During exercise in the heat, body temperature elevations and the associated physiological strain lead to significant decrements in athletic performance (28). Several recent studies have advocated the ingestion of cold fluids or ice slurries to mitigate performance reductions in the heat (2,3,35–37). While the internal exchange of heat energy with the ingested fluid theoretically creates a greater heat sink, numerous investigations have reported sizable reductions in whole-body sweat losses (WBSLs) with cold fluid ingestion (1,2,20,21,36,39), which will in turn lead to a lower evaporative heat loss from the skin. A study from our research group (1) subsequently demonstrated using partitional calorimetry that the lower sweat production with the ingestion of cold (1.5°C) and cool (10°C) fluid relative to a thermoneutral fluid (37°C) leads to a reduction in estimated skin surface evaporative heat loss that is approximately equal to the greater internal heat exchange with the ingested cold fluid, thereby resulting in similar net heat loss and therefore similar changes in body heat storage irrespective of fluid temperature. This finding has been recently corroborated using direct measurements of evaporative heat loss via direct calorimetry (19).
Relative to a cold fluid, ice slurry ingestion approximately doubles the internal heat sink owing to the amount of heat transfer associated with the enthalpy of fusion of ice. However, it is presently unknown whether this far greater internal heat loss with ice slurry ingestion is matched by a proportionally greater reduction in sweating and therefore evaporative potential from the skin. Previous studies have reported smaller changes in body heat storage after ice slurry ingestion (2,35) compared to a thermoneutral fluid, which at a fixed metabolic heat production would require a greater internal heat loss relative to the reduction in skin surface evaporation and thus a greater net heat loss. However, these studies used the thermometric method for estimating heat storage whereby the change in mean body temperature is multiplied by the mean specific heat capacity of human tissue and total body mass (4). The inherent inaccuracies of this method during exercise in the heat are well documented (14,15,38), and the shortcomings of this method have been demonstrated with exercise in different environments (31) and with different ingested fluid temperatures (1).
Reductions in sweat rate and subsequently the potential for evaporative heat loss from the skin with cold fluid ingestion are probably modulated by independent thermoreceptors residing in the abdominal area, since changes in sudomotor output are observed without any forerunning differences in core or skin temperatures (25). However, the dynamic response of local sweat rate (LSR) and any concomitant changes in skin blood flow (SkBF) have not yet been compared between thermoneutral fluid and ice slurry ingestion, and it is possible that thermosensory input from abdominal thermoreceptors is limited. From a practical perspective, it is also important that these comparisons are conducted in warm environmental conditions, since most physical activities that require a form of cooling are typically performed in such environments (33) and are similar to the ambient conditions used in previous ice slurry studies (2,35). Moreover, higher skin temperatures arising in hot climates may mitigate the reductions in local and whole-body sweat rates previously observed in a temperate environment (25).
The first aim of the present investigation was to compare the magnitude of reduction in the potential for evaporative heat loss from the skin with the large difference in internal heat loss induced by the ingestion of ice slurry relative to the ingestion of a thermoneutral fluid (37°C) during exercise at a fixed metabolic heat production in a hot environment. The second aim was to assess whether the maintenance of heat balance after the ingestion of ice slurry is achieved solely via modulations of sudomotor output or whether supplemental modulations of SkBF occur in parallel, and determine whether these thermoeffector modulations occur independently from differences in core and skin temperature. It was hypothesized that, i) relative to 37°C fluid, the large internal heat loss with ice slurry ingestion would be paralleled by a significantly smaller reduction in evaporative potential from the skin, leading to a greater net heat loss and smaller body heat storage, and ii) both sudomotor and local SkBF would be reduced independently of any reductions in core or skin temperature.
The University of Ottawa Research Ethics Board approved the experimental protocol, and the protocol was in accordance with the Declaration of Helsinki. All volunteers in the study provided written informed consent before participating in any data collection. The participants also completed Physical Activity Readiness Questionnaires (PAR-Q) as well as American Heart Association/American College of Sports Medicine Health/Fitness Facility Pre-participation Screening questionnaires before performing any exercise.
Using G*Power 3 software (Heinrich-Heine-Universität Düsseldorf, Germany (9)) a power calculation was performed, which used an α of 0.05, a β of 0.20, and an effect size of 1.08, calculated from the mean difference in LSR after the ingestion of 37°C, and 1.5°C fluids (25), to determine a required sample size of nine individuals for the current study. As such, nine healthy and non–heat-acclimated volunteers were recruited to participate in the experiment (age, 25 ± 5 yr; body mass, 75.9 ± 12.2 kg; height, 1.77 ± 0.07 m; V˙O2peak, 50.9 ± 8.5 mL·kg−1·min−1). The participants were instructed to refrain from consuming alcohol or partaking in any strenuous physical activity for 24 h before testing, to refrain from consuming caffeine the day of testing, and to maintain a consistent routine (e.g., sleep schedules and diet) during the day before and the day of experimental sessions. All sessions were separated by at least 48 h.
During an initial visit to the laboratory, body mass, height, and peak rate of oxygen consumption (V˙O2peak) were measured. After a light warm-up, V˙O2peak was measured using a breath-by-breath metabolic cart (Vmax Encore, Carefusion, Yorba Linda, CA, USA) during an incremental test on an upright cycle ergometer beginning at an external power output of 100 W and increasing by 20 W every minute thereafter until volitional exhaustion.
The study consisted of two experimental sessions, which were performed in a counterbalanced order. During each session, participants exercised at approximately 55% of their V˙O2peak on an upright cycle ergometer for 75 min in a climate-controlled room regulated at 33.5°C ± 1.4°C and 23.7% ± 2.6% RH. These conditions were selected to investigate whether reductions in sweating remained proportional to the ingested heat sink when exposed to a warmer environment while maintaining conditions at a level that should ensure 100% sweat evaporation (29). The absolute external workload was maintained between trials to produce identical power outputs and similar metabolic heat production values. Three 46-