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  • Writer's pictureAlexa

Carb Loading

You might have heard about carb loading, and before I go into it I thought it might be useful to explain a little about why we do it and why it’s useful for endurance sports like the marathon.

Essentially (unless we are sprinters) we have two fuel tanks in us that our bodies use for running;

  1. Body fat. Even on whippets like Mo Farah you have enough body fat to keep you going for days, it’s an excellent endurance fuel because of that. All those long SLOW runs have been training you (in part) to use body fat as fuel more effectively. This will be your primary source of fuel on race day. The only real downside of using body fat as fuel is that it takes the body time and energy to take the fat and turn it into the fuel the muscles use, it’s a long multi step process so on race day you need another, quicker release fuel source…

  2. Glycogen. This is your quicker release fuel source, it works in tandem with body fat and you burn more glycogen the faster you go. So a 5k would have a higher percentage glycogen use than a marathon for example. Your glycogen fuel tank is fairly small, it’s stored in your muscles, blood stream and liver. Glycogen is made by the body from carbohydrates.


I like to think about the glycogen fuel tank as being a bucket with a tap on the bottom. The tap lets fuel out - that’s you burning the fuel as you run, so the faster you go the more glycogen pours out of that tap. You can add glycogen into the tank by taking on fuel as you run. If you run out of glycogen in the tank you “hit the wall” and the body will force you to slow down enough to rely on only the slower release body fat fuel tank. Your brain runs only on glycogen which is why hitting the wall can feel unpleasant! You can’t put fuel into the tank as quickly as we use it when we run, as our digestive system only has a certain throughput.


***As a side note you might have guessed that pacing impacts our glycogen use, going off too fast opens the tap too much early on and risks us running out of glycogen before the end of the race, but that’s a whole other topic I’ll cover in a separate post***


So… after all this; carb loading is making sure that glycogen fuel tank is full at the start of the race! Who’s still with me at this point?! :)


Now you might have heard about a pre race pasta party, or eating a massive plate of pasta the night before. But we’ve just seen that our glycogen fuel tank is quite small, so in reality this has been shown not to work. As our body can only store so much anything else we eat over that amount will just get stored elsewhere; as body fat!


Instead think about carb loading as a little and often drip feed in over the few days before the race to make sure the fuel tank is kept topped up. Have a portion (around a quarter to a third of your plate) of carbs with every meal; most of us do this anyway! Carbs are found in higher levels in rice, pasta, potatoes, bread, cous cous etc.


Ideally you’ve been trying some night before the long run meals in you training so you know what suits your digestion and helps energy levels on the runs. The evening before the race is not the time for trying anything new! over those few days pre race also try to avoid things that are high fat (like fried foods) as these are harder to digest, also spicy foods especially if you aren’t used to them. If you are worried about an on run poo try to cut back on fibre a little in the two days before; so no high fibre foods like dates, dried fruit, bran etc.


A roasting dish or cottage pie just out of the over
The potato in cottage pie could work well as a source of carbohydrates

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