More Ironman days are ruined by fuelling than by anything else. Not by fitness, not by the course, not by the weather. By what somebody put in their stomach and when.
And the mistake that ruins the most races is not eating too little. It is eating too much.
The Two Ways to Get It Wrong
Too little. You run out of available carbohydrate. It arrives somewhere on the run as a sudden collapse in pace, an inability to think clearly, and a feeling that the wheels have come off in a single kilometre. This is the failure people are afraid of.
Too much. Your gut cannot process what you are putting into it. Nausea, bloating, burping, and eventually vomiting. This is the failure people actually experience, and it is more common among athletes who have read about fuelling than among those who have not.
The best race report I read this year is an account of the second one. The athlete, following advice about fuelling around the swim, took an entire hour's worth of carbohydrate and electrolyte before he had even mounted the bike. He spent the next four hours nauseous. He fixed it by stopping intake completely for an hour to let his system clear, then restarting at half the rate, and he finished. Most people in that position stop.
The Numbers
Current evidence supports 60 to 90 grams of carbohydrate per hour for events over two hours. Elite athletes go higher, to 120 grams and occasionally beyond, and they get there through months of deliberate gut training rather than by deciding to on race morning.
For a first full distance, aim for 60 to 80 grams per hour on the bike, and 45 to 60 on the run, where the mechanical jostling and the reduced blood flow make absorption harder.
Across a fourteen-hour day, that is somewhere between 800 and 1,000 grams of carbohydrate. It is a genuinely large amount and it is why this cannot be improvised.
Why the Mix Matters
This is the mechanism that explains most of what follows.
Glucose is absorbed through a specific transporter, and that pathway saturates at around 60 grams per hour. Take in more glucose than that and the excess sits in your gut, drawing water in, which is what produces the bloating and nausea.
Fructose uses a different transporter. Combining the two lets you absorb meaningfully more than 60 grams an hour without the surplus sitting there.
Which is why products designed for endurance use a glucose-to-fructose ratio of roughly two to one, and why taking 90 grams an hour of pure glucose will make you ill while 90 grams an hour of a two-to-one mix will not.
Gut Training Is Real
The capacity to absorb carbohydrate is trainable. The transporters in your gut wall upregulate in response to repeated exposure, the same way your legs adapt to a running programme.
So you build it. Start at 40 to 50 grams an hour on long sessions early in the block, and add roughly 10 grams a month. Give it eight to twelve weeks minimum. An athlete who has trained at 45 grams an hour all winter and attempts 90 on race day has an untrained gut being asked to double its output on the most stressful day of the year.
Practise at race intensity, not on easy rides. A stomach that copes at an easy pace tells you nothing about one working at race effort with reduced blood flow.
The Actual Day
Three days before. Increase carbohydrate at ordinary meals. Not one enormous plate of pasta the night before, which mostly produces a poor night's sleep. Gradual, across three days, at normal mealtimes.
Race morning, three hours before. Something familiar and tested. Porridge, banana, honey. Around 100 to 150 grams of carbohydrate. Nothing new, nothing high in fibre.
Fifteen minutes before the swim. A gel, if it suits you. Some people prefer nothing.
The swim. Nothing.
The first twenty minutes of the bike. This is where the report above went wrong. Do not front-load. Let your heart rate settle, let blood flow redistribute, and start fuelling at a normal rate. Not a compensatory one.
The bike. Your steady rate, 60 to 80 grams an hour, mostly liquid, on a timer rather than by feel. Set an alarm every fifteen or twenty minutes. By hour five you will not remember when you last ate.
The last hour of the bike. Ease off slightly. You want your stomach as clear as possible entering T2.
The run. 45 to 60 grams an hour, smaller and more frequent. Cola from aid stations is genuinely useful late on: carbohydrate, caffeine, and palatable when nothing else is.
Salt, Briefly
Sodium matters for fluid retention and it is heavily over-marketed. Most drink mixes contain enough. If it is hot, or you are a visibly heavy and salty sweater, add more. Loading beyond that has no benefit and contributes to the same bloating as too much sugar.
If It Goes Wrong
Know this in advance, because in the moment you will not think clearly.
If you become nauseous, stop taking anything in for thirty to sixty minutes. Small sips of water only. Ease your effort. Let your gut clear. Then restart at half your previous rate and build back up.
Most people do the opposite. They feel bad, assume they are under-fuelled, and take another gel. That is the decision that turns a difficult hour into a withdrawal.
The Bottom Line
60 to 80 grams of carbohydrate an hour on the bike, 45 to 60 on the run, glucose and fructose together, on a timer, at a rate you have trained your gut to accept over months.
Do not front-load after the swim. If you feel sick, stop eating rather than eat more. And test every single thing in training, at race intensity, because race day is the worst possible place to find out.
