Ironman nutrition guide
Last updated June 2026 · ~7 min read
More long-course races fall apart from a fueling mistake than a fitness one. Run out of carbs and you "bonk"; drink wrong and your stomach shuts down. Here's how many carbs, how much sodium and how much fluid per hour — and how to turn that into a plan you can actually eat.
Carbohydrate: your main fuel
Carbs are what you run out of. Your body stores only ~90 minutes of hard effort as glycogen, so you have to take more in:
- Full Ironman: aim for 60–90 g of carbs per hour. Over 9–13 hours that's a lot — plan it, don't wing it.
- 70.3: you can push 90–120 g/hr if your gut is trained, because the effort is shorter and higher.
- Beginners: start near 60 g/hr and build tolerance in training. Multiple-transportable carbs (glucose + fructose mixes, often "2:1") let you absorb more without GI distress.
- Eat on the bike. It's far easier to digest while riding than running, so front-load calories on the bike to set up the run.
Sodium and electrolytes
You lose sodium in sweat, and sodium is what lets you actually absorb the fluid you drink. Too little causes cramping, bloating and — in extreme cases with overdrinking — dangerous hyponatremia.
- Target ~500–1,000 mg of sodium per hour, toward the higher end in heat or if you're a salty/heavy sweater (white salt marks on your kit after training).
- Get it from drink mix, salt tabs, or sodium-rich gels — and practice the amount in training, not on race day.
Fluid: drink to a plan
- Start around 500–750 ml per hour (roughly one bottle), more in heat and for larger athletes.
- Don't overdrink plain water. Flooding yourself with water dilutes your sodium and is genuinely risky. Pair fluid with electrolytes and let thirst fine-tune the plan.
- Use the bike to hydrate steadily; sip little and often on the run.
A simple race-day template
- Swim: nothing — just a gel ~10 min before the start.
- Bike: the workhorse. Hit your full carb, sodium and fluid targets here every single hour. Set a watch alarm so you never skip.
- Run: harder to digest, so lean on gels, cola and broth from aid stations. Keep sipping; walk aid stations to actually get it down.
Build your fueling numbers
Turn these per-hour targets into exact totals for your race length — total carbs, gels/servings, sodium and bottles to carry:
Frequently asked questions
How many carbs per hour should I eat during an Ironman?
60–90 g/hr for a full, up to 90–120 g/hr for a 70.3 if your gut is trained. Beginners should start near 60 g/hr and build up in training.
How much sodium do I need?
Roughly 500–1,000 mg per hour, more in heat or if you sweat heavily/saltily. Sodium drives fluid absorption and prevents cramping.
How much should I drink?
About 500–750 ml per hour as a starting point, adjusted for heat and size. Pair it with electrolytes and don't overdrink plain water.