Most athletes carb load wrong. The pasta dinner the night before a marathon or Ironman is not carb loading — it is one meal that partially refills glycogen you have already been burning through race week travel, nerves, and short shakeout runs. Real carb loading takes three days and requires numbers most athletes find uncomfortable.

Done correctly, carb loading can add 2–3% to your performance and effectively push back the wall by 30–40 minutes in a marathon. Here is exactly how to do it.

What carb loading actually does

Your muscles and liver store glycogen — the fast-burning fuel that powers moderate-to-high intensity effort. At baseline, you hold about 400–600 grams. With full carb loading, you can push that to 700–900 grams. Each gram of glycogen is stored with roughly 3 grams of water, which is why you feel heavier and fuller after a proper carb load. That water is not dead weight — it contributes to hydration during the race and is metabolized alongside the glycogen.

The extra glycogen does not make you faster. It makes you last longer at the pace you have trained for. The wall is a depletion event; carb loading delays it.

Who should carb load

Carb loading is beneficial for events lasting more than 90 minutes at sustained moderate-to-high intensity. That means marathons, 70.3, Ironman, and ultra marathons all qualify. Half marathons in the 1.5–2 hour range benefit from optimized pre-race fueling but do not require a three-day protocol. If you are racing a 5K, carb loading is irrelevant.

The 3-day protocol

Day 3 before the race

Target 7–8 grams of carbohydrate per kilogram of body weight. For a 70 kg athlete, that is 490–560 grams of carbohydrate across the day. Reduce training to a very easy 20–30 minute run or rest completely. The idea is to stop depleting glycogen while starting to load it.

Good food choices: white rice, pasta, bread, bananas, oats, fruit juice, sports drinks. Keep fat low (fat slows gastric emptying and takes up calorie space you need for carbs), keep fibre low (you do not want GI trouble on race day), and avoid anything new.

Day 2 before the race

Push to 10–12 grams of carbohydrate per kilogram of body weight. This is the hardest day of the load — you are eating significantly more than your normal intake and you will feel full. For the 70 kg athlete, that is 700–840 grams of carbohydrate. Spread it across 4–5 meals and include liquid carbohydrates (fruit juice, sports drinks) to make volume easier to manage.

Training on day 2 should be minimal — a short shakeout at most. Your job is to eat and rest.

Race eve

Drop back to 8–10 grams per kilogram. Keep it familiar: whatever works for you. White pasta with minimal sauce, white rice with chicken, plain bread, banana. No fibre, no spice, no alcohol, no new restaurants. Eat your main meal by 6 pm so you have 12 hours to digest before a 6 am race start.

Avoid the temptation to over-eat at dinner. You did the work on days 2 and 3. Race eve is about maintaining, not pushing higher.

Race morning

Aim to eat 2–3 grams of carbohydrate per kilogram of body weight, 2–3 hours before the start. For the 70 kg athlete, that is 140–210 grams. White toast with honey, porridge with banana, white rice — again, familiar foods only. A small coffee is fine and benefits performance, but avoid anything with milk if your gut is sensitive.

If your race start is before 6 am, a liquid breakfast (maltodextrin powder in water, sports drink) may be easier to get down than solid food at 3:30 in the morning.

Common mistakes

Starting too late. One pasta dinner is not a carb load. Start three days out.

Adding fat with the carbs. Creamy pasta sauces, olive oil, cheese — these slow gastric emptying and compete for calorie space you need for carbohydrate. Keep meals simple.

Adding fibre. Salads, beans, lentils, whole grain bread during load days are asking for GI trouble. Stick to low-fibre sources.

Panicking at the scale. You will gain 1–3 kg during a proper carb load from glycogen and bound water. This is not fat. It will be gone by race finish, and it is working for you, not against you.

Trying something new. Race week is not the time to experiment with new foods, restaurants, or cuisines. Familiar foods only.

Timing it to your race calendar

If your race is on a Sunday, day 3 is Thursday, day 2 is Friday, and race eve is Saturday. That means Thursday through Saturday are load days. Build this into your travel plans — airport and hotel food is notoriously hard to carb load from. Bring your own supplies wherever possible.

A personalized fueling plan that calculates your specific carb load quantities based on your weight, event distance, and race start time removes the arithmetic. Athletes who hit their carb load correctly are not guessing at aid stations — they are executing a plan they already know works.