteins

  • Efficient Use with Carbs: While fats and proteins play important roles in an athlete’s diet, they are more efficiently used in the presence of adequate carbohydrates.
  • Balanced Diet: A diet high in carbohydrates, complemented by sufficient proteins and fats, is recommended for endurance athletes to ensure all-around nutrition and optimal performance.

Nutrient Role in Endurance Running

Nutrient TypeRole in Endurance Running
CarbohydratesQuick energy source, maintains glycogen levels
FatsProvides sustained energy for longer durations
ProteinsAids in muscle repair and recovery

Preparing for Carb Loading

Proper preparation for carb loading is not just about increasing carbohydrate intake; it’s about strategically fueling your body for the intense demands of an ultra marathon.

When to Start Carb Loading


  • Ideal Window: Begin carb loading 3-4 days before your ultra marathon to allow enough time to maximise glycogen stores effectively.
  • Adaptation Period: This period allows your muscles to gradually adapt to storing more glycogen, which is vital for sustained energy during the run.

Gradual Increase

  • Steady Build-Up: Start by slightly increasing your carbohydrate intake each day, then gradually build up. This method helps your digestive system adjust smoothly.
  • Balanced Meals: Integrate carbohydrates as a larger component of your meals, rather than consuming large quantities in one sitting to avoid overwhelming your digestive system.

Calculating Carb Needs

General Guideline

  • Carb Intake Formula: The general recommendation is to consume 8-10 grams of carbohydrates per kilogram of body weight daily. For a 70 kg athlete, this translates to approximately 560-700 grams of carbohydrates per day.
  • Energy Requirements: These recommendations are based on the typical energy demands of ultra runners.

Personal Adjustments

  • Individual Differences: Each athlete’s body responds uniquely to carb loading; some may require more carbohydrates, while others may need less.
  • Training Intensity: Your carb intake should align with the intensity of your training regimen. Higher intensity training may necessitate a higher carbohydrate intake.

Guidelines for Gradual Carb Loading

Incremental Increase

  • Weekly Build-Up: Begin to incrementally increase your daily carbohydrate intake by 10-20% starting a week before the event. This gradual approach ensures your body can comfortably adjust to the increased carb intake.
  • Variety of Sources: To maintain a balanced diet, incorporate a diverse range of carbohydrate sources. This not only keeps your meals interesting but also ensures a wide range of nutrients.

Monitor Response

  • Listen to Your Body: It is crucial to observe how your body reacts to the increased carbohydrate intake. Be vigilant for signs of digestive discomfort or bloating.
  • Adjust as Needed: Be flexible and ready to modify your carbohydrate intake based on your body’s feedback.

Carb Loading Schedule Table

Days Before EventCarb Intake IncreaseExample Foods
7 – 5+10%Whole grain bread, oatmeal
4 – 3+15%Brown rice, sweet potatoes
2 – 1+20%Pasta, bananas, energy bars

Carb Loading Strategy for a Marathon or Ultra

Developing a structured carb loading strategy is critical for maximising its effectiveness, ensuring you are adequately energised for your ultra marathon while maintaining overall health and digestive comfort.

Daily Meal Planning


  • Even Distribution: Aim to evenly distribute your increased carb intake throughout the day. This approach helps maintain steady energy levels and prevents blood sugar spikes.
  • Meal Examples: Incorporate carb-rich foods in each meal, such as oatmeal for breakfast, a sandwich for lunch, and pasta for dinner.

Sample Carb Loading Meal Plan

Meal TimeCarb-Rich FoodsProtein SourcesFat SourcesNotes
BreakfastOatmeal, fruitsGreek yogurtAlmonds, chia seedsHigh in complex carbs and fibre
LunchWhole grain sandwich, quinoa saladGrilled chicken, tofuAvocado, olive oilBalanced with protein and healthy fats
DinnerPasta, brown riceFish, beansCheese, nutsFocus on easy-to-digest carbs


  • Nutrient Integration: While the focus is on carbs, it’s important to integrate proteins and fats for a balanced diet, crucial for muscle repair and overall health.
  • Proportion: Aim for meals where carbohydrates form about 60-70% of your intake, complemented by proteins and fats.

Balancing Carbs with Other Nutrients


  • Muscle Repair: Proteins are essential for repairing and building muscle tissue, particularly important during the recovery phase post-exercise.
  • Sources: Include lean meats, fish, dairy, or plant-based proteins like lentils and beans.


  • Overall Health: Fats are necessary for hormone production, cell health, and nutrient absorption.
  • Healthy Choices: Opt for unsaturated fats found in foods like avocados, nuts, olive oil, and fatty fish.

Hydration Strategy During Carb Loading

Increased Water Intake

  • Digestive Aid: As carb intake increases, so does the need for water to help with digestion and nutrient absorption.
  • Guideline: Aim for at least 2-3 litres of water daily, adjusting based on activity level and personal needs.

