Ironman Nutrition: What Changes When You Step Up From 70.3

Ironman nutrition is not 70.3 nutrition with more hours attached. Carbohydrate targets climb from around 60g per hour to 90g or more, liquid-only fuelling stops being tolerable somewhere in the middle of the bike leg, and a small hourly shortfall you never noticed over five hours compounds into a walk over eleven or thirteen. The change that matters most is timing: the athletes who step up well have their fuelling sorted before week one of the build, not during it.

I have had a lot of conversations with triathletes who race 70.3 beautifully, have a full distance quietly sitting on their list, and assume the nutrition side will carry over. Some of it does. A surprising amount of it does not, and the parts that do not tend to reveal themselves at about the 30 kilometre mark of an Ironman marathon, which is the worst possible place to find out.

This is not about your first triathlon. This is about the step up, and specifically about what stops working when the race roughly doubles.

Why Does Ironman Nutrition Break a Plan That Worked at 70.3?

Because five hours is not long enough to expose a small deficit.

Say you are taking in 60g of carbohydrate an hour on the bike and your body could comfortably use 70g. Over a half distance race that gap is a rounding error. You finish, you feel alright, you conclude your fuelling is sorted. Over a full, that same 10g an hour becomes roughly 110g of missing carbohydrate across the day, which is about four gels you simply never ate.

Your muscles hold a finite amount of glycogen (the storage form of carbohydrate, essentially your petrol tank). At 70.3 you can run partly on what is already in the tank. At full distance you cannot. You are running almost entirely on what you put in during the race, which means the hourly number stops being a nice-to-have and becomes the thing the day is built on.

The plan did not fail. The plan was always slightly short. The distance just gave the shortfall enough time to matter.

How Much Carbohydrate Do You Actually Need Per Hour?

Contemporary guidance for prolonged endurance events sits at 60 to 90g of carbohydrate per hour, with the upper end only accessible if you are using multiple transportable carbohydrates (a glucose and fructose blend, which uses two separate absorption pathways rather than competing for one) and only if your gut has been trained for it (Podlogar & Wallis, 2022, Sports Medicine). Some athletes now push to 120g an hour, though that is the pointy end and not where a first-timer should start.

Two things worth understanding about that range:

• If your race is under about 2.5 hours, then 60g an hour is usually plenty. Glucose-only fuel will get you there.
• If your race is eight hours or longer, then you need to be in the 90g range and you need a glucose-fructose blend to absorb it. A single-source drink physically cannot deliver that much, because the transporter that carries glucose across the gut wall saturates at around 60g an hour.

Daily carbohydrate intake shifts too. Podlogar and Wallis put the range for athletes preparing for demanding endurance work at 7 to 12g per kilogram of body mass a day. For a 70kg triathlete that is 490 to 840g of carbohydrate daily, which is a genuinely different way of eating from what most age-groupers do at half distance. Nobody stumbles into that by accident.

And the run is its own problem. Most 70.3 athletes are getting away with 30 to 40g an hour on the run and finishing fine. Over a marathon at the end of a long day, that number needs to climb, on a gut that is already unhappy and legs that have stopped negotiating.

Why Do You Need Solid Food on an Ironman Bike Leg?

Because palatability collapses long before your energy needs do.

Gels and sports drink work well for a few hours. Somewhere around hour four or five, most athletes hit flavour fatigue, where the thing that tasted fine at the start becomes actively repellent. You are not physically incapable of eating it. You just cannot make yourself, and an athlete who cannot make themselves eat is an athlete who is about to fall into a hole.

This is why full distance athletes move toward real food on the bike: sandwiches, bars, lollies, anything with a different texture and a savoury or neutral flavour to break up the sweetness. It also does something useful mechanically, because chewing and swallowing a solid feels more like eating and less like another chore.

The trap is switching over too fast. Solid food is harder work for a gut that is being jostled around on a bike for five or six hours, and it needs a run-up. Which brings us to the part almost everyone underestimates.

How Long Does It Take to Train Your Gut for Ironman Nutrition?

Longer than you think, and it is the single most skippable-looking step that is not actually skippable.

