Carb loading is the only dietary strategy in endurance sport whose effectiveness has been confirmed in controlled trials for over 50 years. Done right, it boosts muscle glycogen stores by 50–100% and pushes back the wall — the moment of depletion at kilometer 30–35. Done wrong, it triggers diarrhea, adds 1–2 kg of dead weight, and undoes weeks of training.
Glycogen is the storage form of glucose in muscles and the liver. While running at marathon intensity (75–85% of VO2max), the body burns mostly carbohydrates. Stored glycogen lasts about 90–120 minutes of hard work. A runner aiming for a 3:30–4:00 finish inevitably hits the point when glycogen runs out and pace collapses — that is the marathon wall.
The goal of carb loading is to reach the start line with glycogen stores fully topped off. According to the American College of Sports Medicine (ACSM), an untrained person stores roughly 80–100 mmol/kg of dry muscle mass. After a proper load, this number climbs to 180–200 mmol/kg.
The IOC Sport Nutrition Institute and ACSM publish aligned recommendations. They depend on duration of effort and athlete weight.
| Race duration | Carb target | Time frame |
|---|---|---|
| Up to 60 min (10K) | 5–7 g/kg/day | regular nutrition |
| 1–3 hours (half marathon) | 7–10 g/kg/day | 2 days before |
| Over 90 min up to 4 hours (marathon) | 10–12 g/kg/day | 1–3 days before |
| Ultra (over 4 hours) | 10–12 g/kg + on-course carbs | 2–3 days before |
Take a runner weighing 70 kg preparing for a marathon. Target: 10 × 70 = 700 g carbs per day. That is a huge number — about 2,800 calories from carbs alone, which for most people exceeds their normal intake. So fats are deliberately cut to 0.8–1 g/kg, while protein stays at 1.4–1.6 g/kg.
💡 Important: most amateurs overestimate how many carbs they actually eat. A plate of pasta is 60–80 g of carbs, not 200. Track your food for 2–3 days before loading to know your real starting point.
Swedish physiologist Jonas Bergstrom proposed a depletion-based scheme: a week before the race, an exhausting workout drains glycogen, then 3 days of low-carb intake (under 100 g/day), then 3 days of loading. The idea was supercompensation. In practice, the depletion phase weakens immunity, raises injury risk, and is psychologically brutal. Modern Cochrane and ACSM reviews do not recommend the classic version for amateurs.
No depletion phase. Gradual carb increase against a backdrop of declining training volume (taper).
| Day before race | Carbs (g/kg) | Training |
|---|---|---|
| −6 | 5–6 | long run 60–90 min |
| −5 | 5–6 | easy 40 min |
| −4 | 5–6 | rest or 30 min |
| −3 | 8 | easy 30 min |
| −2 | 10 | rest |
| −1 | 10–12 | rest or 15-min shakeout |
| Race day | 2–4 g/kg, 3–4 hours pre-race | marathon |
The day before the race: 10–12 g/kg of carbs plus a 3-minute all-out sprint that morning. The study showed a 90% glycogen rise in 24 hours. Suitable for experienced runners with good weight control. For beginners, the risk of error outweighs the benefit.
Main rule: at the loading stage you need easily digestible carbs with minimal fiber and fat. Whole grain bread and quinoa are excellent food in general, but 700 g of carbs from them means 50 g of fiber and a guaranteed bathroom collapse on race morning.
| Food | Carbs per 100 g | Fiber | Suitable for loading |
|---|---|---|---|
| White rice, cooked | 28 g | 0.4 g | ideal |
| Wheat pasta, cooked | 30 g | 2 g | ideal |
| Boiled potato | 17 g | 1.3 g | good |
| White bread | 49 g | 2.5 g | good |
| Banana | 23 g | 2.6 g | excellent |
| Honey | 82 g | 0 g | excellent (add-on) |
| Brown rice | 23 g | 1.8 g | not the best choice |
| Whole oats, cooked | 12 g | 1.7 g | only at −3 days |
| Legumes | 18–22 g | 7–8 g | avoid on −2/−1 days |
Food composition data: USDA FoodData Central.
Total: ~700 g carbs, ~95 g protein, ~30 g fat, around 3,500 calories. It sounds like a lot, but that is exactly what muscles need. Eat often and in small portions; do not try to fit this into three big meals.
Breakfast 3–4 hours before the gun: 1–4 g of carbs per kg of body weight. For a 70 kg runner that is 70–280 g. Classics: oatmeal in water, white bread with honey, banana, a strong cup of coffee (3–6 mg caffeine/kg — scientifically validated for marathon performance per ACSM).
15–30 minutes before the start — 30 g of fast carbs: a gel, sports drink, or chews. No new foods: anything you plan to consume on race day must be tested on a long run 4–6 weeks before the race.
⚠ Never try a new gel, new pasta brand, new supplement, or exotic cuisine the night before a marathon. GI distress is the cause of DNF for 30–50% of amateur runners according to finisher surveys. It is the most common — and most preventable — reason to drop out.
One serving of pasta is 60–90 g of carbs. That falls short of the 700 g target by 8–10 times. Cramming a loading day into a single meal is physiologically impossible: stomach capacity is limited. Loading is an all-day, every-2-hours pattern, ideally lasting 24–36 hours.
Each gram of stored glycogen pulls 2.7–3 grams of water with it. With a full load, weight may rise by 1.5–2 kg — that is water and glycogen, not fat. After the race, that water leaves within 24–48 hours. This is normal and desired: an extra 1.5 liters of water in the muscles helps fight dehydration on the course.
A meta-analysis by Burke and colleagues (2017, Journal of Physiology) showed that on a long-term keto diet mitochondria adapt to fat oxidation, but running economy at marathon intensity drops by 5–8%. That means 10–15 minutes lost on a marathon. For competitive results, a ketogenic approach loses to traditional carb loading.
💡 Remember: carb loading only works for races longer than 90 minutes. For 5K or 10K it is pointless — your normal glycogen reserves will not be depleted anyway.
Modern recommendations rest on consensus documents:
Bottom line: the loading effect is well documented, but execution requires precision. The biggest failures are undershooting the carb target, eating too much fiber, and experimenting with new foods on race weekend.
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