Tolerance is trainable. Here's the protocol the studies used, and the part most riders skip entirely.
Tolerance is not fixed. It is the most trainable part of a fuelling plan, and almost nobody trains it on purpose. The protocol is unglamorous: raise carbohydrate intake during long endurance sessions, hold it there, and repeat for four weeks.
Tolerance is not fixed. It is the single most trainable part of a fuelling plan, and almost nobody trains it deliberately.
The protocol
Raise carbohydrate intake during long endurance sessions, hold it there, and repeat for four weeks. Symptom scores fall. Measured oxidation rises.
The part riders skip
Neither happens quickly, and neither happens by accident. The number you can hold on race day is a number you decided on a month earlier.