Should You Carb Load Before a Marathon?

Should You Carb Load Before a Marathon?

The energy demands of the marathon are different than that of even the 10K or even half marathon. Part of what makes a marathon so challenging is the chance of bonking; your nutrition before and during the race can mitigate this risk and help you run strong and steady through the end. Carb loading is traditionally viewed as part of a marathon nutrition strategy. But does it effectively work?

What is Carb Loading?

The human body can only store enough glycogen for two hours to two and a half hours of running, and even less than that when you are running at a higher intensity. (And yes, marathon pace is higher intensity if you are racing it!) Glycogen depletion is the reason that many runners hit the dreaded wall. Without enough carbohydrates available via glucose or glycogen, your body resorts to fatty acid oxidization to produce ATP (energy). However, this is a slower process that occurs at lower intensities. Thus, your pace slows down. Your body will also begin to catabolize the muscles in order to synthesize protein into glucose for energy.

Slowing down due to glycogen depletion is also one of your body’s protective mechanisms. If your muscle glycogen is fully depleted, your muscles would be damaged beyond repair. Additionally, hypoglycemia (too low of blood sugar) can cause neural damage. Thus, you slow down to protect your muscles and your central nervous system from damage. Not even the toughest-willed marathoner can overcome that level of fatigue.

You prevent glycogen depletion and optimize your performance by consuming carbohydrates (gels, chews, etc.) during the race. However, this is only part of the puzzle. Carb loading – also known as “glycogen supercompensation” – reduces the risk of glycogen depletion. A combination of proper carb loading and mid-race fueling form a cohesive fueling strategy – one that allows your training to express to its full potential on race day.

However, carb loading is not always how the media portrays it or some runners practice it. It certainly isn’t cramming down a heaping plate of fettuccine Alfredo before your race. Carb loading is the practice of eating more carbohydrates in the two to three days before your marathon in order to stock your muscles with their preferred energy source for the race.

Should You Carb Load Before a Marathon?

Is Carb Loading Effective?

A majority of elite runners do increase the carbohydrate intake in the two to three days before a marathon. Most coaches advocate for a carb load. Why? Because it is effective in reducing your chance of hitting the wall.

Some runners look to fat adaptation as an alternative. However, no research to date indicates that fat adaptation improves performance in the marathon distance. Yes, you would have energy to run the marathon distance. Your speed would be compromised, though.

A study from the International Journal of Sports Medicine examined how carb loading affected runners at the 2009 marathon. This was not a controlled trial in a lab. Instead, this study looked at runners in the real world, thus offering some of the most salient conclusions on carb loading. Of the 250 marathons studied, the researchers found that those who ate more than 7 g of carbs per kg of bodyweight the day before ran about 13.4% faster than their counterparts who did not carb load. 13.4% faster is a significant amount of time in the marathon!

Metabolic adaptations of endurance training make the carb load effective. During marathon training, your long runs burn up a significant amount of your glycogen (stored carbohydrates). As a result of this, your body adapts to increase its ability to store glycogen. By the time race day arrives, your body is able to store more glycogen than previously. Carb loading and the marathon taper combined allow you to take full advantage of this training adaptation on race day.

So in short, carb loading works. Yes, some runners may not need it – there are always variances in individual diet and physiology – but for a majority of runners, a smart carb loading strategy will help you avoid the bonk and optimize both performance and experience in your next marathon.

How to Carb Load Effectively

To effectively carb load, you eat more carbohydrates in the two to three days before your race. How much is more? For the simplest calculation, carb loading is eating approximately 8-10 grams of carbohydrate per kg (2.2 pounds) of body weight. For example, if you weigh 135 pounds (61 kg), you would aim for a minimum of 425 g of carbs in the two days before your marathon. These carbohydrates should be spread out throughout those days, rather than concentrated into one or two meals. Additionally, you should increase your fluid intake, as water is part of glycogen.

Of course, you do not have to download a macronutrient counting app and get caught up in a number game. You can approach carb loading intuitively, by deliberately increasing your portions of carbohydrates and opting for higher carbohydrate foods – for example, oatmeal for breakfast instead of eggs, a bagel or toast for a snack instead of yogurt, etc.  Adding in sports drinks or juice can help sneak in extra carbohydrates.

A change in diet can add unwanted effects, including sluggishness and gastrointestinal upset. Stick to foods that are familiar to you. You do not need to cut out fiber completely and eat only simple carbs. For many runners, this may be too much of a change in routine before a race. You may opt to reduce fibrous foods and eat easily digestible carbs the day before the race. However, be careful not to eat too much fiber as you carb load!

