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.

Keywords: body temperatures; exercise; fluid intake; sweating; thermoregulation.

Fig. 1.

Mean local sweat rate (LSR) after the ingestion of 1.5°C (open circles), 37°C (gray circles), and 50°C (black circles) fluid before and during exercise. Dashed lines denote when fluids were ingested. Values given are the grand mean (A) of the following three locations: forehead (B), upper back (C), and forearm (D). *Where 1.5°C < 37°C < 50°C. ^#Where 1.5°C < 50°C (P < 0.05). Error bars indicate SD.

Fig. 2.

The mean change in LSR following the ingestion of 1.5°C (open circles) and 50°C (black circles) fluid relative to any changes in mean LSR observed during the thermoneutral 37°C fluid control trial. *Time points that 1.5°C < 50°C. Error bars indicate SD. Dashed lines denote when fluids were ingested.

Fig. 3.

Mean body temperatures using a 0.9/0.1 weighting of core to skin temperatures using aural canal temperature (T_au) (left) and rectal temperature (T_re) (right) as an indication of the body core, following the ingestion of 1.5°C (open circles), 37°C (gray circles), and 50°C (black circles) fluid before and during exercise. Dashed lines denote when fluids were ingested. Error bars indicate SD.

Fig. 4.

Absolute LSR after the ingestion of 1.5°C (open circles) and 50°C (black circles) fluid through a nasogastric tube (NG trials) during exercise. Dashed lines denote when fluids were ingested. Values given are the mean (A) of the following three locations: forehead (B), upper back (C), and forearm (D). ^#Where 1.5°C < 50°C. Error bars indicate SD.

Fig. 5.

Mean LSR after mouth-swilling (SW trials) 1.5°C (open circles) and 50°C (black circles) fluid during exercise. Dashed lines denote when mouth-swills were administered. Values given are the grand mean (A) of the following three locations: forehead (B), upper back (C), and forearm (D). Error bars indicate SD.

Fig. 6.

Mean body temperatures using a 0.9/0.1 weighting of core to skin temperatures using T_au (left) and T_re (right) as an indication of the body core, with 1.5°C (open circles) and 50°C (black circles) fluid during the NG trials (top) and SW trials (bottom). Dashed lines denote when fluids were ingested/administered. Error bars indicate SD.