There’s a lot of hype around different types of fuel – supplements, gels and bars for endurance events all designed to help you run faster for longer. It’s all designed around the idea that after a certain amount of time your body runs out of glycogen to use for energy, so in order to be able to continue producing energy efficiently, you need to restock your body with sugars which are converted into glucose and then glycogen which is stored in the liver and muscles, all while you are running.

So, when do you know if you need to refuel during a long run?

Fuel Storage in the Liver and Muscles

Humans store around 100g of glycogen in the liver and 400g of glycogen in muscle, but this varies depending on the individual due to factors like age, gender, genetics and more. The average runner weighing around 70kg is expected to burn 100 calories per mile. 25g of carbohydrates (glycogen) is the rough equivalent to 100 calories. If you do the maths, this means, in theory, you could expect to run 20 miles off full glycogen stores.

However, your brain requires around 125 – 150g of glucose a day to function and this can be taken from the liver and muscle stores. This means your glycogen stores are not always completely filled up and glycogen is typically being used for other metabolic processes occurring in cells within your body.

Realistically, you can expect to be able to run for around 90 minutes at a hard intensity (race pace) before needing to think about fuel. If you’re running at a slower pace and doing say an easy long run, you could expect to be able to run for maybe 2 hours or longer since your body will be metabolising fat for energy.


They say around mile 18 is when you hit the wall – and this is the reason, your body has completely depleted its glycogen stores and now must produce energy via gluconeogenesis where the liver will break down fat and protein to form glucose which will then be used for energy.

Due to having such low glucose levels, runners may experience symptoms of hypoglycaemia where extreme fatigue may occur. It is not uncommon for runners to faint or collapse at this stage – this is known as ‘bonking’ or ‘hitting the wall’.

How to Fuel for a Race or Run Expected to take over 90 Minutes

So, if you’re running for over 90 minutes at a high intensity or training for a race which you expect to take over 90 minutes, it is a good idea to practise fuelling for a race by experimenting with different gels, bars and various energy supplements. Since it takes a while for the supplements consumed to be converted into glycogen, consider taking your first bit of the supplement at around 30 minutes in, but just take a bit.

Try and consume a bit of your energy source every 15 minutes from thereon into the run. This is to prevent gastrointestinal (GI) distress. Some runners find that if they consume too much fuel, then this can upset their GI tract ruining the run. Drink water with your carbohydrate source as this helps your body to digest it properly and to send it out into the body where it can be used to restock your glycogen stores. If your energy source is a gel, you may not need to drink water with it as water is contained within the gel, but for more physical foods then make sure you drink water.

Experiment with different fuels during training runs to see what works with your stomach and what doesn’t. Think about how you felt and whether you felt energised or lethargic.

What fuel do you like to use on your long runs? Let me know down below!