An ultra trail is not a race that you fuel for once. It is a logistics issue involving fuel, digestion, and pacing that lasts 20 to 46 hours. During that time, you burn more carbs than your body stores, so the question is not whether you refuel, but how much per hour and in what form. I will explain how many carbs per hour works, why your stomach sets the true ceiling, and why format is a fueling decision. For the broader basis behind carbohydrates during exercise, I refer you to the complete guide to energy bars and cycling.

How many carbs per hour for an ultra trail?

Sports nutrition science is fairly unambiguous here. For efforts longer than two to three hours, the target zone is between 60 and 90g of carbs per hour, depending on intensity, duration, and how well-trained your gut is. Anyone who stays structurally below 40g depletes their internal sugar reserve (glycogen) faster. Anyone who goes above 90g without a trained gut risks stomach issues. And stomach issues are the most common reason why ultrarunners drop out.

The Mont-Blanc loop of the UTMB covers up to 171 kilometers and 10,000 meters of elevation gain. The fastest finishers take 20 hours, most participants 30 to 46 hours. Over such a time span, fueling is not a preference. It is an operational decision with a direct impact on your finish. King (2019) showed in a dose study that 90g of carbs per hour in a 2:1 ratio was the sweet spot for performance over three hours of cycling. More was not better: at 100g per hour, performance dropped again. Plan your race well yourself with our Fuel Planner tool.

You determine the exact number within that 60 to 90g zone yourself, based on three variables.

• Running intensity: during a technical climb or steep descent, your consumption increases. At a walking pace, it decreases.
• Running time to that point: after ten hours, your internal sugar reserve is low. The pressure on external carbs increases.
• Gut training: an untrained gut tolerates roughly 60g per hour. A trained gut can handle 80 to 90g.

Why digestibility weighs more heavily in ultra trail than in cycling

Cyclists and ultrarunners both burn carbs. But the context of their bodies differs fundamentally. On a bike, your torso remains relatively stable and blood flows quite efficiently to your digestive system, even at high intensity.

Running is different. Every step delivers a vertical impact. Your intestines move along with it. At high intensity, blood flow to your stomach and intestines decreases, because your body prefers to send that blood to your legs. Food that is not digested in time sits there. And over an effort of tens of hours, that problem compounds. The review by de Oliveira (2014) shows that 30 to 90% of endurance athletes experience stomach issues, and that carbs from a single source and overly high sugar concentrations are the main triggers.

Three things make digestibility the critical variable in ultra trail: the impact per step compresses your stomach and increases the risk of nausea, the long duration adds stress to your digestive system, and the changing intensity between climbing, descending, and flat makes a fixed absorption rate difficult to plan. That is why portion size is not a detail. It is strategy. You can read more about why many endurance athletes are swapping out gels in why runners choose a clean-label fruit energy bar.

How does the 2:1 glucose/fructose ratio work during an ultra?

Your body absorbs sugars from your gut via two separate transport channels: one for glucose and one for fructose. Each channel has a maximum capacity. Glucose alone caps out at around 60g per hour. Fructose alone is even lower. If you use both sugars in a 2:1 ratio, you open both channels at once. This allows you to extract up to 90g of carbs per hour from your gut without extra stomach issues, provided your gut is trained.

Currell and Jeukendrup (2008) showed that a 2:1 mix improved time trial performance by 8% compared to glucose alone, because you can process more external carbs and thus save your internal reserve. Rowe (2022) confirmed that effect in runners: 2:1 at 90g per hour improved running time.

I formulate every fruit performance bar with a 2:1 glucose/fructose ratio. No artificials, no synthetic sugars. The 2:1 glucose/fructose ratio is the scientifically backed standard for carbohydrate intake in a ratio that utilizes two transport channels simultaneously. This is supported by Currell and Jeukendrup (2008) and Rowe (2022) as the optimal strategy for long-duration efforts.

How to structure your fueling: sports drink plus bars

Bars do not replace sports drink. They work together. Your primary source during an ultra is a sports drink with 6 to 8% carbs. Count on 500 to 800 ml per hour, good for 30 to 60g of carbs, plus the fluid intake you need for that load. On top of that, add 1 to 2 fruit performance bars per hour, each with 20g of carbs. Together, you are comfortably in the 60 to 90g zone. In the final stage, or during a long technical climb, a gel can catch the peak.

