Ironman nutrition is the difference between crossing the finish line strong and being reduced to walk-shuffle misery at mile twenty. A full 140.6 miles requires 8–17 hours of consistent fuelling, with every hour's nutrition decision affecting the next. This guide maps out the nutrition playbook for all three disciplines.
Most Ironman athletes can tolerate 60–90g of carbohydrate per hour on the bike. Elite athletes with highly trained guts may reach 100–120g; conservative eaters may settle at 60g. The only way to know your personal limit is to test it repeatedly in training.
The bike leg is your primary fuelling opportunity. You have aid stations, you are not breathing heavily, and your stomach is relatively stable. Do the heavy fuelling here. Do not save it for the run.
Start fuelling by km 15 of the bike (mile 10 if US metric).
Consume carbs every 20–30 minutes — do not wait until you feel hungry.
Target 60–90g per hour; adjust based on stomach tolerance and training experience.
Mix gels and sports drinks to hit carb targets without repetition fatigue.
Drink at every aid station, regardless of thirst; dehydration creeps up on long efforts.
More Ironman DNFs are caused by dehydration than by bonking. Your sweat rate varies by fitness level, body size, weather, and intensity. A small, efficient athlete in cool conditions might sweat 500 ml per hour; a larger athlete in heat might lose 2000 ml per hour.
Aim to replace 80–90% of your sweat loss. This usually translates to 400–1000 ml per hour depending on conditions. Drink steadily rather than in large gulps. A common mistake is over-hydrating, which leads to hyponatraemia (dangerously low sodium). Drink to thirst plus roughly 20% more.
Calculate your sweat rate in training: weigh yourself before and after a one-hour workout (accounting for water consumed). The difference is your sweat rate.
On the bike: aim for 400–800 ml per hour in moderate conditions.
On the run: aim for 300–600 ml per hour; reduce if your stomach is fragile.
Include electrolytes (sodium) with every drink to enhance absorption and maintain blood sodium.
Urine colour is your hydration check: pale yellow is ideal; dark yellow means you are behind; clear means you are over-hydrating.
A typical Ironman bike leg is 90 km and takes 4–6 hours for age-groupers. If your plan is 80g of carbs per hour and your bike takes 5 hours, you need to consume approximately 400g of carbohydrate across the bike leg.
This might look like: two gels at km 15 and 25 (40g), sports drink for 50g at km 35–40, two more gels at km 50 and 65 (40g), and 30g from a special needs bag at km 80. Every intake is planned, every quantity is known, and nothing is left to chance.
Map your aid station locations and fuel them into your nutrition plan.
Write down every gel, every hydration point, every nutrition cue.
Account for special needs bag: what time does it arrive, and what do you pull from it?
Have backup gels in your jersey pockets in case an aid station is short of stock.
Confirm the brand available at aid stations matches your training nutrition.
In Race Preprr, import your Ironman bike GPX, drop every aid station location, and assign your nutrition cues to each station. For a 180 km course, seeing ten nutrition cues plotted on the elevation profile removes all guesswork.
T2 is where Ironman nutrition strategy often falls apart. You have been well-fed on the bike. Your stomach is calm. Then you get off the bike and your body says no more food. This is completely normal.
Do not stop fuelling because your stomach signals resistance. The run is 5+ hours of hard work, and you cannot power it on momentum. Start eating within the first 5 km of the run, even if it feels unintuitive. Your body will adjust.
In T2, eat something simple: a small gel, a few sips of sports drink, a banana if available.
Begin run fuelling within the first 5 km — at km 2 is ideal.
Start with liquid calories if solids feel impossible: cola, sports drink, or energy chews.
Gradually shift to whatever works: gels, bars, or a mix depending on stomach tolerance as the run progresses.
The run is where Ironman gets hard. Your legs are heavy, your stomach is tired, and your brain wants to stop. You need carbs to keep going, but your tolerance for them drops significantly compared to the bike.
Most athletes aim for 30–60g per hour on the run — roughly half what they took on the bike. However, this is not a hard rule. If 30g per hour works, stick with it. If you can tolerate 60g, that is fine too. The critical rule is consistency: choose a number and stick with it, do not experiment mid-race.
First half of the run (km 0–10): aim for 30–40g per hour if possible.
Second half of the run: consider dropping to liquid calories only (cola, sports drink) if solid fuel causes nausea.
Slow your pace if fuelling becomes difficult — it is better to finish fuelled and slow than unfuelled and stopped.
Use aid stations: take water, electrolytes, and carbs at every station, not just when you think you need it.
Walking aid stations to eat is fine — it is not cheating, it is logistics.
Mark your Ironman run aid stations in Race Preprr and set your fuelling cue for each one. If the run is 42 km and aid stations are every 5 km, you have nine fuelling opportunities — plan what you take at each one.
Ironman special needs bags are allowed for the bike leg (and sometimes the run leg, depending on the event). Use them strategically for nutrition you cannot get at aid stations: your favourite bar brand, electrolyte pills, extra gels, or even dry socks if your run is chafing.
But do not over-stuff them. You get a brief window to grab the bag, find what you need, and move on. A perfectly planned special needs bag saves precious seconds and mental energy.
Include: backup gels, electrolyte tablets, your preferred bar brand.
Include practical items: blister kit, extra socks, fresh visor for the run.
Avoid: anything you have not tested in training.
Label your bag clearly so volunteers can find it quickly.
Confirm the exact km/mile where it will be available in the race guide.
You have done everything right. Your nutrition plan is flawless. And then your stomach revolts, or an aid station runs short, or you bonked at hour five and did not notice until hour seven.
Have a mental plan B: reduce your carb target to maintenance, switch to liquid calories, walk the aid stations, and focus on finishing. Ironman is long enough that slowing down is always better than stopping.
If solid gels cause nausea: switch to cola, sports drink, or energy chews.
If you are severely bonked: consume whatever carbs are at the aid station (even if not your plan) and walk if necessary.
Remember: finishing an Ironman is hard. Finishing well-fuelled is the achievable goal.
Map every gel, every hydration point, and every special needs item to your course so you can execute your nutrition strategy without thinking on race day.