Carb Loading for Marathoners: Do’s and Don’ts for Training and Racing

The marathon is a unique event: it’s long enough that carb loading becomes an important key to your success on race day.

Because of it’s distance, the marathon is different than shorter races. Your fueling choices before and during the race have a big impact on your overall finish time.

The human body can only store enough carbohydrate in the muscles, blood, and liver (in the form of glycogen) for roughly 20 miles of running, especially at more strenuous intensities.

Run out of fuel and the inevitable crash or “bonk” will happen, typically around the 20-mile mark of the race.

Any marathoner who has “hit the wall” during a marathon will attest to the pervasive fatigue and dead-leggedness that follows. Slowing down is physiologically necessary to conserve the rapidly diminishing fuel stores that power a marathon performance.

My 2:44 at NYC in 2008 was exactly this cliche. I slowed down by almost a minute per mile after mile 20!

So how do we avoid the wall?

First, proper marathon training is essential to teach your body to conserve fuel and use it wisely. If you’re not well trained, no fueling strategy is going to work well for you.

Second, practice your fueling strategy during 3-4 long runs to simulate what you’ll experience on race day.

Finally, carb-load for about two days before the race to boost your carbohydrate stores to help you finish the marathon strong.

For our complete guide, including:

• Actual, hour-by-hour carb loading schedules for the marathon (and shorter races)
• Optimal fueling schedules and time windows for workouts and long runs
• Interviews with top coaches on fueling for every race distance
• An expanded “Fueling Q&A” series on how to control sugar cravings, what supplements are a waste of money, and more…

…be sure to check out Finish Strong, one of our most affordable and practical training programs.

What to Eat Before a Marathon

First, what exactly is carb loading?  

Carbohydrate is the body’s preferred fuel source for high intensity exercise (for any exercise more than an “easy” effort).

And when you carb load then you increase the amount of carbs you eat during the two days before the race.

What kind of results might you get from this approach?

In an interesting study published in the International Journal of Sports Medicine, researchers followed over 250 runners who competed at the London Marathon. They found that only 12% consumed enough carbohydrate before the race – but those who did ran an average of 13.4% faster!

The new guidelines suggest 7-10 grams of carbohydrate for every kilogram (2.2 pounds) of body weight. This is admittedly more carbohydrate than most runners are used to eating; you’ll need to consume an additional 1-2 liters of sports drink during the day to make eating all those carbs possible.

So for two days before the marathon, you should focus on carb loading at this level to ensure your body is fully stocked with the body’s preferred fuel source for high intensity exercise (carbohydrate!).

Take your pre-race fueling seriously. For a 4-hour marathoner, this could mean running more than 25 minutes faster than if you didn’t focus on carb-loading!

Make a Marathon Fueling Plan

A runner once bragged that he was going to use adrenaline to finish his first marathon. I giggled – he was going to have a really hard race.

First of all, adrenaline is not even the right type of fuel for a marathon. It’s for running away from a bear – those “fight or flight” situations.

Once you sprint away or fight for your life, it’s gone and you’ll be exhausted. In other words, it’ll last you through (maybe) mile 2 of the marathon.

Instead, it’s ideal to get the right amount of carbohydrate per hour during the race.

Too little and you risk limiting your potential. Too much, and you risk GI distress.

After you carb-load before the race, here’s a good approach backed by science for most runners:

1. Aim for 45-60 grams of carbohydrate per hour (about two gels or equivalent per hour)
2. Break this up to reduce GI distress into two fueling sessions: one gel every 30 minutes
3. Drink about 4-8 ounces of water with each gel based on conditions and your personal sweat rate

You’ll get plenty of electrolytes from your pre-marathon breakfast and gels so you don’t need sports drink during the race.

In fact, if you have gels and sports drink you might become more dehydrated. Those electrolytes need to be processed with water and your body might leech water from your cells to get it.

With these strategies, fueling for a marathon becomes simpler. And racing the marathon will be a lot more successful!

Just two last reminders…

Practice Your Race Carb Loading Strategy

Just like you practice long runs, pacing, and racing strategy before your goal marathon, you should also practice your exact fueling strategy before the big day. There should never be any surprises when you’re racing.

Aim to run 3-4 long runs using the same fueling schedule you plan to use on race day. Make sure everything is planned: use the same fuel, at the same time of day, in the same timing pattern that you will on the day you race your marathon.

This helps ensure that there won’t be any surprises when you finally line up to race.

Not sure how to plan those marathon long runs? I’ve got you covered:

Once you plan your long runs, you can practice your carb loading strategy during the final several runs.

Don’t Do Anything New on Race Day

You’ve probably heard this before. It’s completely true (no matter the distances): don’t wear new shoes, shorts, or have different foods for breakfast.

And experimenting with new fuels or a different fueling schedule is introducing a potential disaster:

• What if that new fuel makes you nauseous?
• Or how about if it gives you GI problems?
• What if you simply hate the taste?

Stick with foods that you know agree with your stomach and you’ll enjoy the fueling process on race day – because it’s helping power a monster marathon performance!

And don’t forget, our complete fueling program Finish Strong includes a lot more:

• Fueling strategies for every scenario: 5k, 10k, half marathon, marathon, & ultramarathons – plus workout fueling for any length run
• The full Expert Interview series with three top coaches, marathoners, and ultramarathoners
• Side-by-side comparisons of the major fueling brands
• Example fueling plans with multiple goal times in the half marathon and marathon

Here are all the details.

Any questions about carb loading for the marathon, nutrition for runners, or what the eat before the race? Leave a comment below!

A version of this article first appeared on Competitor.