Practically, over the duration of a UTMB run, that looks like this:

• Hours 1 to 6: 60g per hour. Your reserve is still full. Priority: building rhythm and activating your gut. Sports drink plus 1 bar per hour.
• Hours 6 to 20: 70 to 90g per hour. Your internal reserve is dropping, external carbs become your main fuel. Sports drink plus 2 bars per hour, possibly a gel in a tough section.
• Hour 20 and later: 60g per hour. Fatigue lowers your pace and your gut tolerates less. Smaller, more frequent portions work better than large ones. Sports drink plus 1 bar per hour.

The aid stations are located on average every 15 to 25 kilometers. Between stations, you are self-sufficient, so what you carry is the only fuel that counts. Two bars per hour weighs 50g per hour. For a ten-hour block, that is 500g in your vest. Compact for the energy it delivers.

Why the 25g format is functional during ultra trail

Large portions, think of a full gel or a thick 90g bar, require more digestion per intake moment. Your stomach has to process more, empties more slowly, and has to swallow a higher sugar concentration. At low intensity, that is manageable. At hour 20 of an ultra, with thousands of meters of elevation in your legs, it is risky.

A fruit performance bar of 25g delivers 20g of carbs in a solid, chewable form. Chewing is functional: it stimulates saliva production and starts the first phase of your digestion before the food even reaches your stomach. Gels do not do that. The small format is also practical in cold, rain, or fatigue. No sticky hands, no empty packet blowing away. Nutrition during an ultra must work in all conditions, so format is not an aesthetic choice. It is a fueling decision. The full comparison between bar and gel is in energy bar versus energy gel: when what.

Bar versus gel: what do you choose during an ultra trail?

Gels have dominated ultra trail for years, and that is understandable: quick to consume, quickly absorbed, light. But the flip side is just as real. No format is universally superior, but for runners who are out for 30 hours or longer and want clean fuel without artificials, the bar is a functional addition or a complete alternative.

What is taste fatigue and why does it matter during an ultra?

After 15 to 20 hours of exertion, your sense of taste changes. What tastes neutral or sweet in hour one becomes intense and sometimes nauseating in hour twelve. This phenomenon, taste fatigue, is one of the main reasons why athletes stop eating during the night hours. And an athlete who does not eat, does not fuel. An athlete who does not fuel does not reach 80g of carbs per hour. The result is an energy peak followed by a slow, consistent decline in your performance capacity.

Different carbohydrate and flavor reflexes can become tiring after a long exertion. Fruit contains natural sugars and can provide flavor sensations different from strongly concentrated gel flavors or synthetic sweeteners. My fruit performance bars contain no artificials. The taste comes exclusively from fruit and functional ingredients. That is not marketing, that is a different positioning than most gels on the market. I explain the difference from regular sugar snacks in fruit energy bar versus candy.

How do you train your gut for 90g of carbs per hour?

Gut training is a proven approach to increase your absorption capacity. The principle: by repeatedly ingesting higher amounts of carbs during training, your gut adapts its transport capacity. Anyone trying 90g per hour for the first time on race day is asking for stomach issues.

A practical approach for your preparation: start 8 to 12 weeks before race day with conscious carb intake during your long training sessions. Start at 40g per hour and increase weekly by about 10g until you hit your target zone. Train with exactly the format and flavors you use on race day, so there are no surprises. And combine solid and liquid, sports drink plus bars, in rides or runs of three hours or longer to simulate actual race conditions. For additional ideas on portable fuel on the go, see the ideal fuel for on the go.

No gel. No brick. Fuel.

An ultra trail is won or lost in your stomach, not in your legs. The numbers are clear: 60 to 90g of carbs per hour, a 2:1 ratio that opens two channels at once, and a format that digests while you move. Sports drink as a base, 1 to 2 fruit performance bars per hour as a solid component. Clean, chewable, without artificials. During exertion. Period. Fuel your flight.

Choose your fuel