John Davis

Written by John Davis


Does Carbo-Loading for a Marathon Work? If So, What is The Best Way to Do It?

In previous articles, we’ve seen how critical fueling is in the marathon. Without enough carbs to get you to the finish line, you’re bound to “hit the wall“—that is, be forced to switch to burning mostly fats at a huge cost to your pace and of course your overall finish time.

The most obvious way to keep from hitting the wall is to refuel during the race with sports drinks, gels, or solid food.

But this can be difficult, as all three of these can cause gastrointestinal problems, and you can only absorb about 240 calories worth of carbs per hour of running. If you hit the wall early in your marathons, it might not even be possible for you to consume enough carbs during your race to push the wall back beyond 26.2 miles.

Fortunately, there’s another option.

By following a protocol called carbo-loading in the days leading up to the race, you can increase the amount of carbohydrates that your muscles store in the form of glycogen. It’s akin to upgrading your car’s gas tank so it can drive further without needing to refuel.

The Science on carbo-loading

Carbohydrate loading is a systematic and scientific practice that takes course over the weeks and days leading up to competition with the purpose of maximizing the storage of glycogen in muscles.

During intense, continuous endurance exercise, your muscles will become depleted of glycogen after about 90 minutes. Carbohydrate loading is meant to store extra glycogen that your muscles can tap into once the normal stores are used up.

As carbohydrate loading received more attention for its ability to improve athletic performance in endurance events, more research has focused on effective methods.

Do you need to deplete you carbs before loading?

Early theories about carbo-loading postulated that a period of muscle glycogen depletion was necessary before the period of carbo-loading. Under this model, you’d intentionally restrict your dietary carbohydrate intake for a few days, then eat more carbs than usual in the last few days before your marathon.

But the process of depleting your carbohydrate stores is physically and mentally draining as well as unnecessary for peak performance, as argued by physiologist David Costill in a 1978 article in Runner’s World.

Instead, recent scientific research has focused on carbo-loading protocols that only involve a “superloading” portion.

Carb-powered athletes

A typical example of the kind of carbo-loading program studied in scientific literature is described in a 1995 study by Laurie Rauch and other researchers at the University of Cape Town in South Africa. In this study, eight cyclists completed two three-hour trial rides. Before one of these rides, the cyclists increased their dietary carbohydrate intake by about 72%, and before the other, they consumed their normal diets.

After analyzing biopsies from the muscles of the riders, as well as their times for the trial rides, Rauch et al. found that the carbo-loaded athletes were able to ride faster, cover more distance, and put out more power over the final 60 minutes of their trial ride.

The carbo-loading protocol increased the muscle glycogen content of the cyclists by an average of 47%. In a runner, this magnitude of increase would push back the wall by several miles.

A 1997 review article by the same research group surveys several other studies that confirm the benefits of carbo-loading on endurance events that last longer than 90 minutes.

Hawley et al. write that carbo-loading will push back fatigue (i.e. the wall) by about 20%, and can improve your time over a set distance by 2-3% as a result.

This would be a 3-6 minute improvement for a typical marathoner, a pretty significant margin!

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So, how should you go about carbo-loading before your next marathon?

More recent scientific research indicates that athletes should aim to increase their carbohydrate intake to 8 grams of carbs per kilogram of body mass in the final three days before their race, but such a specific target isn’t very much use to anyone but the most nitpicky of calorie counters.

Instead, it’s probably best to aim to increase the amount of carbs you consume by 50-75% over the three days leading up to your marathon.

The cyclists in Rauch et al.’s paper accomplished this by chugging chocolate-flavored mixtures of potato starch and water—not the most appetizing of diets—but a more reasonable approach would be to drink more fruit juices and sports drinks, eat more carbohydrate-rich foods like pasta and rice, and cut down on fat and protein-rich foods like burgers and eggs.

To achieve optimal carbohydrate loading, you don’t need to do any complicated math on your carb consumption per kilo of body mass. The scientific literature indicates that a relatively wide range of loading strategies is effective in increasing your muscle glycogen storage when carried out in the last several days leading up to a long race.

However, if you want an in-depth, step-by-step approach for how to carbohydrate load for your next marathon, please take a look at our nutritionist’s article on this topic found here.

Final notes

  • Try to increase your carb intake by 50-75% for three days before a race lasting over 90 minutes.
  • You’ll want to go easy on fiber-heavy foods, since these can throw off your gastrointestinal system before your race if you aren’t used to eating a lot of them.
  • According to Darlene Sedlock, an exercise physiologist at Purdue University, the form of carbohydrates you load up with doesn’t particularly matter; the simple sugars found in fruit juices and sports drinks should work just as well as the complex carbohydrates found in pasta, rice, oats, and potatoes.
  • A “depletion” period isn’t necessary, and when done properly, carbo-loading can push back the wall, delay fatigue, and improve performance.

Have you tried cabo-loading before? How did it go? Did you notice a difference and what protocol did you follow? We’d love to hear your experiences and suggestions in the comments section.

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1. Rapoport, B. I., Metabolic Factors Limiting Performance in Marathon Runners. PLoS Computational Biology 2010, 6 (10), e1000960.
2. Costill, D. L., Carbohydrate-loading without depletion. Runner's World 1978, p 76.
3. Rauch, L.; Rodger, I.; Wilson, G. R.; Belonje, J. D.; Dennis, S. C.; Noakes, T. D.; Hawley, J. A., The effects of carbohydrate loading on muscle glycogen content and cycling performance. International Journal of Sport Nutrition 1995, 5, 25-36.
4. Hawley, J. A.; Schabort, E. J.; Noakes, T. D.; Dennis, S. C., Carbohydrate-loading and exercise performance. Sports Medicine 1997, 24 (2), 73-81.
5. Sedlock, D., The latest on carbohydrate loading: a practical approach. Current Sports Medicine Reports 2008, 7 (4), 209-213.

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One Response on “Does Carbo-Loading for a Marathon Work? If So, What is The Best Way to Do It?

  1. Over a year ago I switched to a plant based diet (whole grains vegan) for a health concern. The unexpected benefit was that my running became almost easy (I was training for a half marathon, my first in 25 years). I no longer needed to carbo-load, as I was carbo loaded all the time. I just ran 20 miles training for a full marathon including picking up the pace the last few miles and energy was not an issue (though my legs started to get a bit rubbery the last few miles and I do take a 100 cal energy gel every 5 miles). I like that my diet supports my running rather than having to do some special “loading” before a race.

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