Carb Loading for Runners: What It Is, When to Do It, and How to Get It Right

Carb loading is one of those race prep topics that gets talked about a lot and understood poorly. Most runners have heard they should eat more carbs before a race. Fewer know when to start, how much to actually eat, or why it works. And a good chunk end up either doing nothing, or eating an uncomfortable amount of pasta the night before the race and wondering if that counts.

It does not, really. But it is a start.

Here is what carb loading actually involves, who it helps, and how to get it right before you line up for spring race season.

What carb loading actually is

Your muscles run on glycogen, which is the stored form of carbohydrate. Your body can only hold a limited amount of it. For most runners, those stores last roughly 90 minutes to two hours of sustained effort at race pace. When glycogen runs out, the body shifts to burning fat. Fat burns slower. Your pace drops. That is the wall.

Carb loading is a deliberate strategy to top up glycogen stores above their usual level before a race, so you go into the event with as much fuel as possible. Done correctly, research suggests it can improve performance in longer events by around two to three percent. That is meaningful when race times matter.

It is not about eating as much as you can. It is about shifting the composition of what you eat over a specific window so your body can store more glycogen than it usually holds at any given time.

Who actually benefits

Carb loading is backed by solid evidence for efforts lasting 90 minutes or more. That means half marathons, marathons, trail races, and anything longer. If you are racing a 5K or 10K, the strategy is unlikely to make a meaningful difference. Your glycogen stores will comfortably cover those distances without intervention.

For anyone heading into a first marathon or building toward a longer race this spring, carb loading is worth doing properly. For shorter races, a normal diet in the days before is fine. Just avoid anything heavy or unfamiliar the night before, and focus on what you eat the morning of the race instead.

How to carb load correctly

The timing matters more than most runners realise. This is a two to three day process, not a one-meal event. Starting the night before is too late for your muscles to fully load.

Aim to increase your carbohydrate intake to around eight to twelve grams per kilogram of bodyweight per day for two to three days before the race. For a 70-kilogram runner, that is roughly 560 to 840 grams of carbs per day. That is a lot, so the goal is to shift the ratio of carbs in your overall food intake rather than eating significantly more food overall.

What that looks like in practice: white rice, pasta, bread, and potatoes become your main carb sources (easier to digest than high-fibre alternatives). Fruit, particularly bananas and oranges, works well alongside meals. Reduce dietary fibre and fat in the final 24 to 48 hours, not because they are bad, but because they slow digestion and increase the risk of gut problems on race day. Drink more water than usual, because glycogen binds with water when it is stored. You will retain a bit more fluid, which is normal.

Reduce training load over this period too. Light sessions only. This is your taper window, and it works alongside the carb loading rather than against it. Easy running keeps glycogen being topped up without burning through what you have stored.

What most runners get wrong

A few common mistakes are worth knowing before you try this.

Starting too late is the most common. If you are only thinking about this on Friday night before a Sunday race, you have left it too late to fully load. Start on Thursday or even Wednesday evening for a Sunday race.

Eating too much at once is the second. Carb loading does not require you to feel uncomfortably full. Bloating and discomfort going into race day is the opposite of what you want. Spread the intake across regular meals and snacks rather than doubling the size of your dinner.

Trying new foods is the third. Race week is not the time to experiment. Stick to things you have eaten before and know your gut handles well. The goal of race day fuelling more broadly is removing variables, not introducing them.

And the fourth: assuming carb loading replaces a fuelling plan mid-race. It does not. It gives you more fuel to work with. How you use it depends on your pacing strategy and your fuelling plan during the race itself.

Race week gear

While you are sorting nutrition, it is worth finalising your kit. Race week is a bad time to try anything new. The marathon gear checklist covers the full list, but a few things worth having sorted ahead of time:

• Shoes you have already trained in (nothing new on race day)
• Socks that do not cause blisters (again, ones you know)
• A singlet or top you have run in before
• Sunglasses, if you are racing in daylight

Most Australian races start early, which means running directly into low-angle morning sun. It is worth having sunglasses sorted well before race morning rather than grabbing something last minute. Adaptor lenses are a good option for variable conditions, going near-clear indoors at the start and adjusting as light increases out on course. They are built to stay in place, so you do not notice them during the race. If you are not sure which lens suits your race conditions, the lens guide breaks it down by light level and race environment.

What to expect on race morning

You may feel slightly heavier in the days leading up to the race. That is glycogen binding with water, not anything to worry about. A well-loaded runner might weigh one to two kilograms more than usual going into race day. That weight is fuel.

Some runners feel a bit full or sluggish going into race morning. That usually settles once you warm up. Stick to a light, familiar breakfast two to three hours before the start and top up with something small closer to the gun if you need it. For more detail on pre-race morning nutrition, the article on what to eat before a run covers the timing and portions well.

The takeaway

Carb loading is not complicated, but it does require some forethought. Start two to three days out, shift your food toward familiar carb sources, stay well hydrated, and reduce fibre and fat in the final 24 hours. That is it.

If you are heading into spring race season, now is a good time to get this dialled in. The difference between a race where you manage energy well and one where you run out of fuel in the final kilometres usually comes down to preparation. Carb loading is one of the more reliable levers you have access to.

If you want to see how everything fits together before race day, the guide on getting race ready for spring covers training, logistics, and kit in one place. And if you are still working out which sunglasses to run in, the full range is here.