Carb Loading Before a Marathon: How Much, When and What to Eat
Carb loading is the strategy of eating more carbohydrates than usual in the days before a long race so you start with your glycogen stores full. Done well, it helps you delay the dreaded wall at kilometer 30. Done badly, it turns into a huge plate of pasta the night before that leaves you heavy with an upset stomach. In this guide I explain when it makes sense, how many carbs you need, what to eat and a sample carb loading day.
What carb loading is and what it is for
Your body stores carbs as glycogen in your muscles and liver, and it is the main fuel when you run at race pace. Those stores last for about 90 minutes of hard effort. If the race is longer, they run out and your pace collapses.
Carb loading means filling those stores as much as possible before the race, combining more carbs in your diet with less training in those days. That way you have more fuel available and delay the moment it runs out.
When to carb load
It makes sense for races longer than 90 minutes: marathons, ultras, long trail races and, for many runners, the half marathon if they will take close to two hours or more. For a 5K or 10K you don’t need it: with normal eating and a good breakfast your stores are plenty.
If you are preparing your first marathon, it is one of the things that most affects how you reach the finish.
How many carbs you need
The usual recommendation is 10 to 12 grams of carbs per kilo of body weight per day for the 36 to 48 hours before the race. For a 70 kg person, that is 700 to 840 grams a day, quite a lot more than we usually eat.
In the past, people did a depletion phase first, with hard training and few carbs. We now know it isn’t needed: it is enough to cut back training, as in tapering, and increase carbs in the last two days.
What to eat when carb loading
What matters is for most of what you eat to be easily digested carbs and to cut back on fiber, fat and a bit of protein to make room and keep your stomach light. Good options are white rice, pasta, white bread, potatoes, ripe or peeled fruit, juices, honey, jam, low-fiber cereals and sports drinks.
Drinks and liquid foods help a lot to reach the target without feeling stuffed. And spread it over several meals through the day, instead of concentrating it all in dinner.
A sample carb loading day (70 kg)
Breakfast: a bowl of cereal with milk, a banana, two slices of toast with jam and a juice. Mid-morning: a sports drink and a bar. Lunch: a big plate of rice with some chicken and bread. Afternoon snack: a white bread sandwich with honey or jam and a juice. Dinner: a plate of pasta with tomato sauce and bread. Before bed: a yogurt with honey.
It is a reference; adjust it to your weight and what you tolerate, and test it before a long run to see how it agrees with you.
Common carb loading mistakes
The most common one is reducing it to a big pasta dinner the night before: there isn’t time to fill your stores and it leaves you heavy. The second is eating more of everything, including fat and protein, instead of changing the proportions. The third is keeping lots of fiber, salads or legumes, which can cause trouble on race day.
And don’t panic if the scale goes up 1 or 2 kilos: each gram of glycogen is stored with water, and that weight is fuel you are going to use.
Race day
Have breakfast 2 to 4 hours before, with easy carbs like white bread, jam or a banana, and keep taking carbs during the race from the start. It is all in the guide on what to eat before a race, and to know how many gels to take, use the race carb calculator.
Conclusion
Carb loading helps you start races longer than 90 minutes with full glycogen stores. Do it over the 36 to 48 hours before with 10 to 12 g of carbs per kilo per day, choose easily digested, low-fiber foods, spread them over several meals and don’t leave it all for dinner. Choose well, run better.
Frequently asked questions
How many days before a race should I carb load?
For the 36 to 48 hours before the race, that is, the two days before, while you cut back training.
How many carbs should I eat when carb loading?
10 to 12 grams per kilo of body weight per day. For a 70 kg person, 700 to 840 grams a day.
Do I need to carb load for a half marathon?
Only if you will take close to two hours or more. For shorter races, normal eating and a good breakfast are enough.
Is it normal to gain weight when carb loading?
Yes. Gaining 1 or 2 kilos is common, because glycogen is stored with water. It is fuel you will use in the race.
What should I eat for carb loading?
White rice, pasta, white bread, potatoes, ripe fruit, juices, honey, jam and sports drinks, with little fiber and fat.
