How to carb load for a marathon: a plain-English guide

What carb loading actually is

Carbohydrate is your body's main fuel for running. When you eat more than you need right away, some is stored in your muscles and liver as glycogen — a starchy form of energy your body can call on during the race. A marathon is 42.2 km (26.2 miles), and most runners run low on glycogen late in the race. Carb loading is simply topping those stores up before the start.

It is not a single giant bowl of pasta the night before. It is a shift over the last one to two days: more carbohydrate, less fibre and fat, and portions spread through the day so you are not uncomfortably full. It also cannot rescue a training block — think of it as the final top-up, not the engine.

How to carb load, step by step

• Start 1–2 days before the race. The last two days matter most. Don't leave it all to race eve.
• Aim for about 8–10 g of carbohydrate per kg of body weight a day. For a 70 kg runner, that is roughly 560–700 g — a lot, so spread it over meals and snacks. If that feels overwhelming, build up gradually rather than forcing it.
• Lean on low-fibre carbohydrates. White rice, white pasta, white bread, bagels, porridge, potatoes, bananas and juice are easier on the gut than wholegrain versions and big salads.
• Keep fat and fibre modest. Rich sauces, creamy dishes, beans, lentils and large raw salads can make you feel bloated and sluggish. This is not the week to try a new curry.
• Sip fluids through the day. Glycogen holds water, so you may feel a little heavier on the scales. That is normal. Drink to thirst with water or a sports drink, especially in warm weather.
• Practise it in training. Try the foods on a long run before race week, so you know what sits well.

What a carb-loading day can look like

You do not need special products. A simple day might be porridge with banana and juice at breakfast; a white bread sandwich, rice cakes and a banana at lunch; white pasta or rice with a low-fibre tomato sauce at dinner; and snacks such as toast with honey, pretzels or a sports drink. Keep the meals familiar.

If you are vegetarian or gluten-free, the same idea works with rice, potatoes, gluten-free bread or pasta, oats if you tolerate them, bananas and juice. The best carbohydrate is the one your gut already knows. For more on timing around runs, see what to eat before a run.

Common mistakes that make it feel harder

• Eating far more than usual in one sitting. Spread the extra carbohydrate across the day. Feeling stuffed is not the goal.
• Loading with too much fibre or fat. A huge wholegrain salad or creamy pasta bake can leave you bloated on race morning.
• Trying new foods. Race week is not the time for a new gel, juice or fancy energy bar. Use what worked in training.
• Forgetting fluids. Carbohydrate loading and dehydration do not mix. Sip regularly, and include a little sodium (salt) if you are a salty sweater or it is hot.
• Panicking if the scales move. Glycogen holds water. A small rise is expected and is not fat gain.

On race morning, eat the same carb breakfast you practised, 2–3 hours before the start. During the race, start taking carbohydrate early rather than waiting until you feel empty. The load is the base; the fuelling plan is what keeps you going. If you are also working out your taper, keep race week calm and boring.