Carbs per hour ultra trail: what UTMB 2025 teaches you about fueling
TL;DR. The official UTMB Mont-Blanc nutrition guide recommends 40 to 80 grams of carbohydrates per hour. Sports drink is your main source (30 to 60g/hour). Supplement with 1 to 2 fruit energy bars per hour (20g each). The 2:1 glucose/fructose ratio in the bar makes the upper limit safely achievable. Frequency over volume. Plan by time, not by feeling.
The UTMB guideline is clear: 40 to 80 grams of carbohydrates per hour during ultra trail. Portion size and digestibility determine whether you reach that zone. I use a 25g fruit energy bar, good for 20g carbs, integrated into a fueling strategy that you can read about in detail in the complete guide to energy bars. Below is how you actually achieve that zone on the mountain.
What does the UTMB nutrition guide say about carbs per hour?
The official UTMB Mont-Blanc nutrition guide recommends 250 to 300 kcal per hour, including 40 to 80 grams of carbohydrates. That's not a marketing claim. That's the practical translation of what your gut can absorb and process during several hours of exertion. The upper limit of 80g per hour is reserved for trained athletes with accustomed guts and a 2:1 glucose/fructose ratio in their nutrition.
For most ultra trail runners, even on the 55 km OCC or the 170 km UTMB itself, the realistic optimum is between 50 and 70 grams per hour. Lower than 40g/hour, you'll run out of energy. Higher than 80g/hour without preparation, you'll experience stomach issues. Stomach issues are one of the most cited reasons for quitting ultra trail races. Fueling determines the finish.
Why is digestibility different in ultra trail than in cycling?
In cycling, you sit. In ultra trail, you run. That difference is more fundamental than it sounds. Running motion compresses your stomach and intestines differently than cycling. Blood flow to your intestines decreases faster at high intensity for runners than for cyclists. The result: your stomach tolerates less volume per intake.
A cyclist can drink 500 ml of isotonic sports drink in ten minutes on a flat section without a problem. An ultra trail runner cannot, especially not on descents or technical terrain. The mechanical shock of each step affects how quickly your stomach empties. Liquid calories from gels and drinks are quickly available, but also quickly problematic at high volumes. Solid food slows gastric emptying but provides more control over portions.
The practical conclusion: small, frequent portions of solid food perform better in ultra trail than large, incidental calorie bombs. That's why portion size is at least as important as the total grams per hour.
How many carbs does a fruit energy bar provide?
A fruit energy bar weighs 25 grams and provides 20 grams of carbohydrates. The carbs are in a 2:1 glucose/fructose ratio. Glucose travels through one transport channel in your gut. Fructose through another channel. Two separate routes, open simultaneously. Higher total absorption capacity.
One 20g carb bar in a 25g format fits into any running vest pocket, has no packaging issues in cold or humid conditions, and you don't need to wash it down with large amounts of water. Vegan, gluten-free, lactose-free, without artificials. Clean label is not a marketing term here; it's the absence of ingredients that disrupt your digestion when your intestines are already under pressure.
Why is the 2:1 ratio essential above 60g per hour?
Below 60g of carbs per hour, the glucose route alone is sufficient. Above 60g per hour, that route becomes saturated. Without fructose as a second route, carbohydrates accumulate in your gut. The result: too much sugar in your gut, fluid is drawn in, and you experience stomach issues.
The 2:1 glucose/fructose combination (two parts glucose, one part fructose) activates both routes simultaneously. This increases the theoretical maximum absorption from 60g to 90g per hour in trained athletes. For the UTMB zone of 40 to 80g per hour, the 2:1 ratio is not a luxury. It is the prerequisite for safely approaching the upper limit. This has been scientifically established by Currell & Jeukendrup (2008) and confirmed in recent RCTs until 2022.
A fruit energy bar naturally contains this 2:1 ratio. Workable throughout the entire UTMB zone, from a conservative 40g to an ambitious 80g per hour. Read when to use a bar and when to use a gel if you want to understand the difference clearly.
How do you create your fueling plan?
A UTMB plan must work regardless of motivation, feeling, or hunger. At hour 18, your feelings are not a reliable meter for calorie needs. Plan by time, not by sensation. Sports drink forms your base, fruit energy bars are the solid supplement, aid stations provide variety and salt.
Concrete combinations to achieve every goal within the UTMB zone:
| Goal per hour | Sports drink (6-8%) | Fruit energy bars Eagle | Total |
|---|---|---|---|
| 50g (entry) | 500 ml (≈30g) | 1 bar (20g) | 50g |
| 60g (optimal) | 600 ml (≈40g) | 1 bar (20g) | 60g |
| 70g (ambitious) | 500 ml (≈30g) | 2 bars (40g) | 70g |
| 80g (upper limit) | 600 ml (≈40g) + gel finale | 2 bars (40g) | 80g |
Frequency over volume. Distribute your intake throughout the entire ride. One bar every 30 to 40 minutes, sports drink in sips, gel only when intensity truly peaks. At aid stations, replenish with soup, rice, or fruit for salt and variety, but keep your carbs monitored via bars and drinks. On cold passages and night sections, solid food provides more comfort than gels.
Gel versus fruit energy bar during ultra trail?
Gels deliver carbs quickly. That's their strength at high intensity, short duration, and controlled conditions. In ultra trail, the disadvantages outweigh the benefits as the hours tick by. A standard gel contains 20 to 25g of carbs in liquid form. Without extra water, a gel puts pressure on your gut. With water, you have to manage timing and volume, adding one more variable.
