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How To Carb Load Before a Marathon

Amanda Wendorff

Whether you are a new runner looking to tackle a marathon or a seasoned competitor, you've probably heard the term "carb loading" thrown around. What exactly is carbohydrate loading, and why should you care about it for your marathon?

In short, in longer races like half marathons and marathons, increasing your intake of carbohydrates ("carb loading") before a race can ensure that you have the fuel and energy necessary to race hard without experiencing the dreaded bonk.

In this article, we'll explore the science, benefits, and practical tips of carb loading so you can run your best all the way to the finish line. (For a more detailed strategy on fuel during any type of endurance race, including marathons, you can check out our book "Triathlon Nutrition Foundations", which goes in depth to help you nail your fuel for any race.)

What You'll Learn From This Article

Before we dive into the nitty-gritty of carb loading, let's highlight some of the key concepts that will be covered in this article:

  • The various sources of fuel for long-distance running and why carbs are so important for marathon success
  • How many carbs your body needs to complete a strong marathon
  • The benefits of carb loading before a marathon
  • Tips for implementing a smart carb load over multiple days before a race
  • Pitfalls to avoid when carbo-loading before a half or full marathon

Now that you know what to expect, let's dig deep into the world of carb loading and why it's essential for your marathon.

MOTTIV app user Saul Wold is seen running through Assiniboine Park in Winnipeg, Canada during the Manitoba Marathon.

Why Carbs Matter For Marathon Runners

To really appreciate the importance of carbohydrates in your running journey, let's break it down in simple terms.

When running, the body needs a steady supply of energy, especially for longer races like marathons. This energy comes from different sources: fats, proteins, and carbohydrates. All of these macronutrients are important, but for the endurance athlete, carbohydrates are the superstar of the lineup. Why? Because the body is able to convert carbohydrates into energy very quickly and efficiently, which is useful for demanding activities like running.

How The Body Uses Carbohydrates

Let's walk through how it works: first, you consume carbohydrates, whether as part of your daily eating or via sports nutrition products like gels or sports drinks. Through a series of metabolic processes, these carbohydrates get converted into glucose, a type of sugar that the body can use very quickly as a source of energy. This glucose will circulate in your bloodstream and provide energy for the activities you are doing at the moment: thinking, breathing, moving, etc.

When the body has an ample supply of glucose - like right after you eat a meal, it will store the excess glucose in your muscle and liver cells in the form of glycogen. When needed, glycogen can be quickly accessed and converted to energy to keep you moving forward. In simple terms, the more muscle glycogen that is stored in the body, the more energy is available to propel you along the race course.

The Lowdown on Carbohydrate Loading

Now that we've covered the basics, let's discuss why carb loading in the days before the race is essential.

The main goal of carb loading is to ensure that your muscle glycogen stores are loaded before the starting gun goes off. This is important because, in the process of racing a marathon, your body will burn through a huge number of carbohydrates—much more than you could replace with sports nutrition like gels or electrolyte drinks.

Carb loading is like the pit stop where your race car gets fueled up before the big race. With a full tank of glycogen, your muscles have a readily available and easily accessible energy source, making sure you don't run out of gas during your marathon.

How Many Carbs Do You Burn in a Marathon?

To really get a feel for the importance of having an energy reserve of glycogen, let's look at some numbers to see just how many carbohydrates the average runner is burning through during a marathon.

Of course, every runner is different, and the rate at which you burn through carbohydrates is dependent on many things, including how hard you are running, your metabolism, your size, and more. Estimates show, however, that running at a fairly high intensity, such as during a marathon, could burn through 125 grams of carbohydrates, or even more, per hour.

What does that number mean? 125 grams of carbohydrates is equivalent to 500 calories of carbohydrates. Your average sports gel is about 100 calories and 25 grams of carbs. If you were to try to replace all the glucose you are burning during a marathon, you may need as many as five gels per hour -- a Herculean task that may lead to some serious upset stomach!

This is where glycogen stores and carb loading come into the picture.

The Benefits of Carb Loading for Marathoners

Many runners know all about "bonking," or hitting the wall - a very unpleasant experience where your energy levels plummet, and your race day takes a nosedive. This happens when the body reaches a state of carb depletion or runs out of carbs to use for fuel. You definitely want to avoid hitting the wall!

However, as we've seen, the logistics of consuming enough sports food during a race to match your calorie burn can be tricky. That's where your body's glycogen stores come to the rescue.

Glycogen acts as your energy savings account, storing carbs for when you need them most. It's like having a secret stash of energy to tap into during your race. That stash of energy is built up through carb loading. What you eat in the days leading up to your marathon can make the difference between a strong finish and a painful limp to the finish line.

The Basics of Carb Loading

The best way to ensure that you have a full tank of glycogen reserves is by increasing your carbohydrate intake in the two to three days prior to your marathon. Here are some tips for an ideal carb loading strategy:

Spread Carb Loading Over Multiple Meals in 2-3 Days

Rather than gorging yourself at a pasta dinner the night before a race, think of carb loading as a gradual process. Start eating more carbs in the two or three days before your race, increasing the amount of calories coming from carbs gradually.

