Carb loading explained, how to do it properly before a race

Carb loading is the practice of eating a high-carbohydrate diet in the 24 to 48 hours before an endurance event to maximise the glycogen stored in your muscles and liver. Done properly it can increase your stored glycogen by 50 to 100% and delay the point at which you run out of fuel mid-race. Done wrong, it leaves you heavy, bloated, and no better off than if you had eaten normally.

This guide covers exactly how much to eat, when to start, what foods to use, how it changes by race distance, and the mistakes that catch most athletes out.

Does carb loading actually work?

Yes, and the science is well established. Your muscles and liver store carbohydrate as glycogen, which is your body's primary fuel at moderate to high endurance intensities. A typical trained athlete stores around 400 to 500 grams of glycogen. With a deliberate carb-loading protocol, you can push that to 700 to 900 grams.

That extra glycogen is the difference between fading in the final third of a race and finishing strong. The original research by Bergström and Hultman in the 1960s showed that a high-carbohydrate diet roughly doubled muscle glycogen content compared with a normal diet. Decades of work since has refined the protocol, but the core finding holds.

The key point: carb loading does not give you more energy in the tank you can burn faster. It gives you a bigger tank, so you reach empty later.

Who should carb load?

Carb loading only helps for events lasting longer than about 90 minutes of sustained effort. Below that, your normal glycogen stores are more than enough, and loading just adds unnecessary weight.

You should carb load for:

• Marathons and half marathons

• Triathlons at 70.3 and full IRONMAN distance

• Long road, gravel or mountain bike races and gran fondos

• Ultra runs and ultra-distance cycling

• Any race or hard event over 90 minutes

You do not need to carb load for:

• 5k and 10k runs

• Sprint and Olympic-distance triathlons

• Short, high-intensity sessions or races under 90 minutes

• Easy training days

How many carbs do you need to carb load?

The current sports-science consensus is 10 to 12 grams of carbohydrate per kilogram of body weight per day for the 24 to 48 hours before your event.

Body weightDaily carb target (10 g/kg)Daily carb target (12 g/kg)55 kg550 g660 g65 kg650 g780 g75 kg750 g900 g85 kg850 g1,020 g

These are large numbers. A 70 kg athlete targeting 10 g/kg needs 700 grams of carbohydrate in a day, which is roughly 2,800 calories from carbs alone. Hitting that takes planning, and most athletes who think they are carb loading are eating nowhere near enough.

The trick is not to eat more food on top of your normal diet. It is to shift the ratio. Cut back on fat and protein, and replace those calories with carbohydrate. You are changing what you eat, not just how much.

When should you start carb loading?

For most endurance athletes, 36 to 48 hours before the race is enough. You do not need a full week.

The old-school protocol from the 1960s included a brutal depletion phase: a long hard session followed by three days of near-zero carbohydrate to "empty" the muscles, then three days of heavy loading. It worked, but it left athletes exhausted, irritable and prone to illness right before a race. It is outdated and unnecessary.

Modern research, including work by Bussau and colleagues in 2002, showed that trained athletes can fully supercompensate their glycogen stores in as little as 24 to 36 hours of high-carb eating, with no depletion phase at all. For a Sunday race, you start loading on Friday. That is it.

The simple modern carb-loading protocol

1. Two days out: Begin shifting your meals toward carbohydrate. Aim for the 10 to 12 g/kg target. Keep training light or rest.

2. One day out: Continue the high-carb intake. Start reducing fibre to lower the risk of gut issues on race morning.

3. Race morning: Eat a familiar, carb-rich breakfast 2 to 4 hours before the start. This tops up liver glycogen, which depletes overnight.

Note that race morning fuelling is separate from carb loading. Loading fills the muscle tank over the preceding days. Breakfast tops up the liver after the overnight fast.

What to eat when carb loading

The best carb-loading foods are high in carbohydrate, low to moderate in fibre, and easy to digest. As the race gets closer, lower the fibre to reduce the chance of GI distress.