Electrolyte Balance

  • Electrolyte Replenishment: To maintain electrolyte balance, consider sports drinks or electrolyte supplements, especially if you’re sweating more due to increased training.

Common Mistakes in Carb Loading

To ensure your carb-loading strategy is effective and comfortable, it’s crucial to avoid common pitfalls. These mistakes can significantly impact your performance and overall well-being during an ultra marathon.


Avoid Excess

  • Calculated Intake: It’s essential to adhere to a carb intake that aligns with your body weight and training needs. Over Consuming carbs can lead to gastrointestinal discomfort, sluggishness, and even impaired performance. Use a carb-loading calculator or consult with a nutritionist to determine the right amount.
  • Mindful Eating: Eating mindfully means being aware of your body’s hunger and fullness cues. It’s not necessary to force-feed yourself to meet carb targets; instead, focus on feeling comfortably full. Overeating can lead to bloating and lethargy, which are detrimental to runners.

Neglecting Fibre Balance

Moderate Fibre

  • Digestive Comfort: While fibre is a crucial part of a healthy diet, excessive fibre intake during carb loading can cause digestive issues like bloating, gas, and discomfort. This can be particularly problematic during long-distance running.
  • Fibre Sources: In the days leading up to the race, gradually shift your carb sources. Start reducing the intake of high-fibre foods such as whole grains, legumes, and certain vegetables, and transition to lower-fibre, easily digestible carbohydrate sources like white rice, pasta, and ripe bananas. This helps maintain energy levels without overloading the digestive system.

Ignoring Hydration

Stay Hydrated

  • Water Intake: Proper hydration is a vital component of effective carb loading. Carbohydrates store water and electrolytes in your body, so as your carb intake increases, your body’s water requirement also rises. Adequate hydration is crucial for preventing digestive issues and facilitating nutrient absorption. Aim to drink at least 2-3 litres of water daily, and adjust this based on your training intensity and environmental conditions.
  • Electrolyte Balance: Along with water, maintaining electrolyte balance is important. Electrolytes, particularly sodium, potassium, and magnesium, are crucial for muscle function and fluid balance. Use electrolyte supplements or sports drinks strategically to replenish electrolytes lost through increased training and sweating.

Carb Loading and Digestive Health

Effectively managing digestive health is a crucial component of a successful carb-loading strategy, especially when preparing for endurance events like ultra marathons.

Digestive Comfort

Gentle Foods

  • Easily Digestible Carbs: Choose carbohydrates that are easier on the stomach to minimise the risk of gastrointestinal distress. Options like white rice, bananas, and cooked potatoes are excellent because they provide the needed energy without overburdening the digestive system.
  • Meal Timing: Timing of meals is also crucial. Avoid consuming heavy, large meals immediately before training sessions or the event. Instead, opt for lighter, carb-focused meals that provide energy without causing discomfort or sluggishness. Smaller, more frequent meals can be more beneficial than three large ones.

Role of Fibre


  • Fibre Management: Balancing fibre intake is key during carb loading. Fibre is essential for gut health and regular bowel movements, but too much can lead to bloating, gas, and abdominal discomfort, which can be particularly problematic during running. Aim for a moderate fibre intake, and if necessary, slightly reduce high-fibre food consumption in the days leading up to the event.
  • Fibre Sources: Incorporate a mix of soluble and insoluble fibre sources. Soluble fibre, found in foods like oats and apples, can help with slow energy release, while insoluble fibre in vegetables and whole grains aids digestion.

Tips and Tricks


  • Tailoring to Needs: Personalise your carb-loading plan to fit your unique digestive system, training intensity, and past experiences. What works for one runner may not work for another. Pay attention to how different foods and meal timings affect your energy levels and digestive comfort.
  • Experimentation: Use training sessions to experiment with different carb-loading strategies and meal compositions. This can help you identify the best approach for your body.

Last-Minute Adjustments

  • Adaptability: Be adaptable with your carb-loading plan. If you notice certain foods aren’t sitting well, or your energy levels aren’t optimal, don’t hesitate to make adjustments. This could mean altering the types of carbs, the timing of your meals, or the balance of macronutrients.
  • Flexibility: Stay flexible and responsive to your body’s signals during your carb-loading phase. If you feel bloated, sluggish, or unusually fatigued, reassess your diet and make necessary changes. It’s important to listen to your body and adapt as needed.

Alternatives to Carb Loading

Carb loading isn’t the only strategy for ultra running nutrition.

Other Nutritional Strategies

  • Fat Adaptation: Some runners prefer training their body to utilise fat more efficiently.
  • Balanced Approach: A more moderate increase in carbs might suit some athletes better.

