Most athletes should aim for 60 to 90 grams of carbohydrate per hour during an Ironman. Above about 60 g per hour the carbohydrate has to come from a mix of glucose and fructose, because the gut absorbs a single sugar only so fast. Start at the lower end in training and build up: the gut adapts, and the race is not the place to find your limit.
Key facts
- A single carbohydrate source, such as glucose or maltodextrin, is used at up to about 60 g per hour.
- For events longer than about 2.5 hours, the guidance rises to about 90 g per hour, from a glucose and fructose mix.
- The hourly target doesn’t depend on body weight or training status. It depends on how long and how hard you go.
- The gut can be trained: regular practice raises how much carbohydrate you can absorb and lowers the chance of stomach problems.
- 120 g per hour has been tested in trained cyclists, but a benefit for performance hasn’t been shown yet.
How much, by how long you go
| Duration | Carbohydrate per hour | Type |
|---|---|---|
| About 1 hour | A little, or a mouth rinse | Any |
| 1 to 2 hours | Up to 30 g | Any |
| 2 to 3 hours | Up to 60 g | Glucose or maltodextrin |
| Over 2.5 hours | Up to 90 g | Glucose and fructose mix |
An Ironman lasts between 8 and 17 hours for almost everyone, so it sits firmly in the last row. A 70.3 usually does too.
Why the sugar mix matters
Glucose and fructose cross the gut wall through different transporters. One transporter saturates at around 60 g of glucose an hour. Adding fructose opens a second route, so more carbohydrate reaches the muscles and less sits in the gut. Sitting in the gut is what causes bloating, cramps and the sudden stop at the side of the road.
Most sports products that claim 90 g per hour already use a glucose and fructose blend. Check the label for both.
Train your gut like a muscle
The gastrointestinal tract adapts to what you feed it. Eating more carbohydrate regularly increases the transporters that absorb it, and practising fuelling during exercise makes a full stomach feel normal at effort. Both lower the chance of stomach trouble on race day.
- Use your long rides and long runs as fuelling rehearsals.
- Start around 40 to 60 g per hour and add about 10 g per hour every week or two.
- Use the exact products you’ll race with, at race intensity.
- Note anything that upsets your stomach, and change one thing at a time.
From grams to a race plan
Spread the hourly total into small, regular intakes. Every 20 minutes is easy to remember: at 75 g per hour that’s 25 g each time, often one gel or half a bottle of drink, depending on the product. Many athletes take more of their carbohydrate on the bike, where eating is easier than while running, then stay steady on the run with drinks and gels.
Write the plan down before race week: the product, the grams and the time for every intake. Then you only have to follow it.
Longset turns that plan into reminders on your lock screen, so the next intake shows up on time while you ride. It’s on the waitlist now.
Sources
- A step towards personalized sports nutrition: carbohydrate intake during exercise, Jeukendrup, Sports Medicine (2014)
- Nutrition and Athletic Performance: joint position statement, Thomas, Erdman and Burke, Medicine & Science in Sports & Exercise (2016)
- Training the Gut for Athletes, Jeukendrup, Sports Medicine (2017)
- Increased exogenous but unaltered endogenous carbohydrate oxidation with combined fructose-maltodextrin ingested at 120 g/h versus 90 g/h, Podlogar and colleagues, European Journal of Applied Physiology (2022)
FAQ
Should heavier athletes eat more carbohydrate per hour?
No. The limit is how fast your gut can absorb carbohydrate, not your size, so the hourly guidance is the same regardless of body weight or training status.
Do gels, drinks or bars work better?
All three work. Liquids, gels and solid food can all supply the carbohydrate; what matters is the total per hour, the glucose and fructose mix, and what your stomach tolerates at race intensity.
Is 120 grams per hour better than 90?
Not proven. In trained cyclists, 120 g per hour raised how much ingested carbohydrate was burned, but it didn't spare any more of the body's own stores than 90 g, and performance effects haven't been shown yet.
How early should I start practising?
Months before race day. The gut adapts to regular high carbohydrate intake, so build the amount gradually in your long rides and runs rather than trying the full race plan for the first time on the day.