How to Carb Load for an Ultra Marathon: A Runner's Ultimate Guide

November 18, 2025 •

4 min read

Proper carbohydrate loading can improve performance by 2-3% in endurance events lasting over 90 minutes. For ultra marathon runners, this strategic approach to fueling is not just beneficial, but a critical component of a successful race strategy to maximize muscle glycogen stores and sustain energy.

Quick Summary

This guide provides a detailed breakdown of how to prepare your body for an ultra marathon by strategically increasing carbohydrate intake over a few days. It outlines optimal timing, food choices, hydration, and methods to prevent gastrointestinal issues for a strong race.

Key Points

Start 2-3 Days Out: Begin your carb loading 48-72 hours before the race, not just the night before, to effectively top off glycogen stores.
Target 8-12g/kg: Aim to consume 8-12 grams of carbohydrates per kilogram of body weight per day during the loading period, depending on duration.
Prioritize Easy Carbs: Focus on low-fiber, easily digestible carbohydrate sources like white pasta, rice, and bagels, while reducing fat and fiber intake.
Stay Hydrated: Increase your fluid intake, often with electrolytes, to aid in glycogen storage and ensure you start the race well-hydrated.
Practice During Training: Never try new foods or products during race week. Use your long training runs to experiment and perfect your carb loading strategy.
Consider Liquid Calories: Use sports drinks or juices to supplement solid food intake if you struggle to meet your carbohydrate targets without feeling overly full.
Don't Fear Water Weight: The temporary weight gain from water retention linked to glycogen storage is normal and beneficial for hydration during the race.

The Science of Fueling an Ultra

For events that extend beyond the standard marathon, relying solely on race-day fueling is a recipe for disaster. The body's primary fuel source for sustained, high-intensity effort is glycogen, a stored form of carbohydrates in the muscles and liver. While fat is also burned, it is less efficient for higher-intensity segments of a race. An ultra runner's limited glycogen reserves will deplete over hours of running, leading to fatigue and the dreaded "bonk". The goal of carb loading is to super-saturate these glycogen stores in the days leading up to the race, essentially starting with a full tank of premium fuel. This process is now understood to not require a depletion phase, a torturous and unnecessary practice from older protocols. By consuming an increased amount of carbohydrates alongside a reduced training load (tapering), you maximize glycogen storage.

The Strategic Carb Loading Timeline

Effective carb loading is a planned process, not a last-minute binge. The most effective window begins approximately 2 to 3 days before your race.

How Much to Eat

Duration: For a shorter carb load (1-2 days), aim for 10-12 grams of carbohydrates per kilogram of body weight per day.
Duration: For a more gradual approach (2.5-3 days), a target of 7-8 grams per kilogram is appropriate.

To calculate your personal target, multiply your body weight in kilograms by the appropriate number. For example, a 70kg runner targeting a 2-day load would aim for 700-840 grams of carbs daily.

What to Eat and What to Avoid

To achieve these high carbohydrate targets while minimizing gut distress, focus on easily digestible, lower-fiber options. Foods rich in fat, fiber, and protein should be limited as they slow digestion and increase the feeling of fullness.

Recommended High-Carb Foods

White bread, bagels, and English muffins
White rice, pasta, and couscous
Starchy vegetables like potatoes and sweet potatoes
Oats and low-fiber cereals
Fruits and fruit juice
Honey, jams, and maple syrup
Sports drinks, chews, and gels

Foods to Limit or Avoid

High-fat foods (creamy sauces, fried foods)
High-fiber foods (beans, broccoli, kale, most whole grains)
Excessive protein intake (large meat portions)
Cruciferous vegetables
Spicy or acidic foods

Comparison: Carb Loading vs. Fat Adaptation

While carb loading remains a standard approach, some ultra runners explore 'fat adaptation' to train their bodies to use more fat for fuel. Both have pros and cons, and the right approach depends on individual preference and race demands.


