How many carbs per hour on an ultra?
Between 60 and 90 g/h beyond three hours of effort, provided you have trained your gut. Below 60 g/h the deficit builds hour after hour; above 90 g/h you need a glucose-fructose blend to absorb it without gut trouble.
The body oxidises glucose at a rate capped by the intestinal transporter SGLT1, around 60 g/h. Adding fructose, which uses a different transporter (GLUT5), lifts that ceiling: this is why products sold above 60 g/h advertise a glucose:fructose ratio, usually 2:1 or 1:0.8. Without that blend, the surplus sugar sits in the gut, draws water in, and causes exactly the trouble you were trying to avoid.
The commonest mistake is not aiming too high: it is thinking in kilometres. A runner planning "a gel every 5 km" takes in half as much per hour going up as coming down, at the very moment the effort costs most. The target is counted per hour, and the hour depends not on distance but on your real pace on the ground.
On a real race: Mont-Blanc — UTMB®
On Mont-Blanc — UTMB®, a runner with a UTMB® Index of 600 is projected at 37h10. At 60 g/h that comes to 2,230 g of carbohydrate over the race — about fifty gels. The same course asks far less of a faster runner: it is duration that governs, never the kilometres.
| UTMB® Index | Estimated time | Carbs | Fluid | Sodium |
|---|---|---|---|---|
| 500 | 43h01 | 2,581 g | 30.1 L | 21,508 mg |
| 600 | 37h10 | 2,230 g | 26 L | 18,583 mg |
| 800 | 24h36 | 1,476 g | 17.2 L | 12,300 mg |
Temps repris des passages réels des coureurs de chaque tranche de cote sur les éditions précédentes, arrêts aux ravitaillements compris. Besoins calculés à 60 g/h de glucides, 700 mL/h de liquide et 500 mg/h de sodium, par temps tempéré.
In practice: set your hourly target, multiply it by your projected time, then spread the total across the legs — not evenly, but by the duration of each. A three-hour leg needs three times what a one-hour leg needs, even at the same distance. And train the tolerance: 90 g/h is not something you improvise on race day.
RAVIT applies this calculation to 2,778 races across 49 countries, with 6,547 aid stations whose contents are taken from the official rulebooks.