How many carbs per hour should you take in a marathon?

Start at 60 g per hour. Glucose alone is capped near 60 g/hr by intestinal transport; adding fructose lifts the ceiling to roughly 90–120 g/hr, but only if your gut is trained for it.

The limit on race fuelling is not your stomach, it is your small intestine. Glucose crosses the intestinal wall through the SGLT1 transporter, which saturates at roughly 1 g per minute — about 60 g per hour. Feeding more glucose than that does not deliver more energy; it leaves unabsorbed sugar sitting in your gut, drawing water in, which is the mechanism behind most mid-race gastrointestinal misery.

Fructose uses a different transporter, GLUT5, which is not saturated by glucose. Taking both together — in roughly a 1:0.8 to 2:1 glucose-to-fructose ratio, which is what most modern sports products use — raises total absorption to around 1.5 g per minute, or 90 g per hour. The best-adapted athletes reach roughly 2 g per minute, but that is a trained ceiling rather than a starting point.

What you should actually take depends on how long you will be out there. A 3-hour marathoner running at a high fraction of their glycogen-burning capacity benefits from pushing toward 90 g/hr. A 5:30 finisher is running at a lower intensity where fat contributes far more, and 60 g/hr is usually plenty. Bodyweight matters too, though less than intensity and duration.

The rule that matters more than any number: practise it. Absorption capacity is trainable over weeks, and race day is the worst possible time to discover that your gut disagrees with your plan.

Glossary: SGLT1, Dual Transport (Glc+Frc)