Exercise & Training

Gastrointestinal temperature of elite and recreational adult runners during a 100-mile ultramarathon under heat exposure.

Mougin L, Stennett R, et al. • Physiological reports • 2026

TL;DR

Despite ambient temperatures approaching 40°C during a 100-mile ultramarathon, gastrointestinal temperatures exceeding 40°C were rare and transient, with faster runners experiencing greater thermal strain likely reflecting higher metabolic heat production and exposure to the hottest course segments.

Key Findings

Results

Peak gastrointestinal temperature averaged 38.90°C across finishers, with a wide individual range and only a small number exceeding clinically concerning thresholds.

Peak TGI averaged 38.90°C ± 0.55°C across 23 finishers
Individual range spanned from 37.76°C to 40.09°C
9 runners (39%) exceeded 39°C and only 2 runners exceeded 40°C
TGI >40°C were described as 'rare and transient' despite ambient temperatures approaching 40°C

Results

Faster finish times were significantly associated with higher peak gastrointestinal temperatures.

Correlation between finish time and peak TGI: r = -0.57, p = 0.004
Participants were grouped into finish time bands: 14–18 h (n = 4), 19–25 h (n = 6), and 26–30 h (n = 13)
The authors attributed this to higher metabolic heat production and greater exposure to the hottest course segments among faster runners
2 podium finishers and an additional top-10 finisher were included in the dataset

Results

Ambient temperature along the course was strongly associated with higher gastrointestinal temperature.

Correlation between ambient temperature and TGI: r = 0.80, p = 0.008
Ambient temperature ranged from 5°C to 39.6°C across the course
Environmental conditions were recorded every 30 minutes at 15 locations along the course
This suggests that environmental heat exposure meaningfully drove internal thermal load during the race

Results

A notable proportion of runners reached their peak gastrointestinal temperature near the end of the race.

7 runners (30% of finishers) reached their peak TGI in the final 2% of the race
This finding suggests hyperthermia risk may be highest near the race finish
The authors noted that hyperthermia risk may also be elevated 'during the most demanding segments' of the course

Results

Body mass changes were highly variable across participants and were not associated with peak gastrointestinal temperature or finish time.

Mean body mass change was -1.8% ± 3.1% (pre- to post-race)
Individual range of body mass change spanned from -7.4% to +6.4%
No statistically significant associations were observed between body mass change and either peak TGI or finish time
Body mass was measured pre- and post-race in 39 participants who ingested a telemetric pill 30–90 min before the race start

Methods

The study captured gastrointestinal temperature data from 23 of 39 enrolled participants who completed the 100-mile race, including elite and recreational runners.

39 participants ingested a telemetric pill 30–90 min before race start; in-race TGI data were obtained from 23 finishers (54% completion rate for data capture)
The sample included 7 females and 16 males, mean age 42 ± 11 years
The race was the Western States Endurance Run 100 (WSER), a 100-mile ultramarathon held in hot conditions
The study included 2 podium finishers and an additional top-10 finisher, representing elite-level performance

What This Means

This research suggests that even during an extremely demanding 100-mile foot race in hot conditions — with air temperatures reaching nearly 40°C — most runners' internal body temperatures (measured using swallowed sensor pills tracking gut temperature) stayed below dangerous levels. Out of 23 runners who completed the race and provided data, the average peak internal temperature was about 38.9°C, and only 2 runners briefly exceeded 40°C, a threshold often associated with heat illness risk. This indicates that the human body is generally capable of managing heat during prolonged ultramarathon efforts, though individual variation was substantial. Faster runners tended to reach higher peak internal temperatures, which the researchers attribute to their greater pace generating more body heat and their spending more time on the course during the hottest part of the day. Environmental temperature was strongly linked to internal temperature, reinforcing that course conditions matter significantly for thermal strain. Notably, nearly one-third of runners hit their highest internal temperature in the very last stretch of the race, suggesting that the finish line — not the middle miles — may actually be the riskiest point for overheating. Body weight changes varied widely across runners (from losing over 7% to actually gaining over 6% of body mass), but these changes were not linked to internal temperature or race performance, which challenges simple assumptions about dehydration and heat stress in ultramarathon running. This research suggests that race medical teams and runners themselves should be particularly vigilant about heat-related risk during the final stages of long races and during the hottest, most physically demanding segments of a course.

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Citation

Mougin L, Stennett R, Ely B, Morel B, O'Neill L, Esh C, et al.. (2026). Gastrointestinal temperature of elite and recreational adult runners during a 100-mile ultramarathon under heat exposure.. Physiological reports. https://doi.org/10.14814/phy2.71089

Key Findings

What This Means

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