Changes in intragastric temperature reflect changes in heat stress following tepid fluid ingestion but not ice slurry ingestion

This study examined the effects of fluid and ice slurry ingestion on the relationship between intragastric temperature and rectal temperature in humans during physical activity. The purpose was to identify a technique to quantify changes in heat stress in situations when temperature probes are not feasible and when time constraints do not allow for a period long enough for an indigestible temperature capsule to reach the lower gastrointestinal tract. Eight moderately trained male runners inserted a rectal probe and ingested a telemetric capsule before randomized, crossover, pre-exercise ingestion of 7.5 mL x kg^-1 x BM^-1tepid fluid (22°C) or ice slurry (-1°C). Beverage ingestion was followed by a self-paced endurance running time trial. Average intragastric temperature was significantly lower than average rectal temperature across the run following both fluid (37.9 +/- 0.4°C vs. 38.4 +/- 0.2°C; p=0.003) and ice slurry ingestion (37.2 +/- 0.9 vs. 38.3 +/- 0.2; p=0.009). However, a strong relationship was observed between measurements following fluid (r=0.89) but not ice slurry (r=0.18). The average bias +/- limits of agreement during the run was 0.46 +/- 0.50 following fluid and 1.09 +/- 1.68 following ice slurry ingestion, which improved to 0.06 +/- 0.76 and 0.65 +/- 1.42, respectively when analyzed as delta scores. Intragastric temperature appears to not be a valid measure of absolute core body temperature at baseline or during exercise following either fluid or ice slurry ingestion. However, the relative changes in intragastric temperature during endurance exercise appears to be a strong indicator of systemic heat stress during exercise following ingestion of fluid at 22°C, but not ice slurry at -1°C.