Your fuelling strategy for an Ironman event needs to be consistent, focus on your personal requirements and around what your body can cope with.
If you want to know more about general nutrition for performance this study gives the optimal nutrition for general performance.
Ideally you need to be taking onboard at least 60g of carbohydrates per hour. You can train your body to cope with more and it should be a big focus of your training.
*This nutrition plan is a critical component of our Ultimate Guide to Preparing for an Ironman.*
Why Your Nutrition is so Important
Just like a car, if you run out of fuel, you will have to stop. While the body stores in many cases in excess of 50,000 calories worth of fats, it can only store 300-500 grams of carbohydrates, and even at an easy effort, this store only lasts 3-4 hours. With the average ironman time around 13 hours, you will never make it to the finish without some fuel!
Check out our carbohydrate calculator >>
Taking on Carbohydrates for Longevity
The human body can digest and convert carbohydrates into usable energy at a rate of 60g per hour.
This means that, at an easy effort, you will be able to increase that 3-4 hour range to 9-12 hours. Still falling short for those who aren’t speed demons over the distance!
There are some studies which show that you can increase the amount of carbohydrates you can digest by using different sources- adding fructose, fruit sugars for example. These suggest that you can boost your hourly intake up to 90g. This increase will allow you to increase your range to almost 30 hours at an easy effort!
Nailing Your Ironman Hydration
Now you’ve got your carb-loading plan laminated and color-coded. Your gels are arranged by flavour profile. But what about the stuff that keeps your engine from blowing a gasket halfway through the bike leg? Yeah, I’m talking about hydration.
If your fuel plan is the gas in the tank, your hydration strategy is the radiator, the oil, and the entire cooling system. Get it wrong, and you’re not just looking at a bad race time; you’re looking at a full-on system meltdown. It’s a delicate dance, but I’m going to teach you the steps.
If you want to read more about the following tips with science based backing you can see the article by the American College of Sports Medicine.
Why Guzzling Plain Water is a Rookie Move
Picture this: you’re out there, sweating buckets, feeling virtuous as you chug bottle after bottle of plain water. Here’s the problem: when you sweat, you’re losing a heck of a lot more than just H₂O. You’re losing critical minerals—electrolytes. And the undisputed king of these electrolytes, the MVP for any endurance athlete, is sodium.
Sodium is the bouncer for your system. It manages your blood volume (which keeps your heart from working overtime), controls your body temperature, and tells your nerves and muscles how to fire properly. When you only replace the water without replacing the sodium, you’re effectively firing the bouncer and letting chaos reign.
The Big, Scary Myth of “Flushing Yourself Out”
I’ve heard this one on the circuit for years: “I’m just flushing out my system with gallons of water before the race!” Friends, this is one of the most dangerous myths in endurance sports. Let’s bust it right now.
Over-drinking plain water doesn’t “cleanse” you. It dangerously dilutes your blood, leading to a nasty condition called Exercise-Associated Hyponatremia (EAH). Think of it as “water intoxication.” Your body has so much water and so little sodium that it literally starts to short-circuit. The scariest part? The early symptoms can feel just like dehydration—dizziness, nausea, confusion. Treating one like the other can be a disaster. I once saw a guy chugging water in the med tent when he was already sloshing like a human water balloon. It was a classic case of good intentions, terrible strategy.
Check out the full symptoms of EAH HERE to make sure you are clued up.
Your Personal Sweat-o-Meter: Building a Real Plan
So, how do we fix this? We get personal. There’s no magic number for hydration because my sweat rate is different from yours. Your first mission is to figure out your own personal sweat rate. It’s way easier than it sounds:
- Strip down and weigh yourself right before a one-hour training session. Do your best to mimic race-day intensity and weather.
- During that hour, track exactly how much you drink. (Pro Tip: 1 ml of fluid = 1 gram. It makes the math easy!)
- After exactly one hour, towel off, strip down, and weigh yourself again.