Feature	Carb Loading	Fat Adaptation
Primary Fuel Source	Glycogen (stored carbohydrates)	Fat (near-limitless stores)
Energy Release	Rapid, ideal for high-intensity efforts	Slower, more sustained energy for steady pace
Race-Day Intake	Strategic use of carbohydrates to top off stores	Carbohydrates used sparingly or for targeted boosts
Initial Phase	No depletion needed; simply increase carbs during taper	Adaptation period can involve fatigue (keto flu)
Gastrointestinal Impact	Risk of bloating and discomfort if not done carefully	Reduced risk of GI issues due to lower sugar intake
Performance Context	Best for high-intensity segments and peak performance	Better for steady-state ultra-endurance efforts

Preventing Gastrointestinal Distress

GI issues are a leading cause of DNF (Did Not Finish) in ultra races. Proper carb loading is designed to minimize this risk. Training your gut is crucial; practice your carb loading strategy during your longest training runs to see what works for your body.

Strategies to Prevent Distress:

Prioritize Low-Fiber Carbs: Switch from whole grains to white options during your carb load to reduce the work required of your digestive system.
Eat Frequently and in Smaller Portions: Instead of forcing down large meals, eat smaller, high-carb snacks every few hours.
Stay Hydrated: Drink plenty of fluids alongside your increased carbohydrate intake. Glycogen binds to water, so adequate hydration is necessary for storage.
Use Liquid Calories: If you struggle to eat enough solid food, integrate sports drinks, fruit juices, or smoothies to boost your carbohydrate intake.

The Role of Hydration

Proper hydration is intrinsically linked to effective carb loading. As your body stores glycogen, it also stores water (approx. 3-4 grams of water per gram of glycogen), leading to temporary weight gain. This extra fluid is a welcome bonus, aiding in hydration throughout the race. Continue to prioritize fluids, including electrolytes, especially in the final days before the race.

Sample Carb Loading Meal Plan (for a 70kg runner)

This sample provides one way to achieve approximately 700-840g of carbohydrates on a peak day, focusing on low-fiber sources.

Breakfast: 1 large bowl of low-fiber cereal (cornflakes) with milk and a sliced banana. A large glass of orange juice.
Mid-Morning Snack: Two plain bagels with honey or jam. A sports drink.
Lunch: Large plate of white pasta with simple tomato sauce (no heavy meats or excessive vegetables). A few slices of white bread on the side.
Mid-Afternoon Snack: A handful of pretzels and a banana. One or two rice krispie treats.
Dinner: White rice with a simple sauce and small portion of lean chicken. A few plain baked potatoes.
Evening Snack: Rice pudding or low-fat yogurt with a few scoops of honey or jam. A glass of fruit juice.

This is just an example; your personal plan should be developed and tested during training to ensure gut tolerance. For more detailed strategies, consider resources like those provided by sports nutritionists and organizations like Precision Hydration, which offer practical fueling advice for ultra-distance events (Precision Hydration: Carb Loading Guidance).

Conclusion

Successfully navigating an ultra marathon requires more than physical training; it demands a strategic approach to nutrition. By understanding the science behind glycogen stores, planning your carb intake for the 2-3 days leading up to the race, focusing on easily digestible foods, and prioritizing hydration, you can significantly enhance your performance and reduce the risk of race-ending GI issues. Remember to practice your entire fueling strategy during training to dial in what works best for your body, ensuring you arrive at the start line fully primed for success.

Frequently Asked Questions

You should begin your carb loading strategy approximately 2 to 3 days before your ultra marathon. This timeframe is sufficient to maximize your muscle and liver glycogen stores without causing excess discomfort.

For ultra endurance events, a common recommendation is to consume 8-12 grams of carbohydrates per kilogram of body weight per day during the carb loading period. The exact amount can depend on the duration of your load.

Some temporary weight gain and a feeling of fullness or heaviness is normal, as your body retains water with the glycogen. However, focusing on low-fiber, easily digestible carbs helps minimize bloating and GI distress.

While carbohydrates are the priority, you should not eliminate protein and fat entirely. Reduce their intake to a moderate level to ensure there is enough room for the increased carbohydrate volume and to aid digestion.

No, the old practice of glycogen depletion is now considered unnecessary and even detrimental. A proper carb load can be achieved by increasing carbohydrate intake during your taper period without prior depletion.

If you struggle with consuming large amounts of solid food, supplement with liquid carbohydrate sources like sports drinks, fruit juices, and smoothies. These are easily digestible and help increase your carb intake.

On race morning, 3-4 hours before the start, have a high-carbohydrate, low-fiber breakfast that you have practiced in training. Examples include a bagel with jam, oatmeal, or a banana.