What you need to know about carb loading


Carbohydrates have long been king when it comes to race nutrition, but we can’t help but notice that their crown has slipped in recent years. Where once endurance athletes would gleefully gorge on pasta before a big race, they now approach every morsel of pasta with trepidation…“will it speed me up or slow me down?”

Good question. Before we get into the sciencey bit, let’s go right back to basics. Carb-loading (or carbo-loading) is the strategy of consuming large quantities of carbohydrate-rich food to completely saturate the body’s carbohydrate stores. The theory goes that, with increased energy stores in the muscles and liver, the athlete can keep going and going.

Now for the sciencey part…

Our bodies store carbohydrates in the liver and muscles in the form of “glycogen”. Muscle glycogen can be broken down quickly to fuel the muscles during high intensity exercise, while liver glycogen can be broken down to maintain blood glucose levels and avoid hypoglycaemia (low blood sugar), which can cause fatigue and reduced performance. So, by saturating the muscle with high levels of carbohydrates, you can go longer before hitting hypoglycaemic fatigue.

Carb-loading has been around since the mid-60s, but the science and practice has shifted a lot in recent years. The traditional Astrand Method, named after the Swedish scientist Per-Olof Astrand, focuses on the week before a race:

  • Days 1-2: Train super hard to deplete the body’s glycogen, while eating a low-carb, high protein diet (only 10% carbs). This is known as “glycogen stripping”.
  • Days 3-4: Easy workouts with low-carb, high protein diet
  • Days 5-6: More exhausting, glycogen depleting exercise, consuming high carbohydrate diet (80%+) to replenish and increase glycogen stores.


However, the Astrand Method comes with its challenges; the biggest being that you want to go into a race feeling fit and ready, not exhausted from extra training.

That’s where another carb-loading approach comes in. In 1978 research by physiologist Dr. David Costill found that trained muscles develop the ability to store 20-25% more glycogen than untrained muscles, meaning well-trained runners are primed to store more glycogen. However, he argued that carb-loading should focus on the percentage of carbohydrates you consume, rather than trying to exhaust your muscles. His method is this:

  • 4-6 days pre race: Carbs consumed make up 50-60% daily intake.
  • 1-3 days pre race: Carbs consumed make up to 70% of daily intake.


In 2002, an Australian study said you could compress all this into 24 hours. Run hard and fast the morning before your race, immediately start loading and hey presto – your muscles will reach comparable carb levels to the Costill method.

Whichever approach you choose (if any), there is one rule of carb-loading: the amount of carbs you eat needs to be directly related to your weight. It doesn’t appear that anyone really agrees on how much this should be however.

But there are sceptics, of course. Their arguments boil down to how many carbs we can store. In their view, carb-loading the week before a run assumes that we have an unlimited storage capacity for glycogen – which we don’t. Rather, our ability to store energy is relative to our size, and we can’t just eat more food and expect to be able to store more energy.

Another topic of debate is water storage. Science proves that the more glycogen you store, the more water you retain. It is estimated that every gram of glycogen stored is associated with about 2.7g of water. This means you can go into a run weighing a couple of kilos more purely because of water retention. Carb-loading champions don’t deny this, they just say it isn’t a big deal; a little bit of extra water weight never hurt anyone.

Carb-loading critics also point to the associated gut problems, even pointing the finger to carb-loading as the cause of the dreaded runners’ trots. But head of nutrition at Arsenal Football Club, James Collins, says this is because we’re doing it all wrong. Runners are gorging on bucket loads of pasta the night before a run, when what they should be doing is adding a larger portion of whole food carbohydrates. So rather than refined carbs – pasta, bread, rice and so on – they should be choosing starchy veggies (sweet potato, etc.), quinoa and buckwheat. And the secret is to eat them in the right amounts. In other words, if you feel full – stop eating.

Whether you carb load or not, there’s one thing on which everyone seems to agree: don’t dabble in anything new the week before your big event.

*The information contained in this article is general information about nutrition. Before starting any diet or change of eating habits, you should speak to your doctor or an appropriately qualified individual.

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Nicki Letts
Nicki Letts and her partner Mat are trail runners and vanlifers, travelling around in their VW Kombi "The Old Girl" and hitting the trails at every opportunity. They ran their first 100km race back in 2011 and have been enjoying the view from the middle of the pack ever since. Follow their trail adventures at www.runoldgirlrun.com.au

One thought on “What you need to know about carb loading

  1. How does anyone eat 700g of carbohydrate in one day?

    Using whole foods, it seems impossible. 3.5kg of sweet potato is a LOT of eating! Even at its simplest, eating the equivalent of almost 3/4 of a bag of sugar is… insane.

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