- Do the math:
(Pre-workout weight - Post-workout weight) + Fluid consumed = Your Hourly Sweat Rate.
Boom. That number is your baseline. Your goal isn’t to replace 100% of it during the race—that’s a fool’s errand. You’re aiming to drink just enough to prevent losing more than 2-3% of your body weight. For sodium, a good starting point for most athletes is 500-1000 mg per hour. Check the label on your sports drink and have a plan to supplement with salt tabs if you’re a heavy or salty sweater.
My Big Tip: The bike is your all-you-can-drink buffet. It is infinitely easier to carry, open, and sip fluids on the bike than it is while you’re trying not to trip over your own feet on the run. Get ahead of your hydration on those two wheels.
How Carb-Loading Hydrates You
Here’s a little secret that connects your fuel and hydration plans. You know that 1-2 kg you gain during a proper carb-load? That’s not fat, my friend. That’s your secret hydration reservoir!
For every single gram of carbohydrate (glycogen) your body packs away into your muscles, it also grabs and holds onto about 3-4 grams of water. Each little glycogen granule is like a tiny, water-filled sponge.
As you start the race and begin burning through that fuel, that water is released directly inside your muscle cells—exactly where you need it, when you need it most. It’s the most brilliant and efficient hydration delivery system imaginable. So when you see that number on the scale go up pre-race, don’t panic. Smile. You’ve just topped off your internal fuel and water tanks.ost—inside your working muscles. This is your body’s own internal, perfectly-timed hydration delivery system, helping to keep your muscle cells hydrated from the inside out. So, embrace that pre-race weight gain; it means you’re locked, loaded, and hydrated for a strong performance.
The Risks of Heavy Carbohydrate Use
While exercising your blood tends to redivert away from your gastrointestinal system and to your muscles. This reduced blood flow to your digestive system reduces the speed of break down. In some people, this leads to stomach problems. Mix that with a mild bit of dehydration and things become not. So. pretty…
It is the biggest deal of stress to a triathlete, toilet troubles. No one wants to be caught short.
Your body is SMART however, and as long as you train it to deal with increases in stomach content while exercising, it will learn to deal with that content. It will even adapt to take on more than normal if you push it to. Don’t just train your cardio system, train your guts!
Understanding Carb-Loading Weight Gain and Water Release
You’ve probably noticed that after a few days of carb-loading, the number on the scale goes up by a couple of pounds. Don’t panic! This is not fat gain; it’s a clear sign your fueling strategy is working. For every gram of glycogen your muscles store, they also hang onto about 3-4 grams of water. This temporary ‘water weight’ means you are fully hydrated and your fuel tanks are topped off for race day.
So, what happens to this water during the race? As your body breaks down glycogen for energy, this intracellular water is released, becoming an internal source of hydration for your working muscles. Rather than being a negative, this process is a physiological advantage. It’s a built-in mechanism that helps you combat dehydration from the inside out. Understanding this can help you correctly interpret weight changes during your race and trust that your carb-loading has given you both the fuel and the fluid you need to perform. For more on the science behind carb-loading, leading sports nutrition organizations like https://www.sportsnutritionsociety.org/ provide extensive resources.
Getting Your Body to Take on More Carbohydrates
It’s darn expensive, but you need to take fuel on board consistently when you are training. Take on 20g every 20 minutes during sessions of 90 minutes or more. This will have your body get used to the digestion process while under exertion pressure.
After a few weeks of training, you should be able to cope with the intake without the problem of stomach issues. If you are still having problems, it might be worth changing the type of fuel you are taking. This could be as simple as swapping from one brand of gel to another, or from a gel to a bar.
Once you have gotten comfortable with your intake, gradually start taking on more. You can do this by increasing the amount you take on per 20 minutes to 25g or taking on 20g every 15 minutes instead of 20. Personally I would go for the option of 25g every 20 minutes so you are not having to dig into your pockets so often. If you have the choice that is!
