Mavr Sports Nutrition Team
September 24, 2026. 14 min read.
An Ironman is a 3.8 km swim, a 180 km bike, and a 42.2 km run, and for most age-group athletes it takes 10 to 16 hours. In a study of Ironman athletes, men burned about 10,000 kcal during the race and women about 8,600 kcal, but they ate only about 3,900 and 3,100 kcal. You cannot close that gap, and you do not need to. The job of a nutrition plan is to take in enough carbohydrate, fluid, and sodium to hold your pace all day without wrecking your stomach.
This plan covers race week, the morning, each discipline, special needs bags, a sample 12-hour race, and how to practice it. The numbers come from sports nutrition research. They only work if you rehearse them.
2 days before
Target8 to 10 g carbs per kg per day
HowBigger carb portions, carb drinks, low fiber
Breakfast, 3 to 4 hours before
Target1.5 to 3 g carbs per kg, 500 to 750 ml fluid
HowFamiliar, low-fat, low-fiber meal
20 to 30 minutes before the swim
Target20 to 30 g carbs
HowOne gel with a few sips of water
Swim
TargetNothing
HowStart fueled, not hungry
Bike carbs
Target60 to 90 g per hour, up to 120 g if gut-trained
HowDrink mix, gels, and bars every 15 to 20 minutes
Bike fluid and sodium
TargetAbout 400 to 800 ml and 300 to 600 mg per hour
HowSweat rate decides; more sodium for salty sweaters and heat
Run carbs
Target45 to 75 g per hour
HowGels, sports drink, and cola at aid stations
Run fluid
TargetDrink to thirst at most stations
HowWalk the stations if you need to
| Phase | Target | How |
|---|---|---|
| 2 days before | 8 to 10 g carbs per kg per day | Bigger carb portions, carb drinks, low fiber |
| Breakfast, 3 to 4 hours before | 1.5 to 3 g carbs per kg, 500 to 750 ml fluid | Familiar, low-fat, low-fiber meal |
| 20 to 30 minutes before the swim | 20 to 30 g carbs | One gel with a few sips of water |
| Swim | Nothing | Start fueled, not hungry |
| Bike carbs | 60 to 90 g per hour, up to 120 g if gut-trained | Drink mix, gels, and bars every 15 to 20 minutes |
| Bike fluid and sodium | About 400 to 800 ml and 300 to 600 mg per hour | Sweat rate decides; more sodium for salty sweaters and heat |
| Run carbs | 45 to 75 g per hour | Gels, sports drink, and cola at aid stations |
| Run fluid | Drink to thirst at most stations | Walk the stations if you need to |
Even at Ironman effort, a large share of your energy comes from carbohydrate, and your glycogen stores run low after a few hours. Carbohydrate you eat during the race keeps blood sugar up and spares what is left. The limit is your gut. Glucose on its own absorbs at about 60 g per hour. Products that mix glucose and fructose use two different transporters, which lets you absorb and use 90 g per hour or more.
Race data backs this up. In a study of more than 200 endurance athletes, including two Ironman races, higher carbohydrate intake per hour went with faster Ironman finishing times, even though the athletes who ate the most also reported more nausea and wind. In the Ironman energy study, the men who ate more carbohydrate per kg per hour on the run finished faster. The lesson is to eat more, and to train your gut so that more does not hurt.
Treat the last two days like a marathon carb load: 8 to 10 g of carbohydrate per kg per day, low fiber, and smaller portions of fat and protein. For a 70 kg athlete that is 560 to 700 g of carbohydrate a day. Travel, bike check-in, and the athlete briefing make this harder than at home, so plan it.
Most Ironman swims start early in the morning, so breakfast is often before 4:30 am. Aim for 1.5 to 3 g of carbohydrate per kg of body weight, low in fat and fiber, 3 to 4 hours before your start. For a 70 kg athlete that is 105 to 210 g, for example two bagels with jam, a banana, and 500 ml of sports drink. Drink 500 to 750 ml over the morning and stop big drinks about 45 minutes before the swim.
If your stomach will not take that much at 4 am, eat a smaller breakfast and top up with sports drink and a banana later in the morning. Take a gel with water 20 to 30 minutes before the swim start. If you use caffeine, take about 3 mg per kg with breakfast or an hour before the start, and keep a second, smaller dose for the second half of the run.
You cannot eat during the swim, and it takes most athletes one to two hours. Everything you need for that part of the day comes from breakfast and the pre-swim gel. Swallowing lake or sea water makes some athletes queasy early on the bike, which is another reason to start the bike with drink and gels rather than solid food.
The bike is the easiest place to eat and drink. You are sitting, your stomach moves less, and you can carry a lot. Most of your race carbohydrate should go in here. A good target is 60 to 90 g per hour. Athletes who have trained their gut often use 90 to 120 g per hour from drink mixes and gels made with glucose and fructose.
