Survey data from ironman triathlon and ultramarathon consistently identifies gastrointestinal distress as a primary cause of performance decline and DNF — affecting an estimated 30–50% of long-course competitors. Most of these athletes trained extensively. Almost none of them trained their gut.
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The Gut Is Not Automatically Ready for Race-Day Nutrition
The GI system during endurance exercise faces conditions significantly different from rest:
- Splanchnic blood flow is reduced by 40–80% at moderate-to-high exercise intensities as cardiac output is redirected to working muscle
- Gastric emptying rate is altered by exercise intensity, hydration status, and the osmolality of ingested fluids
- Intestinal motility is modified by exercise-induced sympathetic stimulation and prostaglandin release
An athlete who has never practised taking gels at race pace, at race-day carbohydrate rates, during training efforts — is asking their GI system to perform a novel task under stress conditions during the most important performance of their year.
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What Gut Training Actually Does
SGLT1 upregulation: The sodium-glucose cotransporter responsible for intestinal carbohydrate absorption is a trainable protein. Regular exposure to high carbohydrate intake during exercise increases SGLT1 expression in the intestinal brush border, increasing absorptive capacity. Jeukendrup's research group (2010) demonstrated measurable increases in exogenous carbohydrate oxidation rates in athletes who trained with high carbohydrate intake for 4–6 weeks compared to controls.
Gastric emptying adaptation: The stomach adapts to larger fluid and carbohydrate volumes with repeated exposure. Athletes who routinely consume 500–600mL fluid per hour during training develop faster gastric emptying rates at equivalent volumes compared to athletes who train primarily fasted or at low carbohydrate intake.
Reduced prostaglandin response: Intestinal inflammation from exercise-induced ischaemia (reduced blood flow) is partially attenuated with training adaptation. Repeated low-level GI stress from training with food produces a protective adaptation similar to the musculoskeletal adaptation principle.
Psychological habituation: A significant component of exercise-induced GI distress is conditioned — athletes who have repeatedly experienced nausea at high intensity become conditioned to expect it. Successful gut training experiences during training reframe the psychological association between high-intensity effort and GI discomfort.
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The Gut Training Protocol
Start 4–8 weeks before the target race. Gut adaptation requires repeated stimulus over weeks, not days.
Use your exact race nutrition products. Different gels have different osmolalities, carbohydrate types, and flavour concentrations. Adapting to one product does not transfer fully to another. Race week is not the time for a product switch.
Progress carbohydrate rate incrementally:
- Weeks 1–2: Match race-intensity sessions with 30–40g/hr carbohydrate intake (gels + water)
- Weeks 3–4: Increase to 50–60g/hr during long sessions of ≥90 minutes
- Weeks 5–8: Full race-protocol rate (60–90g/hr as required) during the final long training sessions
Test race-morning nutrition. The pre-race meal and its timing affects gastric residual volume at the start. Training starts with the same pre-session meal you plan to eat on race morning.
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The Most Common Gut Training Errors
1. Training fasted, racing fuelled. Athletes who perform all training sessions in a low-carbohydrate fasted state then take 8 gels on race day are conducting their first gut training session during the race.
2. Using training gels but switching brands on race day. Sponsor changes, aid station availability, or bargain product switches introduce a novel GI stimulus at the worst possible time.
3. Practising fuelling at training pace, not race pace. Gastric emptying slows significantly at intensities above approximately 75% VO₂max. A gel protocol that works fine at Zone 2 training pace may produce distress at race-pace effort where blood flow to the gut is further compromised.
For athletes building their race-day fuelling schedule and planning gut training sessions around it — the energy gel calculator at winsport.uk/tools/nutrition/energy-gel-calculator generates the exact gel protocol (count, timing, g/hr) that athletes should practise in training, not just execute on race day.
Have you ever practised your complete race-day nutrition protocol during a training session — same products, same quantities, same timing — or does the gel strategy only come together on race morning?