Cold Defence Thermoeffector Responses are Modified by Ingested Fluid Temperature Independently of Differences in Mean Body Temperature
Authors: Morris Nathan B , Filingeri Davide , Halaki Mark , Jay Ollie 
Published: November 2023

We previously demonstrated that in the heat, warm and cold fluid administration via ingestion and direct perfusion to the stomach via nasogastric tube, but not by mouth swilling, proportionately modifies thermoeffector (i.e. sudomotor) responses in the heat, independently of changes in core and skin temperatures; thereby indicating independent thermoeffector modification by abdominal thermoreceptors. The purpose of the present study was to determine whether these thermoreceptors exhibited similar control over cold defence thermoeffector responses (i.e. shivering). On four separate occasions, nine males (age: 27±5 y, height: 173±7 cm, mass: 75.8±7.3 kg) were cooled by passing 5°C water through a two piece water perfusion suit covering the entirety of the body, save for the hands, feet and head. Participants ingested 1.5 ml/kg (SML) and 3.0 ml/kg (LRG) of 7°C (CLD), 22°C (MOD), 37°C (NEU) or 52°C (HOT) water after 40 and 60 min of cooling, respectively. Rectal (Tre) and mean skin (Tsk) temperature, expressed together (0.9Tre +0.1Tsk) as mean body temperature (Tb), metabolic rate (M), heart rate (HR) and mean electromyography (EMG) from 8 muscle groups were measured continuously. No differences in Tb were observed between trials following the SML (CLD: 35.7±0.5°C, MOD: 35.5±0.5°C, NEU: 35.5±0.5°C, HOT: 35.5±0.4°C; P=0.409) or LRG ingestion (CLD: 35.4±0.5°C, MOD: 35.2±0.5°C, NEU: 35.3±0.5°C, HOT: 35.2±0.5°C; P=0.641). Relative to the NEU (187±42 W), M was lower in the HOT trial (169±39 W; P=0.047) but not different in the CLD (197±66 W; P=0.436) or MOD (184±51 W; P=0.819) trials, following SML ingestion. Following LRG ingestion, relative to NEU (222±54 W), M was lower in the HOT trial (174±35 W; P=0.013) and greater in the CLD (244±56 W; P=0.033) and MOD (255±75 W; P=0.025) trials but not different between the CLD and MOD (P=0.584) trials. No differences in EMG activity, expressed as a percentage of maximal voluntary contraction (%MVC), were observed following the SML ingestion (CLD: 3.5±2.7%MVC, MOD: 3.6±2.6%MVC, NEU: 3.4±2.3%MVC, HOT: 2.6±1.8%MVC; P=0.275). Following the LRG ingestion, the EMG results mirrored those of M, with lower values relative to NEU (4.0±2.3%MVC) in the HOT trial (3.1±2.1%MVC; P=0.015), greater values in the CLD (4.8±2.0%MVC; P=0.025) and MOD (5.2±2.4%MVC; P<0.001) trials, but similar between the CLD and MOD trials (P=0.270). No differences in HR were observed between trials with SML ingestion (CLD: 62±5 bpm, MOD: 61±6 bpm, NEU: 61±6 bpm, HOT: 61±5 bpm; P=0.807), while with LRG ingestion, relative to the NEU trial (63±5 bpm), HR was lower in the HOT (60±7 bpm; P=0.054) and greater in the MOD trial (67±7 bpm; P=0.03) but not in the CLD (65±5 bpm; P=0.27) trial. In conclusion, similar to our previous findings in the heat, cold and warm fluid ingestion modified cold defence thermoeffector responses independently of mean body temperature, however, these thermoeffector responses do not seem proportional to the ingested thermal load/debt.Support or Funding InformationMr. Morris is supported by an International Postgraduate Research Scheme award. Dr. Filingeri was supported by an Endeavour Research Fellowship. Both awards are provided by the Australian Government.