How to Fuel For a Half Marathon

Fuelling for a Half Marathon

This post started out as an email response to a question about fuelling for a half marathon:

Previous advice during a race is not to drink the sugary electrolyte drinks and stick with the water. This stops the energy system switching from fat burning to sugar burning and back. What are your thoughts. Both isotonic drinks and water are available at the drink stations.

First of all the body doesn’t switch from using one fuel to another like a light switch.  It is more a transition from one to another like a facet that controls the amount of hot and cold water flowing to make warm water.

The higher the intensity you are exercising at the greater the proportion of carbohydrate (CHO) is used as fuel.  The lower the intensity the greater the proportion of fat is used at fuel. At some point in the middle it uses 50% from each. The CHO used for fuel is primarily from the muscle glycogen stores or circulating in the blood as your blood glucose. Fat stores get mobilised as Free Fatty Acids (FFA) into the blood system for transport to the working muscles. At no point does the body operate at 100% of one of the fuel sources.

CHO is also stored in other locations in the body and if the blood glucose drops this gets mobilised and returns the blood glucose to a ‘normal’ level. This is where things get interesting. For people who are adapted to eating a high fat diet, their ‘normal’ is different from those that follow a ‘traditional’ diet. But the key thing is that at the higher intensities they will still be burning CHOs.

Even people with less than 10% body fat have a large amount of energy in their fat stores, compared to the small amount of CHO stores on their body. You will still need to keep your CHO stores topped up during the event.

For the half marathon anyone should be able to maintain Level III intensity.  Level IV is a little high and will certainly deplete your bodies CHO stores quickly. Holding back a little and maintaining Level III for the full distance will mean you don’t require a large amount of supplementary CHO but still a bit. Read more about running intensity here.

Aid stations are roughly every 4km, so my advice for you would be to alternate a decent swig at each aid station (certainly not a full 200mL bottle worth), alternating between water and the electrolyte drink. This will keep you topped up without creating any massive swing in anything.

If you would like further advice feel free to contact Coach Ray.

Coach Ray is the Head Coach & Director of Qwik Kiwi – Endurance Sports Consultant.

Coach Ray specialises in assisting first timers and recreational athletes to achieve their sporting goals. He can be contacted at and 021 348 729. Make sure you sign up to his monthly informative newsletter.

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