Most marathons aren't lost in the legs. They're lost to an empty tank around 30 km, or a gut that gives up. Both are largely preventable. Here is how to fuel the week before, the morning of, and the race itself.
Why fuelling matters
At marathon pace your muscles run mainly on carbohydrate stored as glycogen. Those stores last roughly 90–120 minutes at hard effort. Unless you top them up, you slow down sharply — the "wall". The goal is simple: start full, and keep carbohydrate coming in.
Carb loading: the 36–48 hours before
- How much: around 10–12 g of carbohydrate per kg of body weight per day for the final 36–48 hours. For a 70 kg runner that is roughly 700–840 g a day.
- What: white rice, pasta, bread, potatoes, cereal, juice, low-fibre options. Cut back on fibre and fat so the volume is manageable.
- Expect to gain 1–2 kg. That's stored glycogen and water — exactly what you want.
- Don't just eat one huge pasta dinner the night before. Spread it across the two days.
Race morning
- Eat 1–4 g of carbohydrate per kg, 2–4 hours before the start. Toast with honey, a bagel, rice or oats with banana all work.
- Keep it low in fibre, fat and protein. Only eat foods you've tested before a long run.
- Drink 5–7 ml of fluid per kg in the few hours before, then sip to thirst.
During the race: carbs per hour
- Most runners: 30–60 g of carbohydrate per hour.
- Trained guts, faster or longer races: up to 90 g per hour, using gels or drinks that mix glucose and fructose.
- Start early — around 20–30 minutes in — and take small amounts every 15–20 minutes. Waiting until you feel flat is too late.
- A standard gel is roughly 20–30 g of carbohydrate. Check yours and plan the numbers.
Fluids and sodium
Aim to limit losses, not to replace every drop. Drinking far beyond thirst risks low blood sodium, which is dangerous. Heavy or salty sweaters in hot races benefit from sodium in drinks, gels or tablets. A sweat test in training tells you where you sit.
Train your gut
The gut adapts. Practise race-day fuelling on long runs for 6–8 weeks beforehand, building up the grams per hour. Rehearse the exact gels, drinks and timings. Nothing new on race day.
After the finish
Within the first hour or two, have carbohydrate plus 20–40 g of protein and fluids with some salt. Then get back to normal meals.
Quick checklist
- 10–12 g carbs/kg/day for 36–48 hours before
- Familiar, low-fibre breakfast 2–4 hours out
- 30–90 g carbs per hour, starting early
- Drink to thirst, add sodium if you sweat heavily
- Everything rehearsed in training
Figures follow the American College of Sports Medicine and International Society of Sports Nutrition position stands. Individual needs vary with body size, pace, heat and gut tolerance.