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

This study examined gastrointestinal temperature (TGI) during and body mass (BM) changes pre/post the Western States Endurance Run 100 (WSER), a 100-mile ultramarathon in hot conditions. Thirty-nine participants ingested a telemetric pill 30-90 min before the WSER start, and BM was measured pre- and post-race. Environmental conditions were recorded every 30 min at 15 locations along the course. In-race TGI data were obtained from 23 finishers (54%; 7 females; 42 ± 11 years), including 2 podium finishers and an additional top-10 finisher. Participants finished in 14-18 h (n = 4), 19-25 h (n = 6), and 26-30 h (n = 13). Peak TGI averaged 38.90°C ± 0.55°C (range 37.76°C-40.09°C) with 9 runners exceeding 39°C and 2 exceeding 40°C. Faster finish times were associated with higher peak TGI (r = -0.57, p = 0.004). Seven runners (30%) reached peak TGI in the final 2% of the race. Mean BM change was -1.8% ± 3.1% (range -7.4% to +6.4%), with no associations observed between BM change and peak TGI or finish time. Ambient temperature ranged 5°C-39.6°C and was associated with higher TGI (r = 0.80, p = 0.008). Despite ambient temperatures approaching 40°C, TGI >40°C were rare and transient. Faster runners experienced greater thermal strain, likely reflecting higher metabolic heat production and exposure to the hottest course segments. Hyperthermia risk may be highest near the race finish and during the most demanding segments.