Carbohydrate During Exercise — How Much, When, and Why

Why fuel during a run

Your body stores only a limited amount of carbohydrate — roughly enough for 90 minutes to a couple of hours of hard running. As those stores drain, your pace, power, and focus fade; that's the "wall" or "bonk." Taking in carbohydrate during exercise spares your glycogen and keeps blood glucose up, so you can hold effort longer¹. For easy runs under about an hour you generally don't need to fuel — your stores have it covered.

How much — it scales with duration

The longer the run, the more carbohydrate per hour helps. Rough, well-supported targets¹:

• Up to ~1 hour: little to nothing needed — water is usually enough. In a hard effort, even rinsing a carbohydrate drink in your mouth can help.
• ~1 to 2 hours: roughly 30 g of carbohydrate per hour.
• ~2 to 3 hours: up to ~60 g per hour — about the most a single type of sugar can be absorbed.
• Beyond ~3 hours (ultra distance): up to ~90 g per hour can help — but only if you mix carbohydrate types (below).

The trick to absorbing a lot — mix your sugars

There's a ceiling on how fast your gut absorbs a single sugar (about 60 g/h). Push past it with one type and the excess just sits in your stomach and causes distress. The fix is "multiple transportable carbohydrates" — combining glucose and fructose, which use two separate absorption pathways in the intestine, so you can take in up to ~90 g/h without the GI backup¹. Most gels and drinks built for high intake already use a roughly 2:1 glucose-to-fructose blend.

Make it practical

Gels, drinks, chews, or real food all work — use whatever you tolerate. Start early rather than waiting until you're empty, and take small amounts often instead of one big hit. Remember these numbers are starting points: some runners comfortably take more, others need to build up to them. You raise what you can handle by practicing it — see gut training.