Carb Loading for Runners: When Does It Make Sense and How Do You Do It?

What is carb loading and why should you do it?

Carb-loading is deliberately increasing carbohydrate intake in the days before a competition, with the aim of maximally filling the glycogen stores in muscles and liver.

Glycogen is the stored form of carbohydrates in your body and the primary fuel during moderate to high intensity running. The supply is limited: an average runner has enough glycogen for 75 à 90 minutes of intensive walking. Afterwards, the available energy drops rapidly, which is accompanied by a sharp drop in pace and concentration. That's the infamous "man with the hammer" in marathon runners.

By filling the glycogen stores as much as possible before the match, you increase the buffer and postpone that moment as long as possible.

For what distances does carb loading make sense?

This is the question most runners answer incorrectly.

5 km: no carb loading necessary. With an effort of 20 à Your glycogen stores will not be depleted for 30 minutes. Eating normally the day before is completely sufficient.

10 km: marginal benefit. Most runners finish a 10 km well within 75 minutes, with glycogen depletion becoming a factor. A carbohydrate-rich dinner the day before is sufficient, no formal carb-loading protocol necessary.

Half marathon: for runners under 1h30, glycogen management is relevant. Above 1h45 it also becomes the same for recreational runners. An informal carb loading of 48 à 36 hours before the start (more carbohydrates in the meals, less fats and fibers) makes sense.

Marathon: here carb-loading is essential. Almost every serious marathon runner uses some form of carbohydrate loading in the 48 à 72 hours before the start.

How do you do carb loading correctly?

The modern protocol (48 à 72 hours before the start)

The classic carb-loading protocol from the 1970s combined an exhaustion phase (training hard with few carbohydrates seven days before the race) with a loading phase. That is outdated and unnecessarily burdensome.

The current scientifically based protocol is simpler:

72 hours before the match: start with 8 à 10 grams of carbohydrates per kilogram of body weight per day. For a 70 kg runner that is 560 à 700 grams of carbohydrates per day. That is considerably more than a normal diet, but no excuse for uncontrolled eating.

48 hours before the match: same approach. Greatly reduce the training load (taper is already in progress). Less training means less combustion, which promotes glycogen accumulation.

24 hours before the race: Carbohydrate-rich meals, but avoid excessive portions in the evening. Your stomach needs rest. A heavy meal the night before a marathon is a classic mistake.

What do you eat during carb-loading?

Focus on easily digestible, carbohydrate-rich food with little fiber, fats and proteins in large quantities:

• White pasta, white rice, potatoes (without heavy sauces)
• White bread, rice waffles, bagels
• Bananas, grapes, fruit juice
• Oatmeal, cornflakes, rice porridge
• Sports drinks and energy bars as a supplement

Avoid during the loading phase:

• Whole grain products and legumes (lots of fiber, increased gas risk)
• Heavy, high-fat meals that take a long time in the stomach
• New or unknown products
• Large portions of red meat
• Excessive alcohol (see alcohol and running)

Weight gain during carb loading: normal and good

A common complaint: "I am heavier after carb-loading." That's right, and it's no problem.

Each gram of glycogen binds two à three grams of water. A complete glycogen accumulation is associated with a weight gain of één à two kilograms. That sounds like extra ballast, but the water is functional: it contributes to moisture balance during the race and reduces the risk of dehydration.

Those who have completely filled their glycogen stores just before a marathon perform better than those who are "lighter" is at the start but experiences glycogen depletion halfway through.

The night before the match: what do you eat?

The evening meal before a match is not the time to carb-load. Most carbohydrates are already stored in the 48 à 72 hours before. What you eat the night before the race has little influence on your glycogen levels.

What does count: eat familiarly, easily digestible, and not too late. A plate of pasta with a light tomato sauce, rice with chicken, or potatoes with vegetables. No high-fat sauces, no exotic cuisines, no large amounts of meat.

Eat no later than two à three hours before you go to sleep if you have to get up early for the race.

Breakfast on match day

Breakfast on race day replenishes your liver glycogen stores that have decreased during the night. It's not a full carb-loading meal, but it's relevant.

Guidelines:

• Eat two à three hours before the start
• Easily digestible: white bread with jam or honey, oatmeal with banana, rice waffles
• 1 à 4 grams of carbohydrates per kilogram of body weight (for a 70 kg runner: 70 à 280 grams)
• Drink 300 à Add 500 ml of water
• No experiments: eat what you tested in training

You can read more about the complete raceday nutrition on the raceday page.

Frequently asked questions

In summary

Carb-loading has been proven to be useful in races longer than 75à 90 minutes. Start 48 à 72 hours before the start, increase carbohydrates to 8à 10 grams per kilogram per day, choose easily digestible products, and eat a familiar and not too heavy meal the night before the competition.

→ What do you eat on the morning of the race itself? Read the raceday guide.
→ Everything about nutrition related to training in general: the nutrition hub for runners.