[CTA TEST] Carb Loading Before Marathon: The Ultimate Guide

Q: How many days before a marathon should you start carb loading?

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Q: How many carbs should you eat per day to carb load for a marathon?

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Q: What foods are best for marathon carb loading?

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Q: Why do you gain weight when carb loading?

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Q: Can you carb load too much?

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How to top up glycogen in the 2-3 days before race day - the protocol our nutritionists use.

May 02, 2026 5 min. read

Ella Ransley Sport Science Lead

“Most marathoners under-fuel race week. The science is clear: your storage capacity is the ceiling on what you can do at mile 20.
— Ella, Sport Science Lead”

Overview

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Carb loading is the protocol that decides whether you finish a marathon strong or fade at mile 18. It is also the part of race week most amateur runners get wrong - either eating too much for too long, or skipping it entirely and hoping race-day fuelling will cover the gap.

Done properly, carb loading adds around two hours of stored energy at race pace. This guide walks through what it is, when to start, how much to eat, and how to avoid the GI mistakes that ruin race day.

Key points

Start carb loading 2 to 3 days before the race, not earlier.
Aim for 8 to 12 grams of carbs per kilogram of body weight per day.
Use low-fibre, easy-to-digest sources: rice, pasta, white bread, bagels, sports drinks.
Increase fluid intake alongside carbs - 2.7 to 4 grams of water binds to every 1 gram of glycogen.
A 1 to 2 kg gain on the scale during carb load is normal and expected.

Recommended for this protocol

MIX+ Dual-Carb & Electrolyte Drink Mix

40g of carbs per 500ml serve. Hits daily targets without filling the stomach.

View MIX+ →

What is carb loading?

Carb loading is consuming higher-than-normal carbohydrate intake in the 2 to 3 days before an endurance event. The body stores carbs as glycogen in the muscles and liver. Glycogen is the primary fuel for racing at marathon pace, and it lasts approximately 2 hours before depletion forces you to slow.

By eating extra carbs in the days before a race, you push muscle glycogen above its baseline. The result: more available fuel, later onset of fatigue, and a much smaller chance of hitting the wall.

When should you start carb loading?

2 to 3 days before the race. Most marathoners begin Thursday lunch for a Sunday race. Earlier than 72 hours offers diminishing returns - extra carbs just convert to fat or pass through. Less than 48 hours and you don't fully top up glycogen.

The classic 7-day depletion-then-load protocols from the 1960s have been replaced by simpler 2 to 3-day approaches. Modern research shows the same glycogen storage is achievable without the depletion phase, with much less risk of arriving at the start line in poor shape.

How many carbs per day?

Aim for 8 to 12 grams of carbs per kilogram of body weight per day across the loading window. A 70 kg runner targets 560 to 840 g per day. A 60 kg runner: 480 to 720 g. A 80 kg runner: 640 to 960 g.

That sounds enormous on paper. In practice it requires steady portion sizes throughout the day rather than three giant meals - which is uncomfortable to eat and harder to digest. Spread intake across breakfast, mid-morning snack, lunch, mid-afternoon snack, dinner, and a small carb snack before bed.

"Most marathoners under-fuel race week. The science is clear: your storage capacity is the ceiling on what you can do at mile 20."
- Ella, Sport Science Lead

What foods to eat (and what to avoid)

Use low-fibre, easy-to-digest carbohydrate sources. Race week is not the time to load up on whole grains, beans, and large salads - the fibre slows gastric emptying and increases the risk of GI distress on race day.

The carb-loading staples our nutritionists recommend:

Rice (white, basmati, jasmine)
Pasta with simple tomato or olive oil sauces
White bread, bagels, English muffins
Potatoes (white, peeled)
Bananas, white-fleshed fruits
Honey, jam, maple syrup
Sports drinks and drink mix - useful for hitting daily targets without filling the stomach

Foods to avoid in the 48 hours before race day: high-fibre cereals, large salads, lentils, beans, fatty meats, cream sauces, alcohol, and anything you have not eaten before. New foods cause new reactions.

How to avoid GI issues during carb loading

The biggest risk during carb loading is gastrointestinal distress on race morning. Three rules cover most cases:

Practise the load in long-run weeks before race day. Use one or two key training weeks to dial in your carb-load approach. Race week is not the time to test new portion sizes.
Drink water alongside carbs. Glycogen storage requires fluid - 2.7 to 4 grams of water bind to every 1 gram of stored glycogen. Increase fluid intake by 500 ml to 1 L per day during the load.
Stop high-fibre foods 36 hours before race start. Switch to fully-refined carbs in the final day - white rice, white bread, white pasta. The lower the fibre, the cleaner the gut on race morning.

The water-weight gain is the point

Most runners gain 1 to 2 kg during a successful carb load. That weight is bound water held alongside the stored glycogen. It releases as you burn through glycogen during the race - so you finish lighter than you started.

If the scale doesn't move during your carb load, you probably haven't eaten enough. The weight gain is the most visible sign that the protocol is working.

Race morning takes over

Carb loading ends with your final pre-bed snack the night before. Race morning is a different protocol: a 3-hour pre-gun meal of 1.5 to 3 g of carbs per kg, sips of an isotonic drink in the 90 minutes before the start, and an optional caffeinated gel 15 minutes pre-gun for races over 3 hours.

The two protocols stack. A good carb load gives you a fuller starting tank; a good race-morning routine tops up liver glycogen and primes the gut. Neither replaces the other.

Final word

Carb loading is the highest-leverage nutrition decision of race week, and one of the few endurance interventions where the research has converged for decades. Hit 8 to 12 g per kg per day, use low-fibre sources, drink fluid alongside, accept the water-weight gain, and arrive at the start line with two extra hours of fuel in the tank.

Get this right and the rest of race day becomes about pacing and execution, not survival.

FAQ's

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

Start 2 to 3 days before the race. Earlier than that delivers diminishing returns. Most runners begin Thursday lunch for a Sunday race. Aim for 8 to 12 grams of carbohydrate per kilogram of body weight per day across this window.

How many carbs should you eat per day to carb load for a marathon?

Aim for 8 to 12 grams of carbs per kilogram of body weight, daily. A 70kg runner targets 560 to 840g per day. Use easy-to-digest sources: white rice, pasta, bread, bagels, bananas, sports drinks like MIX+, and honey. Keep fibre, fat, and protein moderate.

What foods are best for marathon carb loading?

Low-fibre, high-carb staples: white rice, pasta, white bread, bagels with jam, potatoes, bananas, and sports drinks. STYRKR MIX+ is a useful liquid carb source for hitting daily targets without filling the stomach. Avoid high-fibre options that increase race-day GI risk.

Why do you gain weight when carb loading?

Carbs are stored as glycogen, and 2.7 to 4 grams of water bind to every gram of glycogen. A successful carb load adds 1 to 2kg of water weight before race day - this is exactly what you want. The water releases as you burn glycogen during the race.

Can you carb load too much?

Yes. Eating beyond 12g per kg per day increases GI distress without further glycogen storage. Excess carbs convert to fat or simply pass through. Stick to the 8 to 12g per kg range, hit it consistently for 2 to 3 days, and add fluid to support the glycogen storage process.