Good carb-loading foods:

• White rice and rice-based dishes

• Pasta and noodles

• White bread, bagels, English muffins

• Potatoes, sweet potatoes

• Oats and low-fibre cereals

• Bananas and ripe fruit

• Fruit juice and sports drinks

• Honey, jam, maple syrup

• Pancakes, crumpets

What to ease off as race day approaches:

• High-fibre wholegrains and bran

• Large amounts of raw vegetables and salad

• Beans and pulses

• High-fat foods, which slow digestion and crowd out carbs

• Anything new or untested

The golden rule of race-week nutrition: never eat anything on race day, or the two days before, that you have not tested in training.

Carb loading by race distance

EventCarb loading needed?Suggested protocol5k / 10kNoEat normallyHalf marathonYes, light1 day at 8 to 10 g/kgMarathonYes2 days at 10 to 12 g/kg70.3 triathlonYes2 days at 10 to 12 g/kgFull IRONMANYes2 to 3 days at 10 to 12 g/kgUltra (run or bike)Yes2 to 3 days at 10 to 12 g/kg

Why do I gain weight when carb loading?

Gaining 1 to 2 kilograms during a carb load is normal and expected. It is not fat. Every gram of glycogen your body stores binds roughly 3 grams of water. So when you store an extra few hundred grams of glycogen, you also store close to a kilogram of water alongside it.

That water is not dead weight. It is part of the system, and much of it becomes available as you burn through glycogen during the race. The scale going up before a big race is a sign the load is working, not a problem to fix.

Common carb-loading mistakes

1. Not eating enough carbohydrate. Most athletes who think they are loading hit maybe half the target. 700 to 900 grams is a lot. If you are not slightly sick of carbs, you probably are not loading.

2. Eating more fat and protein on top. Loading is about ratio, not piling extra food on. Adding a huge steak dinner the night before crowds out the carbs and slows digestion.

3. Too much fibre too close to the race. Wholegrains and vegetables are healthy, but a high-fibre gut on race morning is a recipe for mid-race toilet stops.

4. Trying new foods. Race week is not the time to experiment. Stick to what your gut already knows.

5. Starting too late. A single big pasta dinner the night before is not carb loading. The muscles need 24 to 48 hours of consistent high intake to supercompensate.

6. Confusing weight gain for fat gain. The extra kilo is water bound to glycogen. Do not panic and restrict.

Frequently asked questions

How many days before a race should I carb load? For most endurance events, 36 to 48 hours is enough. You do not need a full week, and you do not need the old depletion phase.

How many carbs should I eat to carb load? Aim for 10 to 12 grams of carbohydrate per kilogram of body weight per day. A 70 kg athlete needs roughly 700 to 840 grams of carbs daily.

Does carb loading make you gain weight? Yes, typically 1 to 2 kg, almost all of it water bound to the extra stored glycogen. It is normal and expected, not fat gain.

Should I carb load for a half marathon? A light load helps. One day at 8 to 10 g/kg is plenty for a half. A full marathon or longer warrants two days at 10 to 12 g/kg.

Can I carb load without gaining weight? Not really. The water retention is part of how glycogen is stored. If you store more glycogen, you store more water. That is the mechanism working as intended.

What should I eat the morning of the race? A familiar, carb-rich, low-fibre breakfast 2 to 4 hours before the start. This tops up liver glycogen lost overnight. Race morning fuelling is separate from the multi-day muscle load.

Is carb loading the same as eating a big pasta dinner the night before? No. A single pasta dinner is not enough time or volume to supercompensate muscle glycogen. Proper loading is a 24 to 48 hour process.

Take the guesswork out of carb loading

The hardest part of carb loading is hitting the number. 700 to 900 grams of carbohydrate over a day is more than most athletes realise, and falling short is the single most common reason a load fails.

MOVE Nutrition calculates your exact carb-loading target based on your body weight and race, tracks your intake across the loading window, and builds your daily meals around the number. It also plans your in-race carb, fluid and sodium targets per segment, so the work you put in during the load does not go to waste on the start line.

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