Above hour six, ultra trail runners consistently report higher tolerance for solid food than for gels. Chewing itself stimulates saliva and signals your digestive system that food is coming. A function that liquid intake bypasses.
A 25g fruit energy bar requires chewing. This is functional, not accidental. The solid texture slows down the intake itself, which automatically regulates dosage. No risk of unintentionally consuming too much too quickly, a real risk with gels when your concentration drops. Read why endurance athletes swap gels for clean label fruit energy bars if you want to delve deeper into this.
How do you recognize when your carb intake is too low?
Empty sugar reserves during ultra trail do not always manifest as a classic "bonk". At low to moderate intensity over very long durations, the signal is more diffuse. Look for these signs:
- Gradual drop in your running pace without increased perceived effort
- Loss of concentration and difficulty making decisions (navigation, technical terrain)
- Increased cold sensitivity, even with adequate clothing
- Nausea without clear cause (not overconsumption)
- Muscle cramps due to a combination of depleted glycogen stores and salt loss
If you experience one or more of these signals: immediately consume 20 to 40g of carbs and shorten your interval to every 15 minutes for the coming hour. Two fruit energy bars provide 40g of carbs in a manageable, delayed dose. Supplementing with sports drink does the rest.
What a fruit energy bar is not
Not candy with a sports label. Not a replacement for sports drink. Not a miracle cure that saves a bad fueling strategy. Anyone who goes 12 hours on the mountain without a plan will not have a good day from any bar.
Rather: a functional part of a fueling strategy that works for 40 to 80g of carbs per hour. Vegan, gluten-free, lactose-free, without artificials. Clean label is not a preference in ultra trail. It is strategy. After hour ten, digestibility trumps caloric density. No gel. No brick. Fuel during exercise.
Scientific Basis
2:1 glucose/fructose mechanism. Currell & Jeukendrup (2008) showed in a controlled trial that a 2:1 glucose/fructose drink significantly improved the endurance performance of trained cyclists compared to glucose alone. Jeukendrup & Moseley (2010) substantiated the mechanism via dual transport routes in the gut. Rowe et al. (2022) confirmed in a recent RCT that a multi-transportable carb mix of up to 90g per hour improves performance during prolonged exercise, provided the gut is trained for it.
UTMB Mont-Blanc nutrition guide. The official guideline states 250 to 300 kcal per hour, of which 40 to 80g carbohydrates, for ultra trail performance.
Sources
- Currell K, Jeukendrup AE. (2008). Superior endurance performance with ingestion of multiple transportable carbohydrates. Medicine and Science in Sports and Exercise, 40(2). doi:10.1249/mss.0b013e31815adf19
- Jeukendrup AE, Moseley L. (2010). Multiple transportable carbohydrates enhance gastric emptying and fluid delivery. Scandinavian Journal of Medicine & Science in Sports, 20(1). doi:10.1111/j.1600-0838.2008.00862.x
- Rowe JT et al. (2022). Glucose and fructose hydrogel enhances running performance, exogenous carbohydrate oxidation, and gastrointestinal tolerance. Medicine and Science in Sports and Exercise, 54(1). doi:10.1249/MSS.0000000000002764
- UTMB Mont-Blanc Nutrition Guide. montblanc.utmb.world/nutrition-guide
Frequently Asked Questions
How many carbs per hour do you need during an ultra-trail race like UTMB?
The UTMB Mont-Blanc nutrition guide suggests 40 to 80g of carbohydrates per hour as a workable range. The exact need depends on intensity, duration, and your personal gut training. Those who have trained for high carb intake can reach the upper limit. Those new to fueling during exercise should start at 40g per hour and build up controlled.
Why is a 2:1 glucose/fructose ratio important for UTMB fueling?
Glucose and fructose use two separate transport channels in your gut. With a 2:1 ratio, both are utilized simultaneously, allowing you to absorb up to 90g of carbs per hour without stomach issues. With glucose alone, you'll be limited to around 60g. For a race lasting 20 to 46 hours, this extra absorption capacity is performance-critical. The fruit energy bar provides this 2:1 ratio as standard.
What is the difference between fueling for ultra trail and cycling?
In cycling, you sit stably. Your stomach is at rest. In ultra trail, the exertion constantly changes: climbing, descending, technical terrain. The mechanical stress on your gut is greater. Portion size and digestibility therefore become more crucial than in cycling. Smaller, frequent portions of 20g carbs work better than large, infrequent calorie boosts.
How do I specifically use fruit energy bars within 40 to 80g per hour?
The basis is sports drink (6-8% carbs, 500 to 800 ml per hour, good for 30 to 60g). On top of that, 1 to 2 fruit energy bars per hour (20 to 40g). For the upper limit of 80g, supplement with a gel in the final stages when intensity peaks. No more than 2 bars per hour, otherwise your stomach may become overloaded.
Why do ultra trail runners choose a clean fruit energy bar over a gel?
Gels deliver carbs quickly but often contain synthetic sweeteners and high concentrations that slow down gastric emptying at low intensity, which is exactly what ultra trail demands. A fruit energy bar is functionally just as strong in terms of its 2:1 ratio, but without artificials, vegan, and gluten-free. After hour ten, digestibility wins out over caloric density.
