How to fuel for a marathon

Marathon runners are starting to pay more and more attention to their nutrition. No one ever questions the importance of training, yet the importance of nutrition has often been underappreciated. Or perhaps the topic of nutrition seems to pop up on the radar only close to the marathon.. and this is when many questions suddenly arise. In this nutrition guide we will explain how to best fuel for your marathon. What should you eat the day before, or the morning of the marathon, and what drinks, gels and solids you should take during the marathon, and how often. Keep reading if you want to know the answers.

What can go wrong nutritionally during a marathon?

A large percentage of marathon runners struggle to get their nutrition right, and many who do not have a great run, blame nutrition: they ran out of energy, became dehydrated or experienced stomach problems. They ingested too much, or ingested too little. They tried new products they had not used in training, and so on.

Here are the most common mistakes:

Not having a plan.

Just starting a marathon thinking: "oh I will drink when I get thirsty and I will grab something at feed stations if I need it". It is a gamble. Maybe it will work. If you have prepared for your race, invested so many hours training and you have planned all your training, why not plan your intake, and increase the chances of success?

Sticking to a plan at all cost

Some runners have a plan but are a little too rigid sticking to this plan. If you have a plan, don't stick to it at all cost, and don't be afraid to deviate if you have to. If the weather conditions re warmer than expected, maybe you need to drink more. If you feel stomach discomfort just reduce intake for a while.


Many runners get excited at the expo the day or days before, buy new nutrition products and will have to find out if they work in the marathon. Stick to what you have tested and to what you know will work. Don't experiment in the most important event of the year!

Three common mistakes marathon runners make.

Why is nutrition so important for a marathon?

Let’s start with the most important questions: Why is nutrition important in a marathon and what difference can it make? A marathon will take anywhere from 2-6 hours. Energy expenditure is somewhere between 2200 and 3200 kcal depending mostly on body weight Interestingly it is pretty much unrelated to pace! A runner who uses half as many kcal per minute will be running for much longer, resulting in similar total energy use.

Problems you may run into during a marathon

Poor nutrition can mean that hypoglycemia develops and a runner will be reduced to walking the last part of the marathon, lightheaded and with little or no power left in the legs. Even if this low blood sugar does not occur fuel supply to the muscle may be suboptimal and this can affect performance.

In addition to this, and specially in warm conditions, dehydration may develop: fatigue, headaches and disorientation may be the result.

Thirdly, it is also possible that gastro-intestinal problems develop… and often we see runners on the side of the road vomiting or quickly popping into the porter loos…. The glamour of marathon running….

How can we prevent trouble and increase the chances of a great race?

How do we prevent all of this? How can we make sure we will have a great race?

Besides making sure you are well trained and well recovered at the start of a marathon, performance in a marathon is dependent on 4 main nutrition related factors:

Making sure there is adequate fuel to complete your marathon

a. Enough carbohydrate stored in muscle

b. Enough carbohydrate stored in the liver

c. Enough carbohydrate supply during the marathon

Adequate hydration

This means preventing dehydration throughout the marathon but also preventing overhydration. This is especially important in hot conditions.

Minimise the risk of GI problems

Some runners never have GI problems. They are the lucky ones. The majority of runners will experience at least some discomfort and in some cases serious GI distress. These runners should pay attention to what they eat before and take in during the marathon.

Supplements that may help

There are very few supplements that can help a marathon runner (despite what you may read and despite what companies may claim). Caffeine is the exception, so we will cover caffeine intake below.

Many nutritional factors might affect marathon running performance

Carbohydrate as a fuel

The body uses two main fuels: carbohydrate and fat. Fat is the primary fuel for less intense exercise (low to moderate intensity: often referred to as aerobic) while carbohydrates are the primary fuel for intense exercise moderate to high intensity). Someone who is well trained can burn fat at a higher rate. This will help them to be less dependent on carbohydrate, but when the intensity increases carbohydrate is still the preferred fuel. Carbohydrate can deliver energy much faster to the muscles than fats. Fat is like “diesel” and can support much longer efforts than carbohydrate. Carbohydrate, however can provide much more energy per unit of time.

Carbohydrate stores are relatively small

The body has stores of both fuels, but unfortunately the stores for carbohydrate are much smaller than those of fat. Even the leanest athlete has sufficient fat to sustain the longest races at moderate intensity (thousands of grams of fat). In fact, there would be enough fat in any athlete to run many back-to-back marathons.

The typical athlete will also have 500-800g of carbohydrates stored as muscle glycogen and perhaps 80 grams or so as liver glycogen. These stores are relatively small and can only provide a fraction of the total energy from fat stores). These carbohydrate stores would be sufficient to fuel 2-3 hours of intense exercise.

Avoid hitting the wall

Most marathon runners are familiar with “hitting the wall”. Usually around 20 miles or 32 km. This point in the marathon coincides with the depletion of carbohydrate stores.. and although it may not be the sole reason for the "hitting the wall" phenomenon, with adequate nutrition we can delay this point or even completely prevent it.

A large group of marathon runners setting off on their journey (legs only)

Sweating, temperature regulation and dehydration

We also sweat and this is an important process because it helps us to regulate body temperature. Without it we would soon overheat just like a car engine would overheat without a cooling mechanism. When fluid losses are large this will start to compromise body function. It will increase cardiovascular stress (for example increased heart rate), it will reduce the ability to regulate body temperature, and it will also increase the risk of gastro-intestinal problems. So we must make sure that we drink enough to prevent these large losses. Small losses are fine and maybe even beneficial.


This means that in the lead up to the marathon and during the marathon we must:

1. Make sure we have sufficient fuel in the muscle glycogen tank

2. There is sufficient fuel in the liver glycogen tank

3. We maintain a good level of hydration.

What to eat the day before the marathon?

In order to start a race with optimal glycogen stores it is important to eat carbohydrate-rich in the day or days before the race (It is important to also reduce the amount and the intensity of training to reduce the use of glycogen stores). Traditionally consuming additional carbohydrates has been called carbo-loading. Consuming a little more carbohydrate than normal at the expense of some protein a