Marathon Carb Loading and Race Fueling Strategy

Fueling strategy is extremely important in the full marathon. No matter how well your everyday training has gone, if you arrive at the race under-fueled, you may not reach even 50% of your ability.

I am a recreational runner who started running seriously after joining the workforce. I run about 500 km (310 miles) a month and approach running with a competitive mindset.

I became interested in exercise physiology and studied carbohydrate metabolism in depth. Through that, I came to understand why carbohydrates matter so much in the marathon.

Here I explain carbohydrate fueling strategy for the full marathon: carb loading before the race, meals on race day, how to fuel during the race, and the types of fueling products.

After reading this article, you will understand how to take in carbohydrates for the full marathon. I also believe you will gain the knowledge to choose suitable fueling products.

Why Do Carbohydrates Matter So Much in the Full Marathon?

Because the full marathon lasts more than two hours, it uses up a lot of the muscle glycogen stored in your muscles beforehand.

When muscle glycogen falls below a certain level, the supply of energy needed for muscle contraction stalls and your running motion suffers.

In the full marathon, muscle glycogen often drops below that level right around 30 km (about 19 miles). That is why the way your legs stop after 30 km is called “the 30 km wall.”

Conversely, if enough muscle glycogen remains after 30 km, you are likely to cover the remaining 12.195 km (7.6 miles) without slowing down.

So in the full marathon, not running out of glycogen (carbs, or carbohydrate) until the end of the race is a key point for running a good time.

Running out of carbohydrate in a race can happen for reasons such as going out too fast (overpacing), but this article explains it from the nutrition side.

From the nutrition side, here is how to avoid running out of carbohydrate as much as possible.

Below I explain carbohydrate fueling strategy before and during the race.

Before the Race: How to Carb Load

One way to avoid running out of carbohydrate on race day is to store as much carbohydrate as possible before the race.

Storing carbohydrate before a target event is called carb loading (glycogen loading).

There are several ways to carb load. Here I introduce two: the classic method and the modified method.

Classic Carb Loading

The classic carb loading method works as follows.

Classic carb loading is a method of storing muscle glycogen over one week.

In the first half of the week you deliberately cut carbohydrate and do hard training to deplete muscle glycogen.

In the second half of the week, you do the opposite and raise the carbohydrate share of your diet to 90%. You do not increase total calories; you increase the share of carbohydrate.

Normally I think a calorie ratio of about 6.0 : 1.5 : 2.5 for carbohydrate, protein, and fat is appropriate, so this raises the carbohydrate share considerably.

In practice, think of eating a little less of the side dishes and more staple foods such as rice and udon noodles instead.

Because the classic method cuts carbohydrate in the first half of the week, that part of the week can be quite hard. Depleting muscle glycogen through training can also leave you very fatigued.

Modified Carb Loading

The modified carb loading method is done as follows.

The modified method removes the low-carbohydrate period. Research has shown that 24–36 hours of sufficient carbohydrate intake with a gradual reduction in training is enough to store muscle glycogen well.

From 6 to 4 days before the race, eat almost as usual, and from 3 days before, eat a diet with slightly more carbohydrate.

By gradually cutting training volume through tapering, you build up the muscle glycogen stored in your body.

Heads-Up: You Will Gain Weight When You Carb Load

The thing to watch out for during carb loading is that you will “gain weight” for sure.

Glycogen cannot be stored on its own. It is always stored together with water.

About 3 g of water binds to every 1 g of glycogen, so if you store 200 g more glycogen than usual, your body weight goes up by 800 g (200 + 600), or about 1.8 lb.

If you worry about weight gain and cut back on food before the race, you may not store glycogen properly. Go into it expecting that “your weight will go up.”

Carb Loading With Isomaltulose (Palatinose)

Isomaltulose (sold under the brand name Palatinose) is known as a carbohydrate that is absorbed slowly, has a low glycemic index (GI), and causes only small swings in blood sugar. Recently, many runners mix isomaltulose powder into homemade drinks, and many fueling gels containing isomaltulose have been released.

I thought that using isomaltulose during carb loading might make carb loading go more smoothly, so I ran a carb-loading test with isomaltulose.

