Evidence that transient changes in sudomotor output with cold and warm fluid ingestion are independently modulated by abdominal, but not oral thermoreceptors

Abstract

Two studies were performed to 1) characterize changes in local sweat rate (LSR) following fluid ingestion of different temperatures during exercise, and 2) identify the potential location of thermoreceptors along the gastrointestinal tract that independently modify sudomotor activity. In study 1, 12 men cycled at 50% Vo2peak for 75 min while ingesting 3.2 ml/kg of 1.5°C, 37°C, or 50°C fluid 5 min before exercise; and after 15, 30, and 45-min of exercise. In study 2, 8 men cycled at 50% Vo2peak for 75 min while 3.2 ml/kg of 1.5°C or 50°C fluid was delivered directly into the stomach via a nasogastric tube (NG trials) or was mouth-swilled only (SW trials) after 15, 30, and 45 min of exercise. Rectal (Tre), aural canal (Tau), and mean skin temperature (Tsk); and LSR on the forehead, upper-back, and forearm were measured. In study 1, Tre, Tau, and Tsk were identical between trials, but after each ingestion, LSR was significantly suppressed at all sites with 1.5°C fluid and was elevated with 50°C fluid compared with 37°C fluid (P < 0.001). The peak difference in mean LSR between 1.5°C and 50°C fluid after ingestion was 0.29 ± 0.06 mg·min(-1)·cm(-2). In study 2, LSR was similar between 1.5°C and 50°C fluids with SW trials (P = 0.738), but lower at all sites with 1.5°C fluid in NG trials (P < 0.001) despite no concurrent differences in Tre, Tau, and Tsk. These data demonstrate that 1) LSR is transiently altered by cold and warm fluid ingestion despite similar core and skin temperatures; and 2) thermoreceptors that independently and acutely modulate sudomotor output during fluid ingestion probably reside within the abdominal area, but not the mouth.

Temperature of Ingested Water during Exercise Does Not Affect Body Heat Storage

“…Our mean LSR response is consistent with a recent study (23) showing a transient increase and decrease when ingest ing 50°C and 1.5°C water periodically during exercise, re spectively (Fig. 3).…”

“…After a 15-min baseline period in an upright seated position, continuous semi recumbent cycling at -7 0 rpm was performed for 75 min at 50% o f each participant's respective V 0 2peak. This intensity was equivalent to an external workload of 98 + 22 W. Similar to previous studies (1,23), four boluses o f tap water measured to 3.2 m L k g~1 o f either hot (50°C) or cold (1.5°C) water were consumed 5 min before exercise and at 15, 30, and 45 min during the exercise period. Drinks were consumed within 3 min using a custom mask (plastic tub ing inside mouthpiece with a manual valve) fitted to enable drinking while maintaining a sealed environment and con tinuous measurements o f indirect calorimetry.…”

“…1). However, both Morris et al (23) and Takamata et al (35) noted that the act of drinking and swallowing itself does not alter thennoeffector response in euhydrated individuals. Therefore, the whole-body modulation of evaporative heat loss observed in the present study is likely only mediated by the abdominal thermoreceptors when stimu lated by the heat content of the ingested water.…”

Neurophysiology of Skin Thermal Sensations

“…central nervous system and viscera) as well as in more superficial (e.g. skin and muscle) tissues (25,26,122,144,211,217,218,267).…”

“…thermal sensation) (261); b) modulate autonomic thermal responses (e.g. suppression/increases in sweating due to thermal changes in skin or gastro-intestinal temperatures) (217,222).…”