The good news is that the gut adapts. Gastric emptying (how fast your stomach clears what you put in it) and stomach comfort can both be trained, and repeated exposure to higher carbohydrate loads increases the number of transport proteins available to absorb them (Jeukendrup, 2017, Sports Medicine). Your gut is trainable tissue, the same as your legs.

The less good news is that gastrointestinal problems are extremely common in endurance events, and the disturbance is real rather than imagined. A systematic review of exercise-induced gastrointestinal syndrome found that strenuous exercise measurably compromises gut integrity and function, with heat and dehydration making it worse (Costa, Snipe, Kitic & Gibson, 2017, Alimentary Pharmacology & Therapeutics). You cannot think your way past this one.

Practically, gut training means using your longest session of the week to rehearse race intake, and building the volume gradually over months rather than weeks. If you are 12 weeks out and have never taken more than 60g an hour, you have a project on your hands, on top of the biggest training block of your life.

The other cost nobody warns you about is flavour fatigue in training. Rehearse with the same gel for months and you can burn out on it before you ever get to the start line (annoying).

What Should You Fix Before the Build Starts, Not During It?

This is the whole argument, so I will be blunt about it.

An Ironman program is 16 to 20 weeks of the hardest training you have ever done, usually stacked on top of full time work. It is the worst possible window in which to learn a new skill. Yet that is exactly when most athletes start googling how much carbohydrate they need, because the race entry is what made it feel urgent.

Do it the other way around. Get your numbers, your gut and your daily eating sorted while training is still manageable, so that when the volume climbs the only new thing you are learning is the training.

Julie Whitton, who I spoke to on Episode 257 of the podcast, is a good example of this done properly. She is a paramedic with 21 years of shift work behind her and a nine day roster that changes constantly, and she raced 70.3 for a decade before entering a full. Three years ago she told me on Episode 119 that an Ironman was not the right thing for her at that point in her life. She spent the time in between building her nutrition foundations quietly in the background. Now, 13 weeks out from Ironman Western Australia, she is refining numbers rather than starting from scratch, which is the entire difference.

Her bike fuelling has moved from 70 to 80g an hour. Her run target has gone from around 40g to 60g. Those are her numbers, arrived at over years, not a template. Yours will be different, because this stuff is highly individualised and cookie-cutter plans are what get people into trouble at this distance.

FAQ

Q: Can I just double my 70.3 nutrition plan for an Ironman?
A: No. The hourly carbohydrate rate needs to increase, not just the duration, and above roughly 60g an hour you need a glucose-fructose blend rather than more of a single-source product. Palatability also changes, so a liquid-only plan that worked for five hours will usually fail somewhere in hours five to seven.

Q: How many grams of carbohydrate per hour do I need for an Ironman?
A: Most full distance athletes are targeting 80 to 90g an hour on the bike and 60g or more on the run, though the exact number depends on body mass, gut tolerance and how much gut training you have done. Start at the lower end and build. Never try a new intake rate for the first time on race day.

Q: When should I start gut training for my first Ironman?
A: Ideally before your build begins, and at minimum 12 weeks out. Gut adaptation happens over months of repeated exposure, so trying to compress it into the final block means adding a new stressor at exactly the point your training load peaks.

Q: Do I really need solid food on the bike, or can I stick to gels?
A: Physiologically you could get the carbohydrate from gels alone, but very few athletes can stomach that many hours of sweet liquid. Most full distance athletes bring in solid or savoury options on the bike to manage flavour fatigue, then return to gels and liquid on the run when chewing becomes difficult.

Q: I work shift work. Can I still fuel an Ironman properly?
A: Yes, but it takes planning rather than improvisation. Irregular hours make consistent daily carbohydrate intake the hardest part, so the fix is usually structural: preparing food in advance and matching your bigger eating days to your bigger training days rather than to the clock.

The Bottom Line

Racing a good 70.3 tells you that you can fuel a 70.3. It does not tell you that you can fuel an Ironman, and the gap between those two things is wider than almost anyone expects before they go through it.

If you are looking at a full distance race and quietly wondering whether your nutrition is up to it, that is a reasonable thing to wonder and an easy thing to fix with the right advice and enough lead time. Nutrition is the fourth leg of this sport. Sort it before the build, not during it.

The Triathlon Nutrition Academy is where age-group triathletes learn to do exactly that. Get your name on the list for the next cohort and you will be the first to know when doors open.