Running on Empty

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You wouldn’t get very far if you tried to drive your car with an empty fuel tank, the same applies to your body. Muscle stores of the main fuel for exercise, carbohydrate, can only sustain moderate intensity exercise for around 90 minutes1, so how can you keep yourself revving all the way to the finish line?

Fuel Metabolism

Let’s start with some basic information on how our body provides us with the energy we need to perform activities. There are three energy systems our body uses, each using limited supplies of stored fuels:
  • ATP-CP: adenosine triphosphate is the compound the body breaks down to release energy. There are limited stores of ATP readily available in the muscles, enough to provide around 3 seconds of energy, after which creatine phosphate (CP) is used to produce more ATP, providing another 6-8 seconds. This is the energy system used for activities such as the 100m sprint, a high jump or Olympic weight lifting.
  • Anaerobic Glycolysis: this energy system uses sugar stored in the muscles in the form of glycogen to provide a few minutes of energy. Glycogen can be partially broken down without the need for oxygen, but when sugar is only partially metabolised a by-product is lactic acid, which causes muscle fatigue. Anaerobic glycolysis (also known as the lactic acid system) can only provide enough energy for exercise such as the 400m sprint, but is also called upon during a sprint in the final stages of a race.
  • Aerobic Metabolism: the aerobic energy system can provide energy for much longer activities, kicking in at around 5 minutes of exercise, and keeping us going for hours or even days. This system draws on stores of carbohydrate, fat and, in lesser amounts, protein to keep us fuelled. In low intensity activities (60-70% of maximum heart rate, or 6-7 out of 10 effort) fat is the main fuel metabolised, and provided there is enough oxygen being taken in, this can sustain exercise for days. However, fat is not adequate for moderate to high intensity exercise (70-90% max HR, or 7-9 out of 10) and this is where the metabolism of carbohydrate comes in. With oxygen present, stored glycogen can be broken down without producing lactic acid, and can provide enough energy for around 90 minutes of moderate exercise.

What happens when stores get low?

Just like when your car’s fuel gets low and the warning light comes on, our body provides signals that we need to re-fuel… or stop. You might start to feel weaker, your body will slow down and you may get dizzy.
If you feel at all unwell during exercise, the best thing to do is simply stop. If you’ve been sweating heavily have a salty snack, like ready salted crisps, or a pack of salted nuts to help restore the balance of electrolytes in your body.
DO NOT continue to exercise; it’s much safer to return to your workout on another day, when you are feeling well again.

90 minutes or more

Anyone who has ever run a marathon, or even watched one on telly, will know that it is impossible to complete in less than 90 minutes! So here begs the question, how do I keep fuelled for longer than 90 minutes?
Once your body runs out of stored glycogen, it will turn to metabolising fat. But as we’ve already learned, fat can only sustain low intensity exercise, and because it is slower to metabolise it’s also more likely to result in you “hitting the wall”. Now that’s not going to get you that personal best you’ve been after is it? 
That’s where our good friend carbohydrate comes back in! Studies have shown that intake of carbohydrate in the form of a glucose polymer solution (providing more glucose with less sweetness) fed at regular intervals can increase moderate exercise endurance by an extra hour before fatigue2. Furthermore, a sprint to exhaustion can be increased by 45% in athletes consuming carbohydrate during exercise3.

So if you want to go long, you’ll need to take in some fuel on the go.

What to take on

During moderate to high intensity exercise, blood is diverted away from non-essential organs (the stomach, intestines and kidneys) instead being delivered to the working muscles.  This means that any food or liquid ingested is more likely to cause irritation. Foods and drinks that are easy to digest are preferable. Avoid foods high in fat or fibre as these can be slow to digest and stay in the stomach for longer.
Luckily there is a plethora of different supplements and snacks you can choose from. There are liquid forms of sports nutrition, such as Lucozade, Gatorade, and SIS energy drinks, solids such as energy bars, tablets, jellies, or fruit, sweets etc., or there’s the in-between of energy gels like High5, Zipvit or SIS Go Gel.

Most sports products have recommendations written on them about how much and how often to consume, but as a general rule about 60-80g of carbohydrate per hour should be plenty, which is the equivalent of an energy gel every 20 minutes or so, or a sports drink sipped over the course of an hour.

Here are some pros and cons of the different products available:


Left to right: Lucozade, Gatorade, Powerade

Pros: often readily supplied at races, easy to consume and digest (in most cases)
Cons: difficult to carry, can be extremely sweet, expensive


Variety of energy gels

Pros: easy to carry, isotonic versions are easy to consume and digest,
Cons: concentrate gels can be tricky to digest, can be fiddly, relatively expensive


Jelly baby army!

Pros: sweets and fruit are cheap, can take your mind off of the race
Cons: difficult to carry, tricky to consume and digest, energy bars can be expensive

If you’re taking part in an organised race, such as a half or full marathon, and don’t want to carry your own fuel, check out the organiser’s web page to see what nutrition will be provided on the day and stock up well in advance for your training runs. If it doesn’t work for you, then try a different brand or type of fuel – you may have to make the sacrifice of carrying it yourself.
Each has their ups and downs, but the key is practice, practice, practice. What works for a friend or an elite athlete may not work for you!

Next time: 

Look out for the next post, where we’ll focus on pre- and post- exercise nutrition.

Further Reading:

  1. Hermansen, L., Hultman, E., Saltin, B., 1967. Muscle glycogen during prolonged severe exercise, Acta Physiologica Scandinavica, [online] Available at:[Accessed 8th October 2012].
  2. Coyle, E.F., Coggan, A.R., Hemmert, M.K., Ivy, J.L., 1986. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate, Journal of Applied Physiology, [online] Available at:,%20Muscle%20glycogen%20utilization%20during%20prolonged%20strenuous%20exerc%20when%20fed%20CHO,%20JAP%2061(1)%20165-72,%201986.pdf[Accessed 8th October 2012].
  3. Hargreaves, M., Costill, D.L., Coggan, A., Fink, W.J., Nishibata, I., 1984. Effect of carbohydrate feedings on muscle glycogen utilization and exercise performance, Medicine and Science in Sports and Exercise, [online] Available at:[Accessed 8th October 2012].

Georgina Spenceley
Georgina Spenceley

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