Training the Gut for Athletes — gastric emptying, comfort and intestinal CHO absorption are trainable
Key Findings
GI problems are common in endurance events, so GI function is not always optimal under race conditions.
The GI system is highly adaptable: gastric emptying and stomach comfort can be "trained" and perceptions of fullness reduced; some studies suggest nutrient-specific increases in gastric emptying.
Diet changes absorptive capacity, nutrient-specifically: a high-carbohydrate diet increases the density and activity of intestinal SGLT1 transporters, allowing greater carbohydrate absorption and oxidation during exercise.
When these adaptations occur, GI distress is likely less frequent/severe.
Open questions named by the author: the most effective methods to induce gut adaptation and the timeline of adaptation are not established; more human studies are needed.
Practical Application
Jeukendrup's review concludes that the gut is trainable: repeated carbohydrate intake improves gastric emptying and stomach comfort, and a high-carbohydrate diet raises intestinal SGLT1 density and activity so more carbohydrate is absorbed and oxidised during exercise, likely with less GI distress — but the best method and the timeline are not yet known — practise race fuelling in training rather than meeting it for the first time on race day.
Rehearse the planned race intake on long training sessions, in the conditions the race will bring; the review does not say how many weeks this takes.
Limitations
Review, not a trial. Mechanistic and training claims are synthesised from a small human literature; the author asks for more human studies.
Gut tolerance is highly individual and the review cannot give a personal ceiling; test in training, never on race day.
Absorption capacity is not the same as performance benefit.
How this entry was checked and changed
Why this level
regraded 2026-09-26 from A: narrative review
Content correction 2026-09-27
Headline "GI adapts to 90-100 g CHO/h" and "within 2-4 weeks" -> the review states the gut is trainable (gastric emptying, comfort, SGLT1 absorption) and names the timeline of adaptation as unknown; it gives no dose.
Dose ladder (mouth rinse ≤1 h, 30-60, 60-90, 80-100 g/h), the 60 g/h single-transporter and 2:1 figures and the race-day 90-100 g/h bullet removed -> not in this paper; pointers to SYNTH-02, REF-527, REF-144/333/334 added.
Level A -> B+ (narrative review).