Why the gut can be trained
Gut training is the practice of deliberately consuming carbohydrate at high rates during workouts so the digestive system adapts to absorbing and tolerating more fuel under exercise. The gastrointestinal tract is adaptable: regular exposure can increase the activity of intestinal carbohydrate transporters and improve gastric comfort, allowing an athlete to take in more energy during long events without nausea, bloating, or cramping.
This matters because gastrointestinal distress, not lack of available fuel, is a frequent limiter when athletes try to hit high intake targets in racing.
How it connects to mixed-carbohydrate fueling
Because intestinal transporters for glucose and fructose are partly separate, using a mix of the two raises the ceiling on how much carbohydrate can be absorbed per hour. Gut training combined with mixed-carbohydrate sources is how athletes work toward the higher intake rates used in long-duration events.
Tolerance is individual and adaptation takes weeks of consistent practice. This is general educational information about sports nutrition, not personalized dietary advice.