Taking in 120 g of carbs an hour helped elite marathoners burn more carbs and run the same pace using less oxygen. It also gave them the worst gut symptoms, so most runners should build up to it slowly, if at all.
If you're working out how many carbs per hour to take during a marathon, this is the new study everyone is quoting. Here's what it actually found, and what our sports dietitians would do with it.
What the 120 g/h carb study did
Eight elite male marathoners ran for 2 hours near threshold on three separate days. Each day they took a different carb dose:
- 60 g/h of glucose only
- 90 g/h of glucose + fructose
- 120 g/h of glucose + fructose
The carbs were labelled with a carbon tracer (13C). That meant the researchers could see how much of the carb burned came from what the runners drank, rather than from what was already stored in their muscles.
What they found: more carbs burned, better running economy
More carbs in meant more carbs burned. Total carb use went from about 2.1 g/min at 60 g/h to about 3.1 g/min at 120 g/h, roughly 47% more.
The bigger finding was running economy. At 120 g/h, holding the same pace cost about 2.6% less oxygen than at 60 g/h. Carbs give more energy per litre of oxygen than fat does, so leaning harder on carbs makes each step slightly cheaper. Late in a marathon, that is a real edge.
Why glucose + fructose matters for marathon fuelling
Your gut absorbs glucose through one transporter (SGLT1), which tops out at about 60 g/h. Fructose uses a different transporter (GLUT5). Mixing the two opens a second door, which is why every dose above 60 g/h in this study used both. If your gels are glucose or maltodextrin only, adding more of them mostly adds gut trouble, not fuel.
The catch: gut symptoms and a small study
- Nausea, fullness and gut cramps were highest at 120 g/h.
- Only 8 runners, all elite men, so it may not apply to women, slower runners or hot conditions.
- It measured fuel use and economy, not race times. Nobody ran faster in a race because of it.
Promising, not proof.
The gut symptoms matter more than they sound. A runner who has to slow down or stop with stomach trouble loses far more time than a 2.6% economy gain gives back. More fuel only helps if your gut can handle it on the day.
How many carbs per hour should you take? What we'd do
- Runs under 2.5 hours: 30–60 g/h.
- Longer races: 60–90 g/h from glucose + fructose.
- Build up 10–15 g/h a week on your long runs.
- Only push past 90 g/h once your gut has handled 90 comfortably in training.
Your gut is trainable, but it takes reps, the same as your legs.
How to train your gut: a sample gut-training block
Use your weekly long run to build up, and rate your gut comfort from 0 (fine) to 10 (had to stop) after each one. Only move up when you're at 3 or below.
- Weeks 1–2: 60 g/h. For example, 1 gel + 500 mL sports drink.
- Weeks 3–4: 65–75 g/h. For example, 2 gels + 250 mL sports drink.
- Weeks 5–6: 80–90 g/h. For example, 2 gels + 500 mL sports drink.
- Week 7 on: 90+ g/h, if you want to go there. Add a high-carb gel or drink mix with glucose + fructose.
Spread it out over the hour, every 15–20 minutes, instead of taking it all at once. Practise at race pace too, because your gut handles fuel differently when you're working harder.
Not sure where your ceiling is, or which gels suit your gut? A sports dietitian can build a gut-training block around your race date, your products and how your stomach responds. Book a consultation with one of our Accredited Sports Dietitians in Alexandria, Sydney, and go into race day with a fuelling plan you've already tested.
Frequently asked questions
How many carbs per hour do I need for a marathon?
For most runners racing longer than about 2.5 hours, 60–90 g of carbs per hour from a glucose + fructose mix is the target. For runs and races under 2.5 hours, 30–60 g per hour is usually enough.
Is 120 g of carbs an hour safe?
It isn't dangerous for a healthy runner, but it caused the most nausea, fullness and gut cramps in the study. Only try it once you can handle 90 g per hour comfortably in training.
Can you train your gut to absorb more carbs?
Yes. Increasing your carb intake by 10–15 g per hour each week on your long runs helps your gut adapt, the same way your legs adapt to mileage.
Reference
Ravikanti, S., et al. (2025). 13C-labelled glucose–fructose show greater exogenous and whole-body CHO oxidation and lower O2 cost of running at 120 vs 60 and 90 g/h in elite male marathoners. Journal of Applied Physiology, 139(6), 1581–1595.