What is the SGLT1 limit on carbohydrate absorption?

SGLT1 is the intestinal transporter that carries glucose into the bloodstream. It saturates at roughly 1 gram per minute, which sets a hard ceiling of about 60 g/hr on glucose-only fuelling.

SGLT1 — sodium-glucose linked transporter 1 — sits in the wall of the small intestine and moves glucose from the gut into the blood, using the sodium gradient as its power source. Like any transporter it has a finite number of copies, so above a certain delivery rate it is fully occupied and additional glucose simply waits.

In endurance athletes that saturation point sits near 1 gram per minute. This is the single most important number in race fuelling, because it means a runner drinking a glucose-only sports drink gains nothing from drinking more of it past roughly 60 g/hr — and starts to lose, because the unabsorbed sugar raises the osmolality of the gut contents and pulls water in.

The limit is a property of the transporter, not of your fitness, so you cannot train your way past it with glucose alone. What you can train is the expression of the transporters themselves and general gut tolerance, which is why habitual high-carbohydrate athletes tolerate more. Getting meaningfully above 60 g/hr, though, requires recruiting a second transport route.

Glossary: SGLT1