Guide
Marathon & half-marathon fuelling
Everything here is a training-tested starting point — the defaults our calculator uses, explained. Start at the lower end, test on long runs, and never try anything new on race day.
How many carbs per hour?
The honest answer depends on duration, not the event's name. A 90-minute half and a 2:25 half are different fuelling problems:
| Under 60 min — often not needed (0–20 g/h) |
| 60–120 min — 30–45 g/h |
| 120–150 min — 45–60 g/h |
| 150+ min — 60–90 g/h (90–120 only if gut-trained) |
For most marathoners that means 60 g/h as the working default, and 30–45 g/h for a typical half. 90–120 g/h is real but belongs to gut-trained athletes who have built up to it across a training block.
Why glucose + fructose beats glucose alone
Glucose (usually as maltodextrin) and fructose cross the gut wall through different transporters — SGLT1 and GLUT5. One pathway saturates around 60 g/h; using both is what unlocks higher intakes. Start with a2:1 maltodextrin:fructose mix — gentler and less sweet. The modern 1:0.8 ratio supports 90 g/h+ but earns the label "gut-trained — build up to this in training".
Sodium and fluids
Working range 400–900 mg sodium per hour — the high end for heavy sweaters and warm days. Fluids roughly 400–700 ml/h, adjusted to thirst and weather. Table salt works; sodium citrate tastes gentler at higher doses. If you have high blood pressure, kidney issues or a sodium-restricted diet, talk to a professional before adding sodium to anything.
Race-day timeline
T-48h to T-24h
Carb loading: shift meals towards high-carb, low-fibre food (rice, pasta, bread, bananas). You are topping up glycogen, not overeating — roughly 8–10 g of carbs per kg of bodyweight per day on the final day if your gut handles it.
T-3h
Last proper meal: high-carb, low-fibre, low-fat. Familiar food only. Porridge, toast and honey, rice — whatever you have trained with.
T-1h
Small carb snack if you want it: banana, white bread and jam, or a sports drink. Sip fluids.
T-30min
Optional top-up: a gel or a few sips of carb drink. If you use caffeine, this is the window — 3–6 mg/kg, tested in training, from measured doses only.
During
Start fuelling early — from 20–30 minutes in, not when you feel empty. Hit your g/h target in small regular doses. Fluids 400–700 ml/h to thirst and conditions; sodium 400–900 mg/h if you sweat heavily.
The five common mistakes
- Fuelling too late — glycogen runs down before the first gel goes in. Start in the first half hour.
- A concentration your gut has never met — a strong mix or back-to-back gels without water is how race-day nausea happens.
- New products on race day — the aid-station brand is not automatically your brand. Check what the course stocks and train with it, or carry your own.
- Copying a faster runner’s numbers — 90 g/h is a gut-trained ceiling, not a starting point. Work up from the defaults.
- Ignoring heat — hot days shift the balance towards more fluid and sodium and often less carbohydrate tolerance.
Put numbers on it
The calculator turns these defaults into a gram-accurate recipe and shows what a training block costs in gels versus DIY mix. The prompt generator gets your AI to check current gel prices in your country.
Not medical advice. If you have diabetes or glucose-control issues, kidney disease, high blood pressure, a heart-rhythm condition or relevant allergies, read the disclaimerand talk to a professional first.