Carb Loading Before a Marathon: The Right Protocol

A marathon burns carbohydrates — more than your body normally stores. That is exactly where carb loading comes in: with the right nutrition in the final two days before the race, you fill your glycogen stores beyond their usual level and push the point where energy runs out far back — or out of the race entirely. The good news: the classic six-day protocol with a depletion phase is long outdated; 36 to 48 hours are enough. Here is how the modern protocol works, what belongs on your plate and which mistakes ruin the effect.

What carb loading actually does

Glycogen is the storage form of carbohydrate: around 400 to 500 grams in the muscles plus about 100 grams in the liver are available on a normal diet. A marathon at race pace burns predominantly carbohydrate — at 70 kilograms body weight and a goal time around 3:30 hours you need roughly 2,300 to 2,500 kilocalories, 80 to 90 percent of it from glycogen and blood glucose depending on pace. Without preparation the maths does not add up: that is exactly why the infamous wall hits so many runners between kilometres 30 and 35, when stores run dry.

Carb loading exploits supercompensation: combine heavily reduced training with a high carbohydrate intake in the final days and your muscles load up to twice their normal glycogen content. Studies show performance gains of 2 to 3 percent for race durations over 90 minutes — on a 3:30 marathon that is four to six minutes, without a single extra day of hard training.

The modern protocol: 36–48 hours instead of 6 days

The classic protocol from the 1960s — three days of depletion with hard training and a low-carb diet, followed by three days of loading — works, but is needlessly brutal and error-prone. An Australian study from 2002 (Bussau and colleagues) showed that a single day with 10 grams of carbohydrate per kilogram of body weight, combined with complete rest, fills the stores to their maximum. The depletion phase is unnecessary.

The modern protocol looks like this:

• 48 to 36 hours before the start: cut training down to minimal movement and raise carbohydrate intake to 10–12 g/kg per day.
• Spread it across 6 to 8 smaller meals rather than three large ones — your stomach will thank you, and the stores fill evenly.
• Dial down fat and fibre so the total amount stays edible and your gut stays calm on race day.

For 70 kilograms of body weight that means 700 to 840 grams of carbohydrate per day — an amount that is almost impossible to reach without a plan. 100 grams of cooked pasta delivers about 30 grams of carbohydrate, a banana 23, 150 grams of cooked rice around 40. Without compact, low-fibre sources such as white bread, rice, juice and jam, it becomes very difficult.

The final week at a glance

Carb loading is the final building block of tapering: only the reduction in training volume over the last 10 to 14 days creates the conditions for muscles to store glycogen instead of burning it straight away. Anyone who keeps training at full throttle alongside it is pouring water into a leaky bucket. On the nutrition side, race week looks like this:

• Up to 4 days before the race: normal, balanced eating — about 5 to 7 g/kg per day.
• Day −3: transition — gradually increase carbohydrates, reduce fibre.
• Days −2 and −1: full protocol with 10–12 g/kg per day.
• Race day: breakfast three to four hours before the start, then gels from about kilometre 10 — the in-race fuelling plan follows the guide on fueling for the marathon.

What goes on the plate

The key shift is not “eat more” but “eat denser”: more carbohydrate per 100 grams of food, less fibre, less fat. Proven choices:

• white bread, rolls and rusks with honey or jam
• white rice, plain pasta, couscous and potatoes
• cornflakes and other refined cereals with milk or yoghurt
• bananas, dates, applesauce, fruit juices and soft drinks
• rice cakes, semolina pudding and pancakes

Avoid wholegrain products, legumes, large amounts of raw vegetables, rich sauces and very fatty meat — they deliver too much fibre or slow gastric emptying. If you struggle with a nervous stomach, cover part of the intake through drinks: one litre of apple juice provides around 110 grams of carbohydrate without filling you up.

A sample day for 70 kilograms (roughly 720 g of carbohydrate): breakfast cornflakes with milk, banana and honey (~120 g); snack white bread with jam and juice (~80 g); lunch a large plate of rice with lean chicken (~130 g); afternoon rice cakes with honey and apple juice (~90 g); dinner pasta with tomato sauce and rusks (~140 g); semolina pudding with applesauce before bed (~80 g). Plus about 1.5 litres of diluted juice spread across the day. Don't forget to drink — glycogen is stored together with water, the details are in the article on hydration for runners.

Race day: breakfast as the final load

Three to four hours before the start, eat 1 to 4 grams of carbohydrate per kilogram — at 70 kilograms that is 70 to 280 grams, depending on stomach tolerance and start time. Proven options: white bread with honey, porridge with banana, or rice with a pinch of salt. Keep fat and protein minimal, nothing unfamiliar, nothing experimental. Sip a sports drink until 60 minutes before the start, then no more large amounts. And the most important rule: everything on the race-day table must have been tested during long runs — that dress rehearsal is a fixed part of every marathon build-up.

Common carb loading mistakes

• Loading too briefly. A single carb-rich dinner the night before — the classic “pasta party” — does not fill the stores. The protocol needs 36 to 48 hours.
• Simply eating more of the same. Doubling your usual wholegrain portions lands you with a bloated belly instead of full stores. Choose dense, low-fibre sources.
• Keeping up hard training. Every tempo session in race week burns the glycogen you are trying to store. Rest is part of the protocol, not a sacrifice.
• Trying new foods. Race week is no time for experiments — neither exotic pasta nor unfamiliar gels and drinks.
• Panicking about the scales. One to two kilograms extra in the final days is normal and intended — water bound to glycogen, not fat.

Carb loading for the half marathon and shorter distances

Carb loading only pays off once glycogen becomes the limiting factor — roughly beyond 90 minutes of racing. A half marathon under 1:45 hours runs entirely on normal stores on a normal diet; the full protocol adds nothing here except unnecessary weight. From about 1:45 to 2 hours — exactly the half marathon range for many recreational runners — a reduced version is worthwhile: 7 to 10 g/kg the day before plus a carbohydrate-rich breakfast on race morning. For 10k races and shorter, carb loading is simply unnecessary; there, pacing strategy and daily form decide the outcome, not the state of your stores.

How Peakora plans your carb loading with you

The fueling calculator in Peakora thinks race week through with you: based on your estimated finish time you get a concrete plan for carbohydrates, sodium and fluids on race day — and a reminder during the taper to fill the stores in time. That turns the rule of thumb “10 to 12 grams per kilogram” into a concrete eating and fuelling plan instead of guesswork at the start line.

Frequently asked questions about carb loading

How many carbohydrates do I need for carb loading?

10 to 12 grams per kilogram of body weight per day during the final 36 to 48 hours before the race. At 70 kilograms that is 700 to 840 grams daily — only achievable through compact sources such as white bread, rice, plain pasta, bananas and fruit juice.

How long before the race should carb loading start?

36 to 48 hours is enough. Studies show that a single day with 10 g/kg of carbohydrates combined with complete rest maximises glycogen stores. The classic multi-day depletion phase is unnecessary.

Why do I gain weight during carb loading?

Every gram of glycogen binds roughly three grams of water. Gaining one to two kilograms in the final days is normal, intended and not fat — that water even serves you as a fluid reserve on race day.

Is carb loading worth it for a half marathon?

Only beyond about 90 minutes of racing. Faster half marathoners need no full protocol; from 1:45 hours a reduced version with 7–10 g/kg the day before pays off. For 10k and shorter, carb loading is unnecessary.

What should I eat on the morning of the marathon?

Three to four hours before the start, eat 1 to 4 grams of carbohydrates per kilogram of body weight — low in fat and fibre, such as white bread with honey or porridge with banana. Only eat familiar foods you have tested on long runs.