REF-787 · Pfeiffer et al., 2012 · Medicine & Science in Sports & Exercise, Vol. · Paper · PubMed

Nutritional Intake and Gastrointestinal Problems during Competitive Endurance Events

B+moderate evidenceMeasured on recreational to world-class athletesWritten before October 2026, being re-checked

A total of 221 endurance athletes from two Ironman races, a half-Ironman, a marathon and a 100/150-km cycling race, plus professional cyclists in stage races, reported what they ate and drank and rated 12 gastrointestinal symptoms after competing.

Mean carbohydrate intake was similar across the triathlons (62-71 g/h), lower in the cycling race and lowest in the marathon (35 g/h). Serious GI symptoms were most common in the Ironman races (about 31%) and went with a history of GI problems. In the Ironman races, higher carbohydrate intake went with more nausea and flatulence but also with faster finishing times.

Participants

n=221 endurance athletes, male and female, from:

two Ironman triathlons (IM Hawaii, IM GER), a half-Ironman (IM 70.3), a marathon, and a 100/150-km cycling race;
professional cyclists during stage racing.

Numbers per event, age and performance level are not given in the abstract.

Protocol

Standardised post-race questionnaire quantifying energy, nutrient and fluid intake during the competition.
12 GI symptoms each scored from 0 (no problem) to 9 (worst it has ever been).
Intake compared between events and correlated with GI symptoms, GI history and finishing time.

Key Results

EventMean carbohydrate intake (g/h)Serious GI symptoms
IM Hawaii62 ± 26∼31% across the IM races (P = 0.001 vs the others)
IM GER71 ± 25(as above)
IM 70.365 ± 2514%
100/150-km cycling race53 ± 22 (P = 0.044, lower)4%
Marathon35 ± 26 (P < 0.01, lower)4%
Professional stage racingnot given7%
Carbohydrate intake ranged from 6-136 g/h between events and individual athletes.
Upper and lower GI symptom scores correlated with a reported history of GI distress (r = 0.37 and r = 0.51, P < 0.001).
In both Ironman races, total carbohydrate intake correlated positively with nausea and flatulence and negatively with finishing time (r = -0.55 and r = -0.48, P < 0.001) — more carbohydrate, faster finish.

Practical Application

In 221 athletes surveyed after Ironman, half-Ironman, marathon and cycling races, the Ironman athletes who took in more carbohydrate per hour finished faster (r = -0.55 and -0.48 in the two races) although they also reported more nausea and flatulence; about 31% of Ironman athletes had serious GI symptoms, and a past history of GI trouble was what those symptoms tracked. Eating more during an Ironman went with a faster race, not a slower one — the cost is some stomach discomfort, and the athletes most at risk are the ones who have had gut problems before.
Know the starting point: mean intake was 62-71 g/h in the triathlons and only 35 g/h in the marathon, so many runners race far below common targets.
Ask about GI history first: it was the clearest correlate of symptoms in every data set.
Inference, not tested here: the correlation with finishing time cannot say whether eating more made athletes faster or whether faster, more experienced athletes simply ate more.
Not from this paper; the base's own practice: a target intake is earned over weeks of practice in training (REF-008, SYNTH-02), not chosen on race morning.

Limitations

Self-reported intake and symptoms from memory after the race; no direct measurement.
Cross-sectional and correlational; events differed in duration, intensity and conditions.
Per-event sample sizes and athlete characteristics are not in the abstract.
Written from the abstract and record metadata only (full text not read).
How this entry was checked and changed
Why this level

cross-sectional measurement study, n=221