Carb Loading for Runners: How to + 8 Common Mistakes

Learn how to do carb loading right for optimal athletic performance

When it comes to carb loading, the pre-race pasta dinner and the marathon go hand-in-hand. It’s a long-running joke (pun intended) that if you’re gearing up for a marathon, you’re going to be eating lots of pasta the night before.

But there’s more that goes into carbo-loading for a marathon or half marathon than just loading up your plate the night before.

And, if you plan on doing it all in the meal before the race, you may end up hurting your performance with digestive upset, fatigue, and inflammation.

In this guide, we will discuss what exactly carb loading is, how to do so correctly for optimal athletic performance, and the eight biggest mistakes runners make when carb loading.

Ready? Let’s jump in!

Pancakes are ideal for carb loading

What is Carb Loading?

Carbohydrates in the form of glycogen are your body’s preferred energy source when it comes to running. And for races lasting longer than 90-120 minutes, filling up these fuel stores are key to helping you go the distance.

How do you do this?

By eating carbs to maximize your liver and muscle glycogen stores. You need these carb stores because your body only has limited room for this fuel:

“With only 2,000 calories worth of energy stored from carbohydrates, our body will run out of fuel after approximately 2 hours.” explains exercise physiologist and world-champion triathlete Todd Buckingham.1Mary Free Bed Rehabilitation Hospital: Home. (n.d.). Mary Free Bed Rehabilitation Hospital. Retrieved January 9, 2024, from https://www.maryfreebed.com/

‌”By increasing the number of carbohydrates the body can store, we can run for a longer period of time without running out of energy,”

Thus, carbohydrate loading (or carb loading / carbo loading, for short) involves gradually increasing your total carbohydrate intake prior to an athletic event so that you have excess glycogen in your liver for your muscles to tap into for endurance events and long duration exercises.

“By eating more carbohydrates, you may be able to store more energy in your muscles giving you the stamina to make it through longer endurance events,” explains registered dietitian at Nutrition Rewired, Erin Kenney.2Rewired, N. (n.d.). Home. Nutrition Rewired. Retrieved January 9, 2024, from https://nutritionrewired.com/

What Does Carb Loading Do and How Does It Help Performance?

By carbo loading, you can avoid hitting the wall or bonking during endurance exercise.

Hitting the wall in a marathon or bonking occurs when your body runs out of fuel a.k.a. carbs, and therefore, your pace slows, and your muscles may start cramping and feeling fatigued.

Through carbo loading, endurance athletes have excess carbs stored in the liver making it easier for their bodies to release them during races and long-distance runs.

“When carbohydrates are all used up and blood glucose is low, the runner will rely more on fat for energy, and the pace will slow down because energy (ATP) is produced more slowly from fat than from carbohydrate,” explains Dr. Jason Karp, exercise physiologist, run coach, and author of Running a Marathon for Dummies.

“So, not carb loading before a marathon simply means the carb fuel tank may not be as full as possible, and the runner will fatigue earlier in the race and slow down earlier.” 

Bagels are another ideal carb loading food

Carbs help with hydration, too. For every gram of stored glycogen there are four grams of water stored along with it.

“Carbohydrates help hold water. This hydration is a critical part of race day as well,” says Gina Capone, a runner, dietetic intern, and co-host of the We Run This podcast.3We Run This. (n.d.). Spotify. Retrieved January 9, 2024, from https://open.spotify.com/show/53KeF3aO2jnJC39iCgdQ3U

‌Buckingham notes that while carb loading is key for races longer than two hours, it has been shown to help athletes in shorter, high-intensity races lasting less than two hours, as well.

“The reason for this is that the body will start to slow your pace down if it senses your carbohydrate levels getting too low,” explains Buckingham.

How Should I Adjust My Diet When Carb Loading for a Marathon?

Carbo-loading involves increasing the amount of carbohydrates at every meal and snack in the days leading up to your marathon, half-marathon, triathlon, or whichever athletic event you are preparing for.

Instead of one piece of toast, have two. Instead of a half of a baked potato, eat the whole thing. Have carbs replace fat and fiber in your meals.

Capone suggests high-carb meals in the days before the race.

