Carbohydrates for Runners: Why They Matter and How to Fuel

Many runners fuel carefully with carbohydrates before and during a full marathon. But what about during everyday training?

Carbohydrates are strongly tied to running performance. Fueling with them well can improve the quality of both your training and your recovery.

When I first started running, I barely thought about fueling. I usually ran in the evening after eating lunch, so I never ran short of energy.

But after my child was born, I started running in the early morning, often before breakfast. I began to feel short of energy while running.

Right after waking up, blood sugar is low, and hormone secretion and a dominant parasympathetic nervous system also make it hard to raise your performance, even in key workouts.

If you don’t fuel properly after a run, recovery (your body’s repair) is also delayed, and fatigue carries over to the next day.

Recently, as my running volume has grown, I have felt the nutrition shortfall even more. In the course of researching it, I arrived at the importance of carbohydrates.

This article explains why carbohydrates matter to runners and what roles they play. It then covers carbohydrate strategy for training and for racing.

After reading it, you will understand how carbohydrates work and know the carbohydrate strategy for training and racing.

What Do Carbohydrates Do for Runners?

Carbohydrates are used in many ways in the body, but here we focus on the roles that matter to runners.

Carbohydrates play four main roles for runners.

Let’s go through each one in detail.

Fuel for Your Muscles: Blood Glucose and Muscle Glycogen

There are two main routes by which running uses carbohydrate.

The first uses carbohydrate stored in the muscle as muscle glycogen. The second takes glucose from the blood into the muscle and uses it.

The chart below shows the share of energy supplied by each major energy source over exercise time. It is an example at about 65–75% VO2 max, roughly a fast jog and slower than full marathon pace.

The main energy sources are carbohydrate and fat. Muscle triglyceride and blood free fatty acids are what we call “fat.”

Of the carbohydrate, almost all of it comes from muscle glycogen at the start of a run. As muscle glycogen declines, the supply shifts to blood glucose.

Blood glucose comes from liver glycogen that is broken down and released into the blood. If you take in carbohydrate from outside, it also becomes blood glucose and is sent to the muscles.

The next chart shows the share of each major energy source as exercise intensity (running speed) changes.

At rest (25% VO2 max), you can see that almost all of the energy comes from fat. At high intensity (85% VO2 max), more than 70% of the energy comes from carbohydrate.

If you take in carbohydrate when you are not exercising, it rebuilds the muscle glycogen you lost in running training. Whatever cannot be stored as muscle glycogen is stored as body fat.

Lactate, produced while carbohydrate is metabolized, can also be seen as part of carbohydrate metabolism, but I’ll leave lactate metabolism aside here. If you want to learn more about lactate, see the article below.

Keeping Blood Sugar Up So Your Brain Doesn’t Limit Exercise

When you run for more than two hours, your blood sugar gradually falls. If it falls faster than a certain rate, the central nervous system sends a command to limit exercise (that is, to limit the use of carbohydrate).

Once carbohydrate use is limited, exercise performance clearly drops. Having carbohydrate use limited during a full marathon is devastating.

The slowdown late in a full marathon can be caused by a lack of muscular endurance. But it is also often said to come from legs that stop moving because carbohydrate supply is restricted as blood sugar falls.

If you take even a little carbohydrate while your blood sugar is falling, it turns upward. When blood sugar rises, the central nervous system is less likely to send the command to limit exercise, which extends how long you can maintain your performance.

Carbohydrates Are Needed to Burn Fat

Carbohydrate is needed to burn fat.

In the process of producing energy from fat, intermediate products made from carbohydrate are essential. When carbohydrate is lacking, fat-burning efficiency also drops.

For details, see the article below.

In a full marathon, the point where carbohydrate (muscle glycogen) runs low and your legs stop moving is called “the 30 km wall” (about 19 miles). Because fat-burning efficiency also falls when carbohydrate drops, carbohydrate seems to be a nutrient you must never let run short.

Carbohydrates Protect Muscle From Breakdown

When carbohydrate is plentiful, muscle breakdown is suppressed.

When the body runs short of energy, amino acids begin to be supplied as an energy source. These amino acids are mainly alanine released from muscle. Releasing alanine means breaking down protein, so muscle breakdown progresses.

