Carb Loading for a Marathon: How Much, When and What to Eat

Why Carb Loading Works

Your muscles and liver store carbohydrate as glycogen, which is the body's preferred fuel for sustained, moderate-to-high intensity effort. For anything under about 60–90 minutes, normal glycogen stores are usually enough. A marathon lasts considerably longer than that for most runners, and once glycogen runs low, pace inevitably drops—this is the physiological reality behind "hitting the wall."

Carb loading works by deliberately maximising muscle glycogen stores in the days before the race, so you start with more fuel in the tank before you've taken a single step. The International Society of Sports Nutrition's position stand on nutrient timing confirms that a high-carbohydrate diet in the days before endurance exercise reliably maximises glycogen stores, and that this stored fuel translates into better endurance performance in events lasting over about 90 minutes—exactly the marathon's territory.

How Much: Practical g/kg Guidance

Carb loading targets are usually given per kilogram of bodyweight, which makes them easy to scale to your own size:

• 8–10 g of carbohydrate per kg of bodyweight per day is the standard target for the loading window, per the ISSN position stand above.
• For a 65 kg runner, that's roughly 520–650 g of carbohydrate a day — noticeably more than a normal training diet.
• For a 80 kg runner, that's roughly 640–800 g of carbohydrate a day.

This is a genuine increase, not a token gesture—most runners underestimate how much food is actually required to hit these numbers and need to consciously plan meals and snacks around it rather than just "eating a bit more pasta."

3-Day vs 1-Day Loading: What the Evidence Shows

The traditional approach is a 3-day carb load in the run-up to race day, eating at the higher end of the g/kg range while training volume is already reduced by your taper. This remains a reliable, well-tested method.

A shorter 1-day protocol has also been shown to work for trained runners. A study by Bussau and colleagues found that combining a single day of high carbohydrate intake (around 10 g/kg) with rest raised muscle glycogen from roughly 95 to 180 mmol/kg within 24 hours—with no further increase from two additional loading days. In practice, this means a well-planned final day of eating can be nearly as effective as a longer 3-day approach, which is useful if you'd rather not commit to three full days of deliberately overeating.

Either approach is valid. The 3-day method gives more margin for error if one day's eating falls short; the 1-day method is more convenient and backed by direct muscle-biopsy evidence of its effectiveness.

What to Actually Eat: Realistic UK Meal Examples

"Eat more pasta" is true but unhelpful on its own—hitting 8–10 g/kg takes planning across the whole day, not just a big dinner. Some practical, UK-friendly ways to build carbohydrate-heavy meals and snacks:

• Breakfast: a large bowl of porridge with honey and banana, plus a glass of orange juice; or bagels with jam.
• Lunch: a large jacket potato or rice-based dish, plus extra bread on the side; a bagel or wrap with a carbohydrate-heavy filling.
• Dinner: pasta with a light tomato-based sauce, plus garlic bread or extra rice; keep sauces simple rather than rich and creamy.
• Snacks between meals: bananas, malt loaf, rice cakes, dried fruit, low-fibre cereal bars, pretzels, or a sports drink sipped through the day.
• Fluids: a carbohydrate-electrolyte sports drink alongside meals is an easy way to add carbohydrate without adding bulk, particularly useful on the final day when appetite can drop from nerves.

What to Avoid in the 24–48 Hours Before Racing

• Excess fibre. Wholegrain bread, bran cereals, large amounts of vegetables and pulses can all increase the risk of GI discomfort on race morning. Switch to lower-fibre carbohydrate sources (white rice, white bread, white pasta) for the final day or two.
• Excess fat. Fatty meals digest slowly and can leave you feeling heavy rather than topped up—keep meals carbohydrate-forward and comparatively low in fat.
• Fizzy drinks and excess alcohol. Both can disrupt sleep and digestion in the final stretch before a race; it's not the moment to test how your gut handles either.
• Untested new foods. Race week is not the time to try a food, brand of gel, or restaurant meal you haven't eaten in training. Stick to things your gut already knows.

Race-Morning Breakfast Timing

Eat your final pre-race meal roughly 2–4 hours before the start, giving your stomach time to mostly empty before you're running. A familiar, carbohydrate-rich, low-fibre breakfast works well — porridge, a bagel with jam or honey, or toast with banana are all reliable choices used by experienced marathoners. If your stomach is nervous on race morning, a liquid or semi-liquid option (a carbohydrate drink, or porridge made looser than usual) can sit more easily than solid food. Whatever you choose, it should be something you've already eaten before a long run in training — race morning is the last place to experiment.

Frequently Asked Questions

How many days before a marathon should I start carb loading?

Either 3 days out using a gradual increase, or a single, more concentrated day immediately before the race — both are supported by evidence. Choose whichever fits your routine and appetite better.

Will carb loading make me gain weight?

You may see a small, temporary weight increase of a kilogram or two. This is mostly water stored alongside the extra glycogen, not fat gain, and it's a sign the loading is working as intended.

Do I need to carb load for a half marathon too?

It matters far less. Half marathons typically fall within or close to the range that normal glycogen stores can cover, so the benefit of deliberate loading is smaller than for a full marathon.

What if carb loading upsets my stomach?

Favour lower-fibre carbohydrate sources (white rice, white bread, ripe bananas) over wholegrain options, spread intake across more, smaller meals rather than two or three huge ones, and make sure you've trialled your race-morning breakfast in training beforehand.

Final Thoughts

Carb loading is one of the simplest, most evidence-backed changes you can make in the final days before a marathon. Aim for roughly 8–10 g of carbohydrate per kg of bodyweight over either a 3-day or 1-day window, lean on familiar, lower-fibre foods, and pair it with a proper taper so your legs are as fresh as your glycogen stores. None of it needs to be complicated — it just needs to be planned.