Carb loading over several meals offers a few key benefits:

  • Gradual replenishment: By steadily increasing your carb intake with each meal, you top up your glycogen stores without overwhelming your body.
  • Avoids discomfort: Scarfing down a massive, carb-heavy meal can lead to feeling uncomfortably full or sluggish. Gradual loading minimizes this risk.
  • Sustained energy: Spacing out your carb loading ensures a steady stream of energy in the days before your race, especially when you have other tasks like travel and race prep to tackle.

One of the star meals during carbohydrate loading is the brunch or breakfast you enjoy the day before your marathon. This meal should be packed with carbs to maximize your glycogen stores. Think pancakes, potatoes, or toast, all of which are carb-rich delights. It's a good idea to have this meal after your pre-race run for the day to ensure your body stores as much glycogen as possible.

Stick to Familiar Foods and Keep it Simple.

Be careful not to use carb loading as an excuse to indulge in low-quality foods, as this might make you feel sluggish and uncomfortable during your race. Cinnamon rolls may have a lot of carbohydrates, but they are probably a better choice for after the race. Similarly, avoid new foods that your system is not familiar with. Stomach upset before a marathon is no fun!

Consider incorporating these carb-rich but basic foods into a carb load before your next marathon:

  • Pasta
  • Bagels
  • Rice
  • Quinoa
  • Pancakes
  • Potatoes
  • Farro
  • Oats
  • Bread

What to Eat the Night Before the Marathon

As race day approaches, consider shifting towards lower-residue, easy-to-digest foods with low fiber. For your dinner the night before the race, keep it bland - think white rice, grilled chicken, or white pasta; things that are unlikely to cause gut issues.

Also, keep this meal a little lighter. Your pre-race dinner should satisfy your hunger without making you feel stuffed. You don't want to go to bed with a full stomach, which can disrupt your sleep and impact your race performance.

Don't Stress About Weight

As you load up on carbs, your cells will retain water, resulting in temporary weight gain. In fact, for every gram of stored glycogen, there are four grams of water stored along with it. Don't worry; this won't negatively affect your race performance, and in fact, it will help you arrive at the start line well-hydrated.

MOTTIV app user Chris Bumstead takes a deep breath after completing a race in Mont Tremblant, Quebec, Canada!

Race Day: Carb-Loading Continues

On the day of your marathon, your breakfast takes center stage. Assuming you didn't go for a run after your pre-race dinner, your glycogen stores should be close to full, so think of breakfast as your final top-up of carbs before the race. Aim for 2 to 3 grams of carbs per kilogram of body weight from foods that you have tested before training runs.

Some good pre-race breakfast ideas include:

  • A bagel and almond butter
  • Oats with berries
  • Toast with honey

Of course, remember hydration during this meal! A bottle of sports drink is a good way to get a few extra carbohydrates into the system while also keeping you hydrated.

FAQ for the Runner Planning a Marathon Carbo Load

Q: What is carb loading?

A: Carb loading, also known as carbohydrate loading, is a strategy used by endurance athletes to maximize the storage of glycogen in the muscles and liver before an event such as a marathon or half marathon.

Q: Why is carb loading important for runners?

A: Carb loading is important for runners because it can help increase glycogen stores in the muscles, which can provide additional energy during a long race such as a marathon or half marathon.

Q: How do I start carbo loading before a marathon?

A: To start carb loading before a marathon, increase your carbohydrate intake several days before the race, aiming for 3-5 grams of carbs for every kilogram of body weight per day.

Q: Is it necessary to carb load before a marathon?

A: Carb loading before a marathon  is not strictly necessary for everyone, but it will benefit most runners.

Q: Can I include fat and protein in my carb-loading plan?

A: While the focus of carb loading is on eating more carbohydrates, it's still important to include some fat and protein in your meals and snacks leading up to a race for overall balanced nutrition.

Q: What do I need to know about carbo-loading for a half marathon or marathon?

A: For carb loading before a half marathon or marathon, it's important to plan ahead, gradually increase carbohydrate intake in the days leading up to the race, and focus on easily digestible, low-fiber foods.

Q: Will carb loading for a marathon allow me to run my best time?

A: Carb loading can help ensure optimal glycogen stores and provide additional energy during a marathon, potentially allowing you to run at your best performance level.

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

A: If you run out of glycogen during a race due to inadequate carb intake, you may experience fatigue, difficulty maintaining pace, and reduced performance, highlighting the importance of proper carb loading.


In a nutshell, carb loading is a key factor in optimizing marathon performance by ensuring that you start the race with fully charged glycogen stores. By starting carb loading early, making smart food choices, and following the carb-loading tips provided in this article, you'll set yourself up for a stellar race day. Remember, the longer and faster the race, the more carbs you need, so plan your carb loading with care. When you cross the finish line triumphantly, you'll be glad you did.

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Amanda Wendorff

| Author

Amanda Wendorff is a professional triathlete, focusing on the 70.3 and 140.6 Ironman distances. In the last several years she’s competed in multiple gravel bike races. Top Achievements: Top 3 Ironman Ireland and Ironman 70.3 Coquimbo, Multiple time top-5 finisher, 3rd Overall at Moran 166 Gravel Race in Michigan, Age group podium at Gravel Worlds, Big Sugar, and Ned Gravel in first year of gravel racing.

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