Fuel by duration, not just intensity
Glycogen covers roughly the first hour. Beyond that, carbohydrate keeps blood glucose steady and protects your finishing power.
Jeukendrup's three tiers: under an hour, none. From 1 to 2.5 hours, about 60 g per hour. From 2.5 hours, up to 90 g per hour. Easy sessions need less than hard ones.
Why 90 g/h needs two types of carbohydrate
The gut transporter for glucose saturates at about 60 g per hour. Fructose uses a separate transporter, so a 2:1 glucose-to-fructose blend reaches 90 g per hour. Glucose alone above 60 g per hour sits in the stomach and causes cramps and nausea.
Worked example
An athlete planning fuel for two session lengths:
| 2-hour session, target | 60 g/h |
| 2-hour session, total carbohydrate | 120 g |
| 3-hour session, target | 90 g/h |
| 3-hour session, total carbohydrate | 270 g |
The 3-hour total needs a glucose-to-fructose mix near 2:1 to be absorbed comfortably.
Frequently asked questions
How many carbs per hour should I eat during exercise?
Carbohydrate intake depends on duration. Under an hour, water is enough. From 1 to 2.5 hours, about 60 g per hour. From 2.5 hours, up to 90 g per hour with a glucose-and-fructose mix.
Why can I only absorb 60 grams of glucose per hour?
The intestinal glucose transporter saturates at roughly 60 g per hour, so extra glucose sits in the gut. Fructose uses a separate transporter, so adding it lifts total absorption toward 90 g per hour.
What is the 2:1 glucose to fructose ratio?
The 2:1 ratio is two parts glucose-based carbohydrate to one part fructose, the blend in high-rate sports fuels. It uses both gut transporters and reaches 90 g per hour.
Do I need to train my gut to handle 90 g/h?
Yes. Gut tolerance is trainable. Practice race-day fueling in long sessions, building up over weeks. Never try a new high intake for the first time on race day.
Does easy intensity change how much I need?
Somewhat. Easy effort burns more fat and less carbohydrate, so long easy days sit at the low end of each tier. Hard efforts burn carbohydrate faster and push you toward the top.
Sources
- Jeukendrup A. (2014). "A step towards personalized sports nutrition: carbohydrate intake during exercise." Sports Med 44(Suppl 1):S25-33. Link
- ACSM / ISSN nutrition guidelines. Position stands on carbohydrate intake rates during endurance exercise and multiple-transportable-carbohydrate use.
- How Wout van Aert Trains (the VLamax Fix Behind His Roubaix Win)Van Aert won the fastest Paris-Roubaix ever, then two Vuelta stages. The training idea behind it is one number his team spent years lowering: VLamax.
- How Kristian Blummenfelt Trains (and What He Eats While Doing It)Kristian Blummenfelt runs the Norwegian method in three sports: lactate-guided thresholds, obsessive measurement, and a carb intake that rewrote the rules.
- How Jessie Diggins Trains (Twice a Day, and Why She Calls Food Fuel)America's most decorated cross-country skier trains twice a day, six days a week. The reason she talks about fuelling is the reason the volume is possible.