In a carb-loading trial I did in December 2025, I was not able to gain weight well. So I changed when I take isomaltulose and carb loaded with it again before the Lake Biwa Marathon in March 2026.

This time my weight increased significantly, up to 65.3 kg (144 lb) on race day. I felt no shortage of energy at all during the race and finished in 2:40:15, a personal record. However, I changed many things besides how I take isomaltulose, so I can’t say the personal record came from the intake method alone.

Carbohydrate Fueling on Race Morning

It works well to eat on race morning as follows.

There are two things to watch in pre-race fueling. Finish digesting solid food before the start, and avoid fueling that sends your blood sugar soaring between 30 minutes and 1.5 hours before the start.

If you start running before solid food has been digested, what is left undigested can cause indigestion or diarrhea.

It is best to finish solid food at least 4 hours before. For foods that are mostly carbohydrate with almost no fat, such as daifuku (a rice cake with sweet filling) or castella (a Japanese sponge cake), 2–3 hours before the start is often fine.

I also recommend taking caffeine around 2 hours before the start. Caffeine is an internationally recognized ergogenic aid, approved as a supplement that improves exercise performance.

From 30 minutes to 1.5 hours before the start, it is safer to avoid carbohydrate with a high GI in particular. GI stands for glycemic index, a value that shows how easily a food raises blood sugar, with glucose set at 100.

When blood sugar rises sharply, insulin is secreted to bring it back down. The sharper the rise, the bigger the drop, so hypoglycemia (low blood sugar) can occur.

This phenomenon is called insulin shock.

When blood sugar is low, exercise performance drops. Your blood sugar needs to be reasonably stable by the start of the race.

Conversely, within about 30 minutes of the start, taking high-GI carbohydrate seems to be fine in many cases. Once you start running, you begin using the raised blood sugar as energy, which suppresses insulin secretion and prevents blood sugar from dropping sharply.

Fueling Strategy During the Race

The key points of in-race fueling strategy are as follows.

The first thing to keep in mind during the race is not to make the amount of carbohydrate per dose too high.

The upper limit for carbohydrate absorption is said to be 60 g per hour for a single-source product (glucose only) and 90–100 g per hour for a dual-source product (glucose and fructose). But this is only the absorption rate under good conditions.

It is known that when you take a drink containing carbohydrate, the higher the carbohydrate concentration, the slower it is emptied from the stomach toward the gut.

Ideally, the concentration of what leaves the stomach for the gut should be as low as possible. If you take too many gels at once, the gels stay longer in your stomach, which can also cause diarrhea.

How much gel per dose, and how much fluid to drink with it, is something you need to work out in your everyday training as the best answer for you.

The reason to use fueling products is to replace the carbohydrate you have used. To make up for glycogen use, I recommend using fueling products actively from the early part of the race.

Fueling products take time to be absorbed, so it is best to take them little by little from early in the race.

On Running Scientist, I also cover how to take gels in practice. For details on gels, water, and timing, see the article below.

After the Race: Nutrition for Recovery

Not only after a full marathon, fueling after a race is important for recovery.

After a race, you need to restore muscle glycogen and fix your body’s energy shortage.

When the body is short of energy, recovery from fatigue is delayed and muscle breakdown can progress, so I expect few people choose to restrict nutrition after a race.

Within 1 hour after the race, I recommend taking a fueling product containing carbohydrate and protein at 1.0–1.2 g per kg of body weight. After that, continuing to take 1.0–1.2 g of carbohydrate per kg of body weight until 4 hours after the race lets you restore muscle glycogen quickly.

For a 60 kg (132 lb) runner, that means 60 g of carbohydrate right after the race, then 60–72 g of carbohydrate every hour for the next 4 hours. 60 g of carbohydrate is about the amount in one lightly filled bowl of rice.

Right after the race is when glycogen absorption is at its highest, so fueling properly at this timing can speed up your recovery.

However, your internal organs are under considerable strain after a race, so eating a normal meal is often difficult. It is enough to prepare easy-to-absorb foods like those you ate before the race and eat them after you finish.

Nutrition Matters as Much as Training

From my own experience, nutrition is just as important as the training itself.

To train well, you need enough nutrition and rest (recovery). Especially as your training volume grows, nutrition can fall short, so aim to fuel appropriately for your training volume.

I hope this is helpful to you.