How Early Do You Start Carb Loading? Your Optimal Timeline for Race Day

October 23, 2025 •

3 min read

Research has shown that proper carbohydrate loading can boost endurance performance by up to 3%. Knowing precisely how early do you start carb loading is crucial for maximizing this nutritional strategy before a big race.

Quick Summary

Carb loading typically begins 2-3 days before an endurance event lasting over 90 minutes. It involves increasing carbohydrate intake while tapering exercise to maximize muscle and liver glycogen stores for optimal race performance.

Key Points

Timing is Key: Begin carb loading 2 to 3 days (36-72 hours) before an endurance event lasting over 90 minutes to maximize muscle glycogen stores.
Adjust Intake: Increase your carbohydrate consumption to 7-12 grams per kilogram of body weight per day during the loading phase.
Taper Training: Pair your increased carb intake with a reduced training volume (tapering) to ensure glycogen is stored rather than burned off.
Choose Low-Fiber Foods: Opt for easily digestible, low-fiber carbohydrates like white rice, pasta, and bagels to avoid gastrointestinal issues on race day.
Practice Makes Perfect: Never try a new carb loading strategy on race week; practice during long training runs to find what works best for your body.
Stay Hydrated: Increased carbohydrate stores also require more water retention, so maintain adequate hydration throughout the loading period.

The Science Behind Carb Loading

For any endurance event lasting over 90 minutes, your body's primary fuel source, glycogen, becomes a limiting factor. When you eat carbohydrates, your body breaks them down into glucose, which is then stored in the muscles and liver as glycogen. During a prolonged high-intensity effort, these glycogen stores can become depleted, leading to the dreaded 'wall' or sudden fatigue. The goal of carb loading is to super-saturate these glycogen stores, effectively topping up your fuel tank to delay fatigue and sustain performance.

The Modern Carb Loading Timeline

Modern sports nutrition has moved away from the old, intensive 'depletion and load' method, which often caused gastrointestinal issues. The current best practice focuses on a more manageable, effective approach that aligns with the final tapering phase of training. Athletes should start the carb loading phase approximately 36 to 72 hours (2 to 3 days) before their event. This timeframe, combined with a reduction in training volume, allows the muscles to accumulate and store the extra carbohydrates efficiently.

A Sample 3-Day Carb Loading Plan

Here is a general structure for a 3-day carb load, which should be practiced in training before your main event:

Day 3 Before Race: Begin your carbohydrate increase. Aim for 7-8 grams of carbs per kilogram of body weight. Meals should be carb-focused, reducing some fat and protein. Continue tapering your training, keeping it light.
Day 2 Before Race: This is a peak loading day, with a target intake of 10-12 grams of carbs per kilogram of body weight. Training should be minimal to allow for maximum glycogen storage. Continue to focus on low-fiber, easily digestible foods.
Day 1 Before Race: Maintain a high carb intake, though it may feel like a lot. Keep the evening meal carbohydrate-dense but not overly large, and consume it early to aid digestion. Hydration is key and should continue throughout the day.

How Many Carbs Do You Need?

Your specific carbohydrate target depends on your body weight. Current recommendations range from 7 to 12 grams of carbohydrate per kilogram of body weight per day during the loading phase. This is a significant amount and can be challenging, which is why spreading the intake across meals and snacks is essential.

Optimal Food Choices for Carb Loading

To consume the necessary volume of carbohydrates without GI distress, the focus should be on low-fiber, easy-to-digest sources. This is one of the rare times when choosing refined grains over whole grains is recommended for athletes. Liquid carbohydrates from sports drinks and fruit juices can also be a simple way to increase intake without feeling overly full. Specific food examples and a table comparing what to eat and avoid can be found on {Link: STYRKR website https://styrkr.com/blogs/training-and-nutrition-hub/carb-loading-before-marathon}.

Common Carb Loading Mistakes

Avoiding common mistakes is crucial for successful carb loading. Waiting too long to start, failing to taper exercise, and consuming high-fat or high-fiber foods are frequent errors that can lead to reduced glycogen stores, discomfort, or sluggishness. Practicing your carb load during training is essential to find what works best for you.

Is Carb Loading Necessary for Shorter Events?

Carb loading is generally not necessary for events lasting less than 90 minutes. For shorter races like a 5K, a balanced diet is sufficient. Attempting to carb load for short events can lead to unnecessary weight gain and a sluggish feeling. This strategy is best for longer endurance events where glycogen depletion is a significant factor.

Conclusion

For endurance events longer than 90 minutes, start your carb loading process 2 to 3 days beforehand. Focus on increasing your carbohydrate intake to 7-12 g/kg of body weight per day, while simultaneously reducing your training volume. Choose low-fiber, easy-to-digest foods like white pasta, rice, and bananas, and don't forget to stay well-hydrated. Successfully carb loading ensures your glycogen stores are fully topped up, helping you avoid the wall and reach your peak performance on race day. Learn more from sports nutrition experts at {Link: Sports Dietitians Australia website https://www.sportsdietitians.com.au/sda-blog/carb-loading-success/}.

Frequently Asked Questions

You should start carb loading 2 to 3 days, or 36 to 72 hours, before your marathon. This timing, combined with a tapered training schedule, is sufficient to top up your muscle glycogen stores for race day.

Starting too late, for example just the night before, is ineffective because your body needs more time to fully maximize its glycogen stores. This can lead to a less-than-optimal energy reserve for your endurance event.

The best foods are high in carbohydrates but low in fiber, fat, and protein to aid digestion. Examples include white pasta, rice, bagels, bananas, sports drinks, and honey.

Yes, you will likely gain 1-2 kilograms during carb loading. This is temporary weight from the extra water your body stores with glycogen and is a positive sign that you are successfully fueling for your race.

No, carb loading is generally unnecessary for events lasting less than 90 minutes, such as a 5K or 10K. For these shorter distances, simply having a normal, balanced diet with a good race day breakfast is sufficient.

While the focus is on increasing carbs, you should still include some protein and healthy fats for a balanced diet. However, you should reduce your intake of fat and protein to make room for the extra carbohydrates without consuming excess calories.

Yes, drinking your carbs from sources like sports drinks, fruit juices, or even soft drinks can be an effective way to consume large volumes of carbohydrates without feeling too full from bulky food.