Carbohydrate loading — or glycogen supercompensation — is one of the most well-studied performance strategies in endurance sport. Done correctly, it can measurably improve your marathon time. Done wrong, it leaves you bloated, heavy, and worse off than before. Here's what the science says.
What Is Carb Loading?
Carb loading is the practice of systematically increasing carbohydrate intake in the days before a race to maximise muscle glycogen stores. Glycogen is your primary fuel source at marathon pace — and you can only store about 90 minutes' worth at race intensity. The goal is to push that ceiling as high as possible before you even cross the start line.
Research shows that glycogen supercompensation can increase muscle glycogen stores by 20–40% above baseline, which translates to a meaningful delay in fatigue onset.
Who Should Carb Load?
Carb loading is scientifically validated for events lasting 90 minutes or more. This includes marathons, half marathons for slower runners, triathlons, and ultramarathons. For 5K or 10K races, carb loading provides no measurable benefit — the race ends before glycogen becomes limiting.
The Modern Protocol: 3-Day Loading
The original 7-day depletion-and-reload protocol from the 1960s has been replaced by simpler, more effective approaches. Current best practice:
- 3 days before race: Increase carbohydrate intake to 8–12g per kg of body weight per day
- 2 days before race: Continue the same high-carb intake, reduce training volume
- Day before race: 8–10g/kg, choose familiar low-fibre, low-fat carbohydrate sources
- Race morning: 1–4g/kg carbohydrates, 3–4 hours before start
For a 70kg runner, 10g/kg = 700g of carbohydrates per day. This is a significant increase from typical intake (~300–400g/day) and requires intentional meal planning.
What to Eat
The goal is high-carbohydrate, low-fibre, low-fat foods that are easy to digest and familiar to your stomach:
- White rice, white pasta, white bread (low-fibre)
- Potatoes (without the skin)
- Bananas, white rice cakes, bagels
- Sports drinks and energy gels (easy carb top-ups)
- Pancakes, waffles with honey or syrup
Avoid high-fibre foods (beans, bran, cruciferous vegetables) and high-fat meals in the final 48 hours — these slow gastric emptying and can cause GI issues on race day.
The Weight Gain Is Normal
Every gram of glycogen is stored with approximately 3g of water. Loading carbohydrates means gaining water weight — typically 1–2kg for most runners. This is normal and expected. The temporary increase on the scales does not represent fat gain, and the stored water actually helps maintain hydration during the race.
Common Mistakes
- Eating too much fat: High-fat meals slow carb absorption and cause GI distress
- New foods: Never try unfamiliar restaurants or dishes during carb loading week
- Skipping breakfast on race day: Top up glycogen with a familiar 500–800 calorie carb-rich meal 3–4 hours before start
- Over-eating on race eve: Spreading carb intake across 3 days works better than a massive pasta dinner the night before
Calculate Your Exact Targets
Use the NorthLine Carb Loading Calculator to get personalised daily gram targets based on your body weight and race distance. Then use the Race Day Nutrition Planner to build your in-race gel schedule.
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