Fuel by duration, not just intensity
The main driver of how much carbohydrate you need during exercise is how long you are out there. Your muscles and liver store enough glycogen to cover roughly the first hour, so short sessions need little or no fuel. As duration climbs, topping up carbohydrate keeps blood glucose steady and protects your finishing power.
Andrew Jeukendrup's widely used guidelines set three tiers: under an hour needs essentially no carbohydrate, 1 to 2.5 hours calls for about 60 g per hour, and sessions of 2.5 hours or more can use up to 90 g per hour. Intensity matters too, but mostly at the margins: an easy short ride needs less than a hard one of the same length.
Why 90 g/h needs two types of carbohydrate
Your gut can only absorb glucose at about 60 g per hour, because the transporter that carries it saturates. Fructose uses a separate transporter, so adding it lets you absorb more total carbohydrate. That is why high-rate fuels and gels blend glucose and fructose, typically around a 2:1 ratio, to reach 90 g per hour without overwhelming the gut.
Below 60 g per hour, a single glucose-based source is fine. Only when you push toward 90 g per hour does the multiple-transportable-carbohydrate mix become necessary. Going above your gut's capacity with single-source carbohydrate just leaves it sitting in your stomach and invites 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?
It depends on duration. Under an hour, water is usually enough. From 1 to 2.5 hours, aim for about 60 g of carbohydrate per hour. For sessions of 2.5 hours or longer, you can absorb up to 90 g per hour, but only with a glucose-and-fructose mix that uses two gut transporters at once.
Why can I only absorb 60 grams of glucose per hour?
The intestinal transporter that carries glucose into the blood saturates at roughly 60 g per hour. Eating more glucose alone does not raise absorption; it just sits in the gut. Adding fructose, which uses a separate transporter, lets total carbohydrate absorption climb toward 90 g per hour on long efforts.
What is the 2:1 glucose to fructose ratio?
It is the blend used in high-rate sports fuels to push absorption past the glucose-only ceiling. Roughly two parts glucose-based carbohydrate to one part fructose lets you use both gut transporters at once, reaching up to 90 g per hour. Below 60 g per hour a single carbohydrate source works fine.
Do I need to train my gut to handle 90 g/h?
Yes. Tolerating high carbohydrate rates is a trainable skill. Practise your race-day fuelling in long training sessions, starting modestly and building up over weeks. An untrained gut often rebels at 90 g per hour, so never try a new high intake for the first time on race day.
Does easy intensity change how much I need?
Somewhat. At easy effort you burn proportionally more fat and less carbohydrate, so short easy sessions need little or no fuel. The duration tiers still apply, but on long easy days you can sit at the lower end of each range. Hard efforts burn carbohydrate faster, pushing you toward the top end.
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.