How to Fuel an Ironman: Full-Distance Triathlon Nutrition
There is an old line in long-course triathlon that the swim is a warm-up, the bike is a nutrition test, and the marathon is where you find out whether you passed. It is a cliché because it is true. Most age-groupers who blow up at 30km of the run did not lack fitness — they lacked carbohydrate, sodium, or a gut that could cope with what they threw at it. A full-distance day burns somewhere between 8,000 and 11,000 kilocalories for a typical 10-to-13-hour finisher, and your body carries perhaps 2,000 kcal of stored glycogen. The gap is enormous, and it has to be closed while you are moving.
The good news is that fuelling a full-distance race is not mysterious. It is arithmetic, rehearsal and discipline. Below is the approach I use and coach: work out the numbers, practise them in training until they are boring, then execute on the day without improvising. Learning how to fuel an Ironman properly is probably the single biggest performance gain available to most amateur athletes — bigger than a new wheelset, and considerably cheaper.
Start With the Numbers, Not the Products
Your total glycogen store — muscle plus liver — is roughly 400–500g of carbohydrate if you are well trained and properly loaded. That is around 1,600–2,000 kcal. On race day you will be working at 60–75% of your maximum aerobic capacity, burning perhaps 700–900 kcal an hour depending on body mass and pace. Do the sums and you run empty somewhere around the 2.5-hour mark unless you keep feeding.
Three numbers matter more than any product choice:
- Carb loading: 8–12g of carbohydrate per kilogram of body mass per day for the 36–48 hours before the race. For a 75kg athlete that is 600–900g a day — far more than most people actually manage. Cut fibre and fat back to make room for it.
- Race morning: 1–4g/kg, roughly three hours before the gun. Most athletes settle on about 2g/kg — 150g for a 75kg athlete — of low-fibre, familiar food. White bread and jam, rice pudding, a banana and a sports drink.
- On the move: 60–90g of carbohydrate an hour on the bike, 45–60g an hour on the run. Below 60g/h you are running a deficit that compounds all day; above 90g/h you need a genuinely trained gut.
Write those figures on your top tube. Everything else about how to fuel an Ironman is execution, and vague intentions dissolve at hour six.
How to Fuel an Ironman Bike Leg Without Ruining Your Run
The 180km bike is where the race is fuelled. Blood flow to the gut is higher on the bike than on the run, you are in a stable position, and you have five to seven hours to get the work done. Whatever you fail to take in here, you will not recover later.
Target 80g of carbohydrate an hour if you have practised it, 60–70g if you have not. The practical trick is to break the hour into blocks: 20g every 15 minutes, prompted by a beeping watch. Nobody eats 80g in one sitting. Everyone can manage a mouthful four times an hour.
The reason 90g/h is possible at all comes down to transporters. Glucose is absorbed via SGLT1, which saturates at roughly 60g an hour. Fructose takes a completely separate route through GLUT5, so a product blending glucose and fructose in something close to a 1:1 ratio lets you push total absorption towards 90–100g/h rather than leaving the surplus sitting in your stomach. Anything relying on glucose alone hits a hard ceiling at 60g an hour, no matter how determined you are.
Keep hydration and calories on separate lines where you can. A bottle of drink mix plus chewable carbohydrate gives you far more control than one super-concentrated bottle you either finish or do not. And front-load: take your full hourly target from the first 20 minutes on the bike, while your stomach is fresh and the pace is still settling.
The Run: Damage Limitation, Not Heroics
Off the bike, gut blood flow drops sharply and the mechanical jostling of running makes everything harder to keep down. Realistically you are aiming for 45–60g/h across the marathon, taken in small, frequent doses at aid stations roughly every 1.5–2km.
- Do not wait until you feel bad. Once you are hypoglycaemic and nauseous you cannot eat your way out of it — absorption takes 15–20 minutes you no longer have.
- Caffeine is real, but dose it. Around 3mg/kg, roughly 200mg for a 70kg athlete, measurably lowers perceived effort. Take it from about 15km into the run, not at the start of the bike.
- Have a plan B texture. When sweet liquid stops going down, something you can chew changes the sensory experience enough to keep the calories flowing. Plenty of athletes who cannot face another gel at 32km will still happily chew.
Sodium, Fluid and the Mistakes That Actually End Races
Sweat sodium concentration varies enormously between individuals — anywhere from about 400mg to over 1,500mg per litre. That is why blanket advice is useless. In warm conditions 500–1,000mg of sodium an hour is a sensible working range, at the higher end if you routinely finish long rides with salt crusted on your face.
For fluid, 500–750ml an hour suits most athletes, adjusted for heat and humidity. Weigh yourself before and after a three-hour training ride, add back whatever you drank, and you have your sweat rate in litres per hour. Aim to finish the day down no more than 2–3% of body mass. Drinking to the point of weight gain is genuinely dangerous — exercise-associated hyponatraemia puts more long-course athletes in the medical tent than dehydration does.
Finally, train the gut. Riding all your long sessions fasted and then attempting 90g/h on race day is the most common self-inflicted disaster in the sport. Spend the last eight weeks of your programme taking your full race-day carbohydrate rate on every long ride and every brick run. Intestinal transporters upregulate with repeated exposure, exactly like any other trainable system.
What We Use at FTP Endurance Fuel
We built our pastilles around this exact problem: a chewable, dose-controllable carbohydrate that still works when your stomach has had enough of sweet liquid. Each pastille delivers a known quantity of carbohydrate in a 1:1 glucose-to-fructose blend, so you can stack them to 80 or 90g an hour on the bike without exceeding what your gut can absorb. Counting pastilles is a great deal easier at hour seven than guessing at half a gel.
The Tart Cherry Pastilles also carry Montmorency tart cherry, which is rich in anthocyanins — the polyphenols repeatedly shown to reduce markers of oxidative stress and muscle damage after prolonged exercise. That matters less mid-race and rather more in the 48 hours afterwards, when you are trying to get down a flight of stairs. The Mixed Fruit Pastilles exist because flavour fatigue is real: after eight hours of the same taste, variety stops being a luxury.
We sell them in 1kg bulk packs rather than single sachets for two reasons. It works out substantially cheaper per gram of carbohydrate, which matters when a full-distance build means five months of long sessions. And it produces far less packaging waste than the equivalent stack of individual wrappers — something you appreciate when you are picking foil out of a jersey pocket at the end of a ride.
Knowing how to fuel an Ironman is not about finding a magic product. It is about knowing your numbers, rehearsing them until they are automatic, and refusing to deviate on race day. Get 80g an hour in on the bike, 50g an hour on the run, sodium matched to your own sweat rate, and you will pass more people in the final 10km than any amount of aero kit will buy you. If you want to test what chewable fuelling feels like before committing to a full pack, start with a taster pack on your next long ride — then build your race-day plan around what your gut actually tolerates.
