Why the gut is a trainable organ for fueling
Tolerance to carbohydrate during exercise is not fixed. Regularly practicing race-pace fueling appears to upregulate intestinal carbohydrate transporters and improve gastric comfort, allowing the athlete to absorb and tolerate more carbohydrate per hour over a period of weeks. This is why elite endurance athletes can take in amounts that would cause distress in an untrained gut.
Reported progress often moves from roughly 30 g of carbohydrate per hour toward 90 g or more per hour, though the upper end requires using multiple sugar types that use different transporters.
Why glucose and fructose are combined
A single transporter for glucose saturates at a limited rate, so adding fructose, which uses a separate transporter, allows higher total absorption without simply piling up undigested sugar in the gut. Training sessions are used to rehearse exactly the products and amounts intended for race day.
This is general educational information, not medical advice. Aggressive fueling can cause significant gastrointestinal distress, and anyone with digestive disorders should progress carefully and consult a qualified professional.