Time following ingestion does not influence the validity of telemetry pill measurements of core temperature during exercise-heat stress: The journal toolbox.

In studies of human thermoregulation, ingestible temperature pills are being increasingly used as a convenient alternative to more clinically relevant indices of deep-body (core) temperature (e.g., rectal temperature). It remains unclear whether the time between pill ingestion and the measurement period influences the validity of telemetry pills as a surrogate index of core temperature. We therefore assessed the influence of pill ingestion timing on the agreement between rectal temperature (criterion method) and ingestible pill temperature during exercise-heat stress. To achieve this, nine young men (21-31 years) completed two trials involving 15-min rest, 90-min exercise at an average metabolic heat production of 200 W/m (~40% peak oxygen consumption), and 45-min recovery. Core temperature was measured throughout using rectal temperature and four telemetric temperature pills (VitalSense®) ingested 12, 6, 3 and 1 h(s) prior to the start of each trial. Data from the two trials were combined and averaged over the final 10-min of rest, exercise, and recovery for analysis. Our primary finding was that the mean squared difference between rectal temperature and each pill did not differ significantly across ingestion times during rest, exercise or recovery (p = 0.056), with those errors ranging from 0.1-0.2°C, 0.2-0.2°C, 0.1-0.2°C, and 0.1-0.2°C for the pills ingested 12, 6, 3, and 1 h(s) before data collection, respectively. While there is a need for larger confirmatory studies, our findings indicate that pill ingestion timing does not significantly influence the validity of telemetry pill temperature as an index of core temperature.