Half Marathon Carb-Loading Dos & Don’ts

Should you carb up for a half marathon? We’ve got the low-down about eating before the big race

What is carb-loading?

Most people associate long distance running races with carb-loading on a big plate of pasta or rice the night before. The logic behind this seems to be filling up with easy-access energy before a long output of energy (like a half marathon), to “top up” the energy stores so you can go the distance without running out of fuel.


But does it really work, or is carb-loading a running myth we should leave behind?


The science of glycogen and energy

Here’s the science. The human body stores carbohydrates (sugars) as glycogen in muscle tissue and (to a lesser extent) in the liver. These stores of glycogen are easy to access as energy during long runs. It’s much easier for our body to use glycogen for fuel than any other form of energy, such as fat or muscle tissue.

Most of us can store a maximum of 500g glycogen in our bodies, which can fuel us for about 90 minutes of running.


So what happens if you want to run for more than 90 minutes?

Hitting the wall

If you’ve ever felt like you’ve run out of energy during a really long run, you know how it feels to hit the wall. It’s the sudden, dragging feeling of just having nothing in the tank. That’s what happens when we have burned through our glycogen stores.

In a pinch, your body can switch to using fat for fuel, but this is such an inefficient process that you will struggle to run through it.


How to carb load properly

That’s why it makes sense to carb load – but you must do it properly. Don’t just stuff yourself with pasta and potatoes the night before the race and think “job done”.


  • You can’t fill your glycogen stores in one meal, so take your focus on the pre-race dinner
  • Look at what you’re eating for 72 hours before the race
  • Focus on increased carbs and reduced fats (so you balance out your calories – if you just eat more carbs, you’ll end up overeating)
  • Get 70% of your calorie goal from carbohydrates in the final 72 hours
  • Choose easy to digest carbohydrates – this will be different for everyone – but might mean avoiding very fibrous carbohydrate choices like lentils.
  • Steer clear of junk food carb sources – these tend to be high fat anyway so you will need to avoid these to keep calories in check
  • Eat a reasonable meal the night before the race – if you’ve been gently carb-loading for 3 days, you don’t need to go crazy with this meal
  • Eat carbs with your breakfast on race day, leaving 2-3 hours between breakfast and the race

And don’t worry if you feel heavy during a carb-loading period. Carbohydrates literally pull in water, but this will soon settle back to normal. It won’t be fat gain.