Energy shortfalls during running are mainly caused by a shortage of carbohydrate.

When muscle glycogen and liver glycogen fall and the carbohydrate stored in the body runs low, protein breakdown becomes dominant and amino acids start to be used as an energy source.

If you fuel with enough carbohydrate, protein breakdown is suppressed. As a result, muscle breakdown is held back, which can prevent a loss of muscle strength.

Carbohydrate shortage and muscle breakdown are explained in the article below.

Adaptations That Happen When You Run Low on Carbohydrates

For marathon runners, running short of carbohydrate is not only a disadvantage. There are benefits too.

Adaptations that occur when carbohydrate is scarce include the creation of new mitochondria and improved mitochondrial function.

When carbohydrate runs short (meaning energy runs short), AMPK (AMP-activated protein kinase) is activated. AMPK activates PGC-1α, a transcription factor for mitochondria.

As a result, adaptations such as mitochondrial biogenesis and improved mitochondrial function occur.

However, although these changes in metabolic signaling have been confirmed in the laboratory, whether they translate directly into better performance is a separate question.

A meta-analysis of endurance athletes (pooling nine studies) found that carbohydrate-restricted training had no significant performance advantage over training with normal carbohydrate intake※1. Activated mitochondrial signaling does not necessarily mean a faster race time.

On the other hand, AMPK also causes suppression of mTORC1 (mammalian target of rapamycin complex 1). mTORC1 is the enzyme complex that switches on protein synthesis, so the chain mTORC1 suppressed → protein synthesis suppressed → muscle breakdown more likely occurs.

Running short of carbohydrate also makes it harder to raise your performance in hard training. This is because energy production in the mitochondria is most efficient when carbohydrate is plentiful.

So running short of carbohydrate has both benefits and drawbacks.

Carbohydrate Strategy for Training

Based on what I have covered so far, let’s first think about carbohydrate strategy for training.

Before that, let’s check a guide to how much carbohydrate you need per day. Position stands from international bodies such as the American College of Sports Medicine (ACSM)※2 give the following guidelines for daily carbohydrate intake by training intensity.

• Moderate intensity (about 1 hour of training per day): 5–7 g per kg of body weight
• High-intensity training (1–3 hours per day): 6–10 g per kg of body weight

For a 60 kg (132 lb) runner training 1–2 hours every day, that is a guide of 360–600 g of carbohydrate per day. One bowl of cooked rice (150 g, about 5 oz) contains about 55 g of carbohydrate, so that equals 6–11 bowls.

Securing this amount consistently through everyday meals is the foundation for maintaining the quality of your training and recovery※3.

Before Training

I think whether you need carbohydrate before training depends on the type of training you are doing.

Carbohydrate is recommended mainly before moderate- or higher-intensity running training. Specifically, before training at moderate intensity or above, it is advisable to fuel with enough carbohydrate.

There are two reasons. A carbohydrate shortage may keep your performance from rising, so you cannot train at your target intensity. It may also delay your recovery after training.

If you run in the evening, for example, after a proper lunch, it may not be much of a problem. But especially when you run right after waking up in the early morning, I think you need to take something before you run.

If you have enough time before training, eat a normal meal about three hours before. Ideally, you would also have taken in enough carbohydrate by the day before.

If you have only an hour or less, bananas or running gels, which turn into energy quickly, are recommended.

During Training

As with before training, decide whether to take carbohydrate during training based on what the workout is.

Depending on the workout, though, there is hardly any time to take carbohydrate, so in many cases it seems you don’t really need it.

However, if you couldn’t fuel with enough carbohydrate before training, for example, it seems better to take carbohydrate little by little during training as well.

In particular, at or above lactate threshold (LT) intensity, most of your energy comes from carbohydrate. Taking carbohydrate as needed so you don’t run short is expected to improve your training performance and speed up your recovery.

After Training

After training, carbohydrate for recovery is a must. This is because once training ends, you need to recover from muscle fatigue as soon as possible and suppress protein breakdown.

During running training, deliberately running low on energy may produce benefits such as mitochondrial biogenesis and improved mitochondrial function. But these are adaptations that occur in the muscles being used.

After a run, first take at least the minimum carbohydrate you need. Replace the carbohydrate you lost as soon as possible after training and start recovering.