Three to Four Days Out From Race Day

Your plate leading up to the race should consist of:

  • 50% carbohydrates (rice, legumes, whole grains, startchy vegetables like sweet potatoes, etc.)
  • 25% protein (lean meat, fish, poultry, cheese, etc.)
  • 25% vegetables (leafy greens, cruciferous vegetables, etc.)

The Morning of the Race

  • 3-4 hours before the race: 1.5-2.5 grams of carbs for every 1 kg of body weight
  • 1 hour before the race: A smaller carbohydrate snack (banana, bar, piece of bread)
  • 5-7 mL of water/electrolytes fluid per 1kg of body weight  

Sports dietitian Dana Eshelman notes that research4Wee, S.-L., Williams, C., Tsintzas, K., & Boobis, L. (2005). Ingestion of a high-glycemic index meal increases muscle glycogen storage at rest but augments its utilization during subsequent exercise. Journal of Applied Physiology99(2), 707–714. https://doi.org/10.1152/japplphysiol.01261.2004 shows a 15 percent increase in glycogen storage with a carb-heavy breakfast in the 3 hours before an event. But, don’t do this without practice.

“Carb heavy meals, without practice and nutrition training on the gut, can cause extreme stomach upset,” she warns.

How Long Before A Race Should I Carb Load?

Carbohydrate loading generally begins about 3-4 days out from race day (although some can start with a high-carbohydrate diet as many as 7 days out).

At this time, you gradually increase your carb intake per meal while eating less fat and fiber.

When to Start Carb Loading For a Half Marathon

Carb loading ahead of a half-marathon looks similar to carb-loading for a marathon.  About 3 days out from the race, aim for 70 percent of carbs from your total calories. Eat this along with low-fat unprocessed foods and lean protein, says Nelson Joseph at CardioZero.

What Food is Best for Carb Loading?

It’s important to keep in mind that our body stores carbohydrates best from starchy carbohydrates, says Kenney.

Good simple carbohydrate fuel sources for enhancing exercise performance include . . .

Related: 11 Carbo Loading Sources For Runners

Examples of simple carbs for carb loading such as sugar, fruit, cookies, bread and garins.

The idea is to eat more carbs than you normally would but still keep a balanced plate. Some meals to eat in the days leading up to the race include:

  • Pasta with vegetables and shrimp
  • Farro or rice with chopped apples, arugula, shaved parmesan and goat cheese, pecans, salmon or chicken
  • Potato boats stuffed with spinach, mushrooms, tomatoes, and mozzarella

The day before the race, stick to only simple carbohydrates because they are easy and quick to break down and use as energy. Complex carbohydrates are slower to break down in the body.

The 8 Big Carbohydrate Loading Mistakes Runners Make

#1: Eating All of Your Carbs At Once

The most common mistake runners make ahead of their marathon or half-marathon is that they plan to carbo load the night before at the infamous pasta dinner.

This is a big mistake because it does not give your body ample enough time to fill its stores. It also can leave you lethargic the next morning.

Runners should aim to build up carbs the 3 to 4 days ahead of the race, rather than all at once.

#2: Depleting Carbs Before Loading Carbs

It was once believed that in order to fully stock your fuel stories, runners needed a “carb depletion phase” before loading up on carbs. This involved doing high intensity exercise just 7 days prior to your race, and then not eating carbs for 3 days.

Carb depletion involves a risk of injury and decreased confidence because it can make runners feel, well, terrible—cranky, sore, and fatigued.

The risks aren’t worth the potential dip in physical performance and high carbohydrate intake should be the best plan of action in the days leading up to the race.

#3: Overeating

Still, carb loading does not mean you “stuff yourself” to get fuller muscle glycogen levels.

You are simply adjusting each meal to have a larger carbohydrate component. Overeating (which is common due to nervousness) is not what you want, notes Buckingham.

#4: Freaking Out About Weight Gain

On the other hand, gaining weight while carbo loading is natural. That’s because carbs help you retain water.

Remember that for every gram of stored glycogen, you’re storing 4 grams of water. If you gain some weight, that’s actually a good thing. It means your body has the fuel and hydration ready to race.

