REF-980 · Miall et al., 2017 · Scandinavian Journal of Medicine & Science in Sports, Vol. · Paper · PubMed

Two weeks of repetitive gut-challenge reduce exercise-associated gastrointestinal symptoms and malabsorption

Blimited evidenceMeasured on trained to highly trained athletesWritten before October 2026, being re-checked

Endurance runners (n=18) did a gut-challenge trial: 2 hours of running at 60% VO2max eating, every 20 minutes, a gel-disk with 30 g carbohydrate (glucose-fructose 2:1, 10% w/v), then a 1-hour effort bout.

They were then randomised, blinded, to carbohydrate (90 gCHO per hour) or placebo (0 gCHO per hour) taken in a 1-hour run at 60% VO2max each day for two weeks, and did the trial again. With carbohydrate, gut discomfort, nausea and total, upper- and lower-gastrointestinal symptoms all fell; with placebo they did not.

Peak breath hydrogen dropped from 13±6 to 6±3 ppm and effort-bout distance rose from 11.7±1.5 to 12.3±1.3 km, in both cases on carbohydrate only.

Participants

Endurance runners (n=18), randomly assigned to a carbohydrate or placebo gut-training group. Group sizes, sex and fitness are not given in the abstract.

Protocol

Gut-challenge trial (GC1, repeated as GC2): steady-state running for 2 hours at 60% VO2max, with a formulated gel-disk of 30 g carbohydrates (glucose-fructose 2:1, 10% w/v) taken every 20 minutes; a 1-hour running effort bout came after.
Intervention: blinded carbohydrate (CHO, 90 gCHO per hour) or placebo (PLA, 0 gCHO per hour), consumed during 1-hour running at 60% VO2max equivalent, daily over a period of two weeks.
Measures: gastrointestinal symptoms, feeding tolerance, breath hydrogen (H2), effort-bout distance.

Key Results

In GC2, CHO (but not PLA) lowered nausea (P=.05), gut discomfort (P=.012) and gastrointestinal symptoms: lower (P=.008), upper (P=.015) and total (P=.009).
H2 peak on CHO: 13±6 ppm in GC1 to 6±3 ppm in GC2 (P=.004); on PLA 11±7 ppm and 10±10 ppm (no change).
Effort-bout distance on CHO: 11.7±1.5 km in GC1 to 12.3±1.3 km in GC2 (P=.035); on CHO only.
Feeding tolerance did not differ between GC1 and GC2 on CHO or PLA.

Practical Application

"Will practising my race intake reduce the stomach trouble?" In 18 endurance runners randomised and blinded to carbohydrate (90 g per hour) or placebo during a daily 1-hour run for two weeks, the carbohydrate group had fewer gut symptoms, less nausea and less carbohydrate malabsorption in a 2-hour fuelled run, and ran further in the following 1-hour effort (11.7 to 12.3 km); the placebo group did not change.
The dose that did it was the target rate itself, practised daily for two weeks — not a slow build. Inference, not tested here: an athlete who cannot tolerate that rate at the start would need a ramp, which is base practice in SYNTH-02.
The authors word the performance result cautiously ("may have performance implications"); treat symptoms and malabsorption as the finding.
Not from this paper: the three-arm trial from the same laboratory, including a food arm, is REF-979.

Limitations

n=18 in two groups; group sizes are not in the abstract.
Laboratory running at 60% VO2max; no race, no heat, no cycling; no follow-up after the two weeks.
The performance change was tested within the carbohydrate group; the abstract gives no between-group statistic for distance.
Symptoms are self-reported.
Inference, not stated in the abstract: the cohort may overlap with REF-979 (same laboratory, same protocol, same period) — do not count the two as fully independent.
Header year follows the Crossref issued date (online 2017); the print issue is February 2018.
Written from the abstract and record metadata only (full text not read).
How this entry was checked and changed
Why this level

parallel RCT, n=18 (group sizes not in the abstract)