Nutrition Diet: How many days to carb load before marathon for peak performance?

October 8, 2025 •

3 min read

Effective carb loading has been shown to improve endurance performance by up to 3% in events lasting over 90 minutes. This makes understanding how many days to carb load before a marathon a critical component of any endurance runner's race preparation.

Quick Summary

The ideal carb loading period before a marathon is typically 2 to 3 days, focusing on increased carbohydrate intake while tapering training. This strategy maximizes muscle glycogen stores, which are crucial for maintaining energy and avoiding fatigue during long-distance endurance events. Food choices should favor easily digestible, low-fiber carbs to prevent gastrointestinal issues.

Key Points

Timing is key: Start your carb loading 2 to 3 days before your marathon, not just the night before.
Focus on easy-to-digest carbs: Prioritize low-fiber sources like white rice, pasta, and potatoes to avoid gastrointestinal issues.
Reduce fats and fibers: Trim down your fat and fiber intake during the loading phase to make room for more carbohydrates and ease digestion.
Don't overeat: Aim to replace calories from fat and protein with carbs rather than simply eating more overall to prevent bloating.
Hydrate adequately: Increase your fluid intake, as your body stores extra water with every gram of glycogen.
Expect temporary weight gain: Don't be alarmed by a slight weight increase; it's a sign that your glycogen and water stores are being maximized.
Practice makes perfect: Test your carb loading strategy during your longest training runs to see what works best for you on race day.

For marathoners, the energy stored in your muscles and liver—known as glycogen—is the primary fuel source for powering through the race. While consuming carbohydrates during the race is vital, arriving at the starting line with fully saturated glycogen stores can be the difference between hitting a personal best and 'hitting the wall'. The strategic process of maximizing these stores is known as carb loading.

The Optimal Timing for Carb Loading

Many runners mistakenly believe that a single, large pasta dinner the night before the race is sufficient for carb loading. In reality, modern sports nutrition recommends a more gradual approach over several days to effectively fill the body's energy reserves without causing discomfort.

Why 2-3 Days is the Sweet Spot

A carb loading period of 2 to 3 days before your marathon is generally considered optimal. This timing aligns with a tapering training schedule, allowing your body to efficiently store increased carbohydrate intake as glycogen. This gradual approach over 48 to 72 hours helps maximize muscle and liver glycogen stores while potentially minimizing digestive discomfort that can occur with a shorter loading period.

Don't Skip Race Morning Fueling

Even after carb loading, race morning fueling is essential for topping off liver glycogen stores that deplete overnight. Aim to consume 1-4 grams of carbohydrates per kilogram of body weight 1-4 hours before the race, sticking to familiar, easy-to-digest foods.

The Right Carbs and What to Avoid

Effective carb loading involves increasing the percentage of calories from carbohydrates while reducing fat and fiber intake. Focusing on specific types of carbohydrates is key.

Best Foods for Carb Loading

Good choices include low-fiber options like white pasta, white rice, bagels, white bread, potatoes, sweet potatoes, and oats. Bananas also offer easily digestible carbohydrates. Liquid carbohydrates from sports drinks and juices can help increase intake without feeling overly full.

Foods to Limit or Avoid

Limit high-fiber foods such as whole grains, legumes, and raw vegetables, which can cause digestive issues. High-fat foods like heavy sauces and fried items should also be limited due to slower digestion. While protein is important, excessive amounts during carb loading can displace carbohydrates. Always stick to foods you've tested during training to avoid race day surprises.

Hydration and the Weight Gain Paradox

Adequate hydration is crucial during carb loading, as glycogen storage involves storing water as well.

Why You'll Gain a Little Weight

A temporary weight gain of 1 to 2 kilograms is normal and indicates successful glycogen and water storage. This extra fluid and stored carbohydrate are beneficial for endurance performance.

Carb Loading Strategy Comparison Table


Feature	2-Day Carb Load	3-Day Carb Load
Carbohydrate Target	~10-12g per kg of body weight per day	~7-8g per kg of body weight per day
Pacing of Intake	More aggressive intake per meal, requires more focused eating.	More gradual and potentially easier to manage without feeling bloated.
Energy Benefit	Maximum glycogen storage potential for well-trained athletes.	Still highly effective; a good option if high volume is difficult.
Feasibility	Can be challenging to consume the required volume of carbs, especially with a busy schedule.	More manageable for many runners, especially those new to carb loading.
Practice during training	Important to practice this higher volume to gauge gut tolerance.	Easier to practice and implement consistently during long run simulations.

Conclusion: Practice Your Carb-Loading Plan

Implementing a carb loading strategy 2 to 3 days before a marathon is a key part of race preparation. Focus on digestible carbohydrates, reduce fat and fiber, and stay hydrated. Practice your plan during training to find what works best and ensure you are well-fueled for race day. By following these guidelines, you can help optimize your performance and avoid common pitfalls.

For more detailed sports nutrition advice, you can consult reputable organizations like Sports Dietitians Australia: https://www.sportsdietitians.com.au/sda-blog/carb-loading-success/.

Frequently Asked Questions

Carb loading is an eating strategy used by endurance athletes to maximize the amount of glycogen stored in their muscles and liver in the 2-3 days before an event like a marathon.

While not strictly necessary for everyone, carb loading is highly beneficial for most runners participating in endurance events lasting over 90 minutes. It can significantly boost glycogen stores, helping to delay fatigue and improve performance.

Good foods for carb loading include easily digestible, low-fiber sources such as white pasta, white rice, bagels, potatoes, oatmeal, and bananas. Sports drinks and fruit juice are also effective for increasing intake without excessive bulk.

Carb loading is not typically necessary for shorter races like a 5K or 10K, as they don't deplete your glycogen stores to the same extent as a marathon. A normal, balanced diet is usually sufficient.

Feeling bloated can be a sign that you're effectively storing glycogen, which requires water. However, it can also happen if you increase your fiber intake too much. Sticking to lower-fiber carbs can minimize this.

Yes, a balanced intake of protein and fat is still important, but your focus should shift to prioritizing carbohydrates. You should aim to increase your carb intake by reducing your fat and protein portions slightly.

Your last meal should be carbohydrate-heavy but relatively small to allow for proper digestion before you go to bed. Eating earlier in the evening can also prevent a full stomach from disrupting your sleep.