When to start carb loading for Ironman?

December 28, 2025 •

3 min read

Proper carbohydrate loading can improve endurance performance by 2-3% during long events like an Ironman, making the timing of this strategy critical. Starting too late or incorrectly can lead to digestive issues or missed opportunities for maximum glycogen storage. Mastering the 'when' is just as important as the 'what' to ensure you arrive at the starting line fully fueled and ready to perform.

Quick Summary

This guide outlines the optimal timing for carb loading for an Ironman, detailing a strategic approach beginning 36-72 hours before the race. It covers the specific dietary changes required, the science of glycogen storage, and practical tips to ensure maximal fuel reserves without gastrointestinal distress.

Key Points

Start 36-72 Hours Before: Begin strategically increasing your carbohydrate intake and lowering fiber content 2-3 days out from your race.
Aim for 8-10g/kg: Consume 8-10 grams of carbohydrates per kilogram of body weight each day during the loading phase to maximize glycogen stores.
Favor Low-Fiber Carbs: Switch from high-fiber whole grains to simple carbs like white rice, white bread, and sports drinks in the final days to prevent digestive upset.
Hydrate Adequately: Increase your fluid intake, as each gram of stored glycogen is bound with water, and dehydration is a risk during the loading process.
Taper Your Training: Reduce your training volume significantly during the carb loading period to help your muscles store glycogen more efficiently.
Practice Makes Perfect: Test your carb loading plan during a high-volume training weekend several weeks before the race to ensure tolerance and effectiveness.

Carb loading is a crucial performance strategy for endurance athletes, particularly those undertaking the immense physical challenge of an Ironman triathlon. It is not just about a huge pasta dinner the night before, but a strategic, multi-day process designed to maximize the glycogen stored in your muscles and liver. While an average person might have about 2,500 calories of glycogen stored, proper loading can significantly increase this reserve, delaying fatigue and preventing the dreaded 'bonk'. For every gram of glycogen stored, your body also stores around three grams of water, which aids hydration during the race.

The Carb Loading Timeline for Ironman

Unlike shorter races, a full Ironman requires a more deliberate carb loading strategy. Experts often recommend a multi-day approach to ensure your body has ample time to adjust and store glycogen effectively. The timeline can be broken down into three key phases leading up to race day.

72 Hours Before: The Transition Phase

Approximately three days before your Ironman, it's beneficial to start transitioning your diet. The primary goal during this phase is to decrease your fiber, fat, and protein intake while gradually increasing your total carbohydrate consumption. Reducing high-fiber foods (like whole grains, beans, and many vegetables) helps clear your digestive system, minimizing the risk of GI issues on race day. This is also when your training volume should be significantly tapered, further encouraging glycogen storage. Aim to consume around 8-10 grams of carbohydrates per kilogram of body weight per day. For a 70kg athlete, this means 560-700 grams of carbs daily. Spreading this intake across smaller, more frequent meals and snacks is more manageable and prevents uncomfortable bloating.

36-48 Hours Before: Peak Loading

This is the most critical window for maximizing glycogen stores. Your carbohydrate intake should be at its highest, still focusing on lower-fiber options. Athletes find it challenging to eat enough high-carb food due to bulkiness, so incorporating liquid carbs is a practical solution. Sports drinks, fruit juices, and simple carbohydrate supplements can help meet your targets without making you feel excessively full. Hydration is also paramount during this period, as you're storing water alongside glycogen. Ensure you are drinking more fluids than usual and possibly adding electrolytes.

Race Day Minus One: Light and Simple

The day before the race, you don't need to overeat. The heavy lifting of carb loading should be largely complete. Your focus now is on consuming easy-to-digest, low-fiber carbs and avoiding large meals that could disrupt sleep.

Breakfast: A substantial, carb-heavy meal. Pancakes, rice, or bagels are excellent choices.
Lunch: A normal-sized meal of white rice or pasta with a simple, lean protein.
Dinner: An early, light, and low-fiber dinner. Think white rice and a small amount of lean, grilled chicken. Finishing eating early (e.g., by 7 PM) allows for proper digestion.
Snacks: Frequent, small, carbohydrate-rich snacks throughout the day, like pretzels or fruit snacks.

Comparison of High vs. Low Fiber Carbs for Carb Loading


Food Type	High Fiber (Best for Training)	Low Fiber (Best for Race Week)
Grains	Whole-wheat pasta, brown rice, oats, whole-grain bread	White pasta, white rice, white bread, low-fiber cereal
Fruits	Apples, pears, raspberries	Bananas, peeled peaches, canned fruit in syrup, fruit juices
Vegetables	Broccoli, beans, lentils, raw greens	Peeled potatoes, sweet potatoes, strained tomato sauce
Snacks	Granola bars, trail mix with nuts	Pretzels, rice cakes, sports nutrition products

Practicing Your Race Week Nutrition

Just like you wouldn't try a new pair of shoes on race day, you should never attempt a new carb loading strategy for the first time before your Ironman. Practice your nutrition plan during a big training weekend a few weeks before the race. This allows you to test your food choices, gauge your gut's tolerance for higher carb intake, and refine your plan based on how you feel. Pay close attention to any bloating, discomfort, or energy level fluctuations. The practice run is just as important as the final taper.

Conclusion

Effective carb loading for an Ironman is a methodical process that begins 36-72 hours before the race. By strategically increasing your low-fiber, high-carbohydrate intake while tapering your training, you can significantly boost your muscle glycogen stores. Remember to practice your strategy in training, spread your meals out to avoid discomfort, and focus on simple, easily digestible foods, especially in the final 24 hours. This disciplined approach to fueling will set you up for a powerful and consistent performance on race day. Learn more about optimal race fueling directly from the source Ironman's race day nutrition guide.

Frequently Asked Questions

You should begin your carb loading strategy 36 to 72 hours before your Ironman race. This multi-day approach allows for maximum glycogen storage without the digestive issues that can accompany a single, large meal.

During the peak loading phase (36-48 hours before), aim for 8-10 grams of carbohydrates per kilogram of your body weight per day. A 70kg athlete should aim for 560-700 grams of carbs daily.

Focus on low-fiber, simple carbohydrates during your carb load. Good options include white rice, white pasta, bagels, fruit juice, sports drinks, and pretzels. Avoid high-fiber foods like whole grains and legumes, as these can cause GI distress.

Yes, absolutely. You should practice your carb loading plan during a heavy training weekend 4-6 weeks before your race. This ensures your body responds well to the foods and intake levels you've planned, preventing any unpleasant race-day surprises.

It is normal and expected to gain a few pounds during carb loading. For every gram of glycogen you store, your body stores approximately three grams of water. This is a positive sign that your fueling is on track, and it contributes to hydration.

No, a huge meal the night before is a common mistake and can lead to bloating, discomfort, and poor sleep. Your main carb intake should be earlier in the day, with a lighter, low-fiber dinner the night before the race.

Due to the water-binding properties of glycogen, you should increase your fluid intake during the loading period. Staying well-hydrated is critical, and adding electrolytes can be beneficial.