If you can eat a normal meal right after training, you don’t need any special fueling. But if there will be a gap before your next meal, it is best to fuel with an easy-to-take source of carbohydrate.

Is Training With Low Carbohydrates Worth It?

Studies of endurance athletes have repeatedly shown that even when training in a low-carbohydrate state (train low, meaning training with low carbohydrate availability) activates mitochondria-related signaling, it does not lead to better endurance performance※1.

In addition, competitive runners who ate a low-carbohydrate diet (under 1.5 g/kg per day) for two days after a glycogen-depleting workout ran a 1,500 m time trial about 2% slower than when they had taken enough carbohydrate※4.

2% looks small, but converted to a full marathon time it is about 3 minutes 40 seconds for a runner finishing in 2 hours 50 minutes.

On training days with intervals or threshold runs at or above LT intensity in particular, being short on carbohydrate may keep you from reaching your target intensity and reduce the benefit of the training itself.

For runners who pursue performance, at least, deliberately running short of carbohydrate seems to cost more than it gives. That is where the current evidence points.

Carbohydrate Strategy for Race Day

Basically, you need carbohydrate during a race only at the full marathon distance or longer. Up to the half marathon, it is said that the meal you eat before the race is enough. Here I’ll cover carbohydrate fueling for the full marathon.

Carbohydrate strategy for the full marathon is covered in detail in the next article. It also explains pre-race carb loading (glycogen loading) and in-race gel strategy in detail.

Below is an overview.

Before the Race

In a full marathon, stored muscle glycogen and liver glycogen are almost completely depleted, so how much carbohydrate you can store in your body before the race matters.

Storing as much muscle glycogen as possible before a race is called “carb loading (glycogen loading).”

Try to finish your race-morning breakfast 4–5 hours before the race. This is because it takes a relatively long time for the blood sugar rise from a meal to settle.

It is said to be best to avoid carbohydrate 30 minutes to 1 hour before the race. If you take carbohydrate then, “insulin shock” (a sharp drop in blood sugar) can occur just as the race starts.

During the Race

In a full marathon, carbohydrate fueling during the race is important.

Elite runners can prepare their own special drinks containing carbohydrate and fuel at the aid stations. Recreational runners must rely on fueling products they bring themselves or on the shared aid stations.

The limit on how fast a person can absorb carbohydrate is 60 g per hour when taking glucose alone, and up to 90–100 g per hour when glucose and fructose are combined in the right ratio.

However, taking high-concentration carbohydrate can cause diarrhea because of the osmolality difference with body fluids when it is absorbed in the gut. That is why some runners get an upset stomach when they take Maurten gels.

You need to take countermeasures, such as taking it with water and practicing fueling in training.

Fueling with carbohydrate promotes the use of blood sugar and spares muscle glycogen, and liver glycogen also doesn’t have to be used, so you can slow down the loss of carbohydrate in your body even a little.

Carbohydrate fueling during a race also prevents blood sugar from falling. When blood sugar falls too far, the central nervous system sends a command to limit exercise, and muscle movement is restricted.

Even a small amount of carbohydrate during the race raises your blood sugar back up, which makes it harder for the central nervous system to send that command.

After the Race

After a full marathon, you should take in enough carbohydrate no matter what.

In a full marathon, your body’s glycogen stores drop sharply. The damage to your body is also large, so you need to start recovering quickly and recover as soon as possible.

The energy burned in a full marathon is about 2,500 kcal, of which about 2,000 kcal is carbohydrate. That is roughly 500 g of carbohydrate, so you need to replace at least the carbohydrate you used.

References

※1 Gejl KD, Nybo L (2021) “Performance effects of periodized carbohydrate restriction in endurance trained athletes – a systematic review and meta-analysis” Journal of the International Society of Sports Nutrition

※2 Thomas DT, Erdman KA, Burke LM (2016) “American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance” Medicine & Science in Sports & Exercise

※3 Hearris MA et al. (2018) “Regulation of Muscle Glycogen Metabolism during Exercise: Implications for Endurance Performance and Training Adaptations” Nutrients

※4 Venckunas T et al. (2024) “Effect of Low vs. High Carbohydrate Intake after Glycogen-Depleting Workout on Subsequent 1500 m Run Performance in High-Level Runners” Nutrients