Mastering Carb Loading for Peak Performance

As we’ve explored in this guide, carb loading is a pivotal element in the nutrition plan of endurance athletes, particularly those preparing for a marathon or ultra marathon. When executed correctly, it can significantly enhance performance by maximising glycogen stores in the muscles. This strategy, often culminating with a pasta dinner the night before the race, is about more than just eating a big meal; it’s about a structured increase in carb intake leading up to the race day.

However, carb loading doesn’t follow a one-size-fits-all approach. It requires personalisation based on individual needs, race distance, and the runner’s usual diet. The key is to start carb loading correctly, which typically means gradually increasing carbs in the days prior to the race, avoiding anything new on race day, and ensuring you don’t run out of fuel. British marathon runner Ron Hill exemplified the potential of effective carb loading in his illustrious career, demonstrating the benefits of a well-planned nutrition strategy.

As a certified running coach would advise, it’s important to listen to your body and make adjustments based on how you feel. This means being attentive to signs of distress on race day and adjusting your intake of rich foods, low GI foods, or extra carbohydrates as needed.

Remember, carb loading before a marathon or an ultra marathon is as much about science as it is about art. The balance of grams of carbs per pound of body weight, timing (like 3-4 hours before the race or even 1 hour before the race), and the type of carbs consumed can all impact performance.

Q: What is carb loading and why is it important for endurance athletes?

A: Carb loading is the process of increasing your carb intake in the days leading up to a race in order to maximise muscle glycogen stores. It is important for endurance athletes because glycogen is a primary fuel source for prolonged physical activity, and having ample glycogen stored can help delay fatigue during long distance races.

Q: How can I carb load correctly for a big race?

A: To carb load correctly, you should gradually increase your carb intake in the week leading up to the race, with a focus on the last 2-3 days before the event. It’s important to not drastically change your diet but rather to include more carb-rich foods such as pasta, rice, potatoes, and fruits.

Q: What should I eat on the night before a race to fuel properly?

A: The night before a race, it’s recommended to consume a meal that is high in carbohydrates and moderate in protein and low in fat. Foods like pasta with a light tomato-based sauce, rice dishes, or a grain bowl can be good choices for carb loading and ensuring you have enough energy for the next day’s race.

Q: What are some key things to know about carb loading for runners?

A: When carb loading for runners, it’s essential to know that it’s not about just eating more carbs, but rather increasing your overall carb intake while maintaining a balanced diet. It’s also important to note that this strategy is more beneficial for endurance races like marathons, ultra marathons, and long distance events rather than shorter races like 5k or 10k runs.

Q: Is there anything new I need to know on race day regarding my carb intake?

A: On race day, there’s nothing new that should drastically change your carb intake compared to your carb loading routine. It’s important to stick to foods and drinks that have worked well for you during training and avoid trying new foods or supplements that could potentially cause stomach upset or digestive issues during the race.

Q: What happens if I run out of glycogen during a long distance race?

A: If you run out of glycogen during a long distance race, you may hit the dreaded “wall” or “bonk,” where you’ll experience a significant drop in energy levels and physical performance. This can lead to fatigue, mental fogginess, and a struggle to maintain your pace, emphasising the importance of effective carb loading for endurance events.

Q: Can I carb load for shorter races like a half marathon or 10k?

A: While carb loading can benefit athletes participating in longer races like marathons or ultra marathons, it may not be as necessary for shorter races such as a half marathon or 10k. These events typically do not deplete muscle glycogen stores to the same extent as longer races, so a balanced diet leading up to the race should provide sufficient energy.

Q: How does carb loading impact race nutrition and performance for endurance sports?

A: Carb loading can positively impact race nutrition and performance for endurance sports by ensuring that muscle glycogen stores are maximised, thus delaying fatigue and potentially improving overall race performance. By increasing carb intake in the days leading up to the race, endurance athletes can enhance their energy reserves and support sustained effort during the event.

Q: What should I eat in the hours before the start of a race to support my carb loading efforts?

A: In the hours before the start of a race, you should focus on consuming easily digestible carb-rich foods such as a bagel with jam, a banana, or a sports drink. These choices can help top off your glycogen stores without causing digestive discomfort, providing an additional boost of energy for the upcoming race.

Q: How can I increase my carb intake during race week leading up to a big event?

A: To increase your carb intake during race week, you can incorporate foods like whole grain bread, oatmeal, quinoa, sweet potatoes, and plenty of fruits into your meals and snacks. This gradually elevates your carbohydrate consumption and supports adequate glycogen storage for the upcoming race without dramatically altering your overall diet.

Author - Mathew Stuckey

Mathew Stuckey is the founder of Ultramarathon Central, an online platform dedicated to supporting and inspiring ultra runners from all walks of life. With a passion for pushing the limits of what's possible, Mathew has taken on some of the toughest ultra events in the UK, including the Monster Triathlon.

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