Carb Loading Errors to Avoid

How you carb load matters. Some common errors – including taking carb loading to an excess – may negate the performance benefits of carb loading and leave you feeling bloated and sluggish.

  • Over-eating: Carb loading is not license to indulge in enormous portions and sugary foods. You are still fueling your body for performance. Yes, you want to eat more carbs than normal – but not so many that you feel bloated, sick, and sluggish.
  • Eating “junk” food: While pastries, cookies, and fries may be high in carbohydrates, they are also high in fat. These foods will increase risk of GI upset, particularly bloating.
  • Eating lots of fiber: Ignore those ill-informed Instagram posts that proclaim complex carbohydrates are superior for carb loading. If anything, simple carbs are preferred, as less is lost in the digestive process. Additionaly, simple carbs reduce the risk of GI upset on race day. If you increase your intake of complex carbs, you increase your fiber intake…and we all can imagine how that’s not pleasant during a marathon.
  • Eating only carbohydrates: You may be eating more carbs, but you shouldn’t eat only carbs. Protein and fat are still vital!
  • Deliberate depletion: The old-school approach of deliberately depleting carbohydrate stores before a carb load through means of a long workout and low carb diet is not necessary. This approach was based on studies in the 1960s done on sedentary individuals. Runners, on the other hand, partially deplete their carb stores repeatedly through training. The body adapts to increase its ability to store glycogen. Additionally, a sudden switch to a low-carb diet can render you moody and tired shortly before the race – not exactly how you want to feel.
  • Not drinking enough water: Your body requires water to aid in the storage of glycogen. Without drinking enough fluids, carb loading may not be as effective. Also, you may be slightly dehydrated going into your marathon. Neither are ideal!
  • Worrying about weight: You will gain a small amount of water weight with carb loading. This will not harm your race – at most, it’s a couple of pounds. It is far preferable to go into your marathon with a couple of extra pounds yet plenty of energy than at a slightly lower weight but without enough energy.
  • Waiting until the night before the marathon: Much like studying for an exam, cramming is not an effective approach to carb load. Eating a huge, heavy meal the night before the marathon can increase your chances of GI upset on race day. Instead, spread the carbohydrates out throughout the entire day before the race.

Carb loading is only part of the picture for achieving a marathon PR or avoiding the wall. Your mid-race nutrition, proper nutrition during training, and a sound training plan all will affect the outcome of your race. Carb loading alone won’t compensate for training errors. But for runners who have trained well, carb loading reach the potential of your training on race day.

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Do you carb load before a race?
What’s your favorite food to eat during the carb load?

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13 Responses

  1. Carb loading is tricky for a lot of runners mainly because they think it’s a free for all to eat ALL the carbs. I know I did when I first started running! I do up my carb intake prior to a race but it’s usually nothing crazy.

  2. That is an interesting study! I used to wait until the night before the race to carbo load but now I do it a few days before hand and I can really feel the difference in the way I race and run!

  3. I was chatting with an elite triathlete this weekend and it was interesting that he doesn’t really carb load- he just encourages eating as usual. BUT, for him, eating as usual does include a lot of carbs. Obviously if you are skimping on the carb before a race (or any workout) you’re going to feel it!

  4. I try to eat more carbs the week before a longer race, and I would say I eat moderately high carb to begin with, but I doubt I have ever managed ~400 g in a day! I will have to try adding in liquid carbs next time.

  5. That’s interesting about the deliberate depletion – I see many runners do this and I was always wondering about its effectiveness.
    I’m glad to see that it’s not necessary. I would find the depletion process (effectively a keto diet) quite challenging.

  6. Great info! I’ve always done a carb load before marathons and recommend that athletes I coach do as well. I think the key is to not go overboard and to spread out the extra carbs over the meals/snacks in the days leading up to the race. Also making sure you’re not eating anything new that can upset your stomach is so important!

  7. carb loading is tricky. Learning how to do it so it works best for your body is key. Also learning what amount of fuel you need for certain distances is super important. I am working on this with a few of my clients now and it is a lot of trial and error to get it right

  8. Always. But I’m a creature of habit. Alway pizza. If it’s not available then pasta. Nothing fancy. Just tomatoes and cheese.

  9. This is such a tough one and thanks for shedding light on it. I must admit I’ve made many of the errors you’ve highlighted we should avoid! But I’m a work in progress and posts like this really help to make things even clearly for me.

  10. I usually stick with a lot of beige food leading up to a race. I can get away with a burger, but anything else usually sends me to the restroom with GI issues. It’s just how my body processes 🙁

  11. I’ve always carb-loaded before big races and it’s worked well for me. I agree that it’s not a good idea to over-indulge or wait until the night before to get started.

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