#5: Eating Too Much Fiber

As you increase your carbs, runners should reduce their fiber intake in the last three days prior to a race. This is because fiber can be taxing on the GI system.

Therefore, instead of carbo loading with whole wheat toast that has a high fiber content, choose white bread instead.

This gives your GI system a break and reduces the chance of GI distress during the race, says Buckingham.

A plate of pasta

#6: Not Practicing Carb Loading Before Race Day

You never want to “try” something new on race day.

Thus, it is best to practice your carb loading nutrition strategy ahead of time on a couple of long runs so that you know what to expect and how your body will respond to the higher carb diet.

Practicing will help you dial in your carbohydrate loading plan just like you dial in your race nutrition plan. Then, once you know what works best for you, pre-plan your meals the last three days so you don’t stress about what to eat.

Related: Marathon Training Meal Plans (free download!)

#7: Not Eating Enough Carbs

Runners typically eat healthy.

But eating lots of vegetables and smoothies isn’t going to give your body the carb fuel it needs for your marathon or half-marathon.

In order to properly carb load, runners should aim for 8-12 grams of carbohydrate per kilogram of body weight, explains Buckingham. For a 160-pound individual, that’s 580-870g or 2,300-3,400 calories from carbohydrates alone.

“This is also why runners need to limit the number of other foods they’re eating, like fat and protein,” says the sports physiologist.

“Eating 2,000-3,000 calories from carbohydrates without any fat or protein is already close to the daily calorie recommendation. If runners do not also limit their fat and protein consumption, they could see excess weight gain leading up to the race.”

A person hydrating.

#8: Not Drinking When Carb Loading

Not all carbs are created the same, notes Capone.

There are different types of sugars: fructose, sucrose, glucose. Different carbohydrates have different transporters into the bloodstream. Therefore, runners need to be mindful of what they’re drinking with what carbs they’re eating.

“Sports drinks, such as Gatorade, contains fructose, glucose, and sucrose allowing it to be one of the best liquid sources of carbohydrates for athletes. Sodium helps with the transportation of glucose, meaning sodium is an important part of carbohydrate utilization,” she explains.

So, while consuming carbohydrates is important, it’s also important to be hydrating properly with sugars and salts during activity too, not just before the race.

Of course it is!

Carb loading properly prior to race day can be the difference between a PR versus a DNF for runners.

If you want to comprise the perfect sports nutrition plan for your goals, consider meeting with a nutritionist who can assess your dietary needs and guide you to the perfect training diet.

Here are a few day-to-day popular diets for runners to browse through:


  • 1
    Mary Free Bed Rehabilitation Hospital: Home. (n.d.). Mary Free Bed Rehabilitation Hospital. Retrieved January 9, 2024, from https://www.maryfreebed.com/
  • 2
    Rewired, N. (n.d.). Home. Nutrition Rewired. Retrieved January 9, 2024, from https://nutritionrewired.com/
  • 3
    We Run This. (n.d.). Spotify. Retrieved January 9, 2024, from https://open.spotify.com/show/53KeF3aO2jnJC39iCgdQ3U
  • 4
    Wee, S.-L., Williams, C., Tsintzas, K., & Boobis, L. (2005). Ingestion of a high-glycemic index meal increases muscle glycogen storage at rest but augments its utilization during subsequent exercise. Journal of Applied Physiology99(2), 707–714. https://doi.org/10.1152/japplphysiol.01261.2004
Photo of author
Whitney Heins is the founder of The Mother Runners and a VDOT-O2 certified running coach. She lives in Knoxville, TN with her two crazy, beautiful kids, pups, and husband. She is currently training to qualify for the US Olympic Trials marathon.

3 thoughts on “Carb Loading for Runners: How to + 8 Common Mistakes”

  1. I didn’t know the issue about the importance of reducing fibre. Interesting. Am doing your ultra plan for improvers. Not so much a plan, but a way of life! It is REALLY GOOD. Everyone in my club is amazed at the distances I’m covering on LSRs. Nice one, Thomas.

  2. I personally think carb loading doesn’t need to be so difficult or scientific. Eating a good pasta dinner the night before a marathon seems to work just fine. In 180 marathons, I haven’t ever bonked or wish I had more glycogen stores in me.


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