60 g per hour
One 750 ml bottle of sports drink (about 45 g) plus half a bar
80 g per hour
One 750 ml bottle of drink mix with 60 g carbs plus one gel
100 g per hour
One bottle with 80 g of glucose-fructose mix plus one gel
| Target | One way to hit it every hour |
|---|---|
| 60 g per hour | One 750 ml bottle of sports drink (about 45 g) plus half a bar |
| 80 g per hour | One 750 ml bottle of drink mix with 60 g carbs plus one gel |
| 100 g per hour | One bottle with 80 g of glucose-fructose mix plus one gel |
Sodium matters more over this length of time. Aim for about 300 to 600 mg per hour from drink mix, gels, or capsules, and more if you sweat heavily or the race is hot. In a half-Ironman trial, triathletes who took salt capsules lost less body mass, kept higher blood sodium, and finished faster than a placebo group. Salt helps you hold fluid and keeps drinks tasting good, but it does not replace drinking, and it does not protect you from the dangerous low sodium that comes from drinking far more than you sweat.
Most athletes need about 400 to 800 ml of fluid per hour on the bike. A sweat rate test on a long ride at a similar temperature tells you where you sit. Weigh yourself before and after long rides: losing more than 2 to 3 percent of body weight means you drank too little, and gaining weight means you drank too much.
Most Ironman races let you leave a special needs bag partway through the bike and the run. Use it for things you cannot carry all day: refills of your drink mix, extra gels, spare salt capsules, and one food you know you will crave, such as pretzels, a small sandwich, or sweets. Do not build the core of the plan around it. Bags get delayed, and rules differ between races, so check your athlete guide.
Running shakes the stomach, and blood flow to the gut drops as you get hot and tired, so most athletes take in less on the run. Aim for 45 to 75 g of carbohydrate per hour. Ironman runs have frequent aid stations, often about every mile or two kilometers, usually with water, sports drink, gels, cola, fruit, and salty snacks. Check your athlete guide for the exact list.
This example is for a 70 kg athlete with a 1:15 swim, a 6-hour bike, and a 4:30 run, starting at 7:00 am.
3:30 to 4:00 am
Breakfast of 150 to 200 g carbs, 500 ml fluid, usual coffee
6:35 am
One gel with water
7:00 to 8:15 am
Swim
Bike hours 1 to 3
80 g carbs per hour from drink mix plus a bar or gel every 20 minutes, 500 to 700 ml fluid, about 500 mg sodium per hour
Bike hours 4 to 6
80 to 90 g carbs per hour from drink mix and gels, no solids in the last 45 minutes
Run hours 1 to 2
A gel every 30 minutes plus sports drink at stations, about 60 g per hour
Run hours 3 to 4.5
Cola and gels, 45 to 60 g per hour, salty snacks at every second station
| Time | What |
|---|---|
| 3:30 to 4:00 am | Breakfast of 150 to 200 g carbs, 500 ml fluid, usual coffee |
| 6:35 am | One gel with water |
| 7:00 to 8:15 am | Swim |
| Bike hours 1 to 3 | 80 g carbs per hour from drink mix plus a bar or gel every 20 minutes, 500 to 700 ml fluid, about 500 mg sodium per hour |
| Bike hours 4 to 6 | 80 to 90 g carbs per hour from drink mix and gels, no solids in the last 45 minutes |
| Run hours 1 to 2 | A gel every 30 minutes plus sports drink at stations, about 60 g per hour |
| Run hours 3 to 4.5 | Cola and gels, 45 to 60 g per hour, salty snacks at every second station |
That adds up to about 700 to 850 g of carbohydrate during the race, roughly 2,800 to 3,400 kcal, plus breakfast. It is far less than you burn, and it is enough to keep moving at a steady pace.
You cannot absorb 90 g of carbohydrate per hour on race day if you have only tried 40 g in training. The gut adapts: regular practice with high carbohydrate intakes increases how much you absorb and reduces stomach problems. About a third of Ironman athletes report serious gut symptoms in the race, and athletes with a history of gut problems are the most likely to get them again. Start 8 to 12 weeks out.
60 to 90 g per hour on the bike, up to 120 g if you have trained your gut with glucose-fructose products, and 45 to 75 g per hour on the run. Most athletes get the majority of their race carbohydrate on the bike.
In one study, men burned about 10,000 kcal and women about 8,600 kcal during an Ironman, while eating about 3,900 and 3,100 kcal. Your number depends on your size, speed, and time on course.
Most athletes do well with about 300 to 600 mg of sodium per hour, and heavy or salty sweaters may need more, especially in the heat. Test your sweat rate in training, and use sodium to support drinking to thirst, not to replace it.
1.5 to 3 g of carbohydrate per kg of body weight, 3 to 4 hours before the swim, low in fat and fiber: bagels or white toast with jam, a banana, rice or oats, and sports drink. Take a gel 20 to 30 minutes before the start.
Some solid food early on the bike helps many athletes with hunger and flavor fatigue. Keep it easy to chew and low in fat, eat it in the first half of the bike, and switch to gels and drinks as the effort rises and before the run.
Refills of your drink mix, extra gels, salt capsules, and one food you know you will want, such as pretzels, a small sandwich, or sweets. Treat the bag as a bonus, not the core of the plan.
General guidance for healthy adults, not medical advice. Talk to a doctor or registered dietitian if you have a health condition, take medicines or have special dietary needs. Caffeine advice is for healthy adults only. EFSA's safe limit is 200 mg in one dose and 400 mg in a day.
Mavr turns your race distance, expected splits, body weight, and sweat profile into carbs, fluid, and sodium per hour for the bike and run, and fuels your long training days on the way there.
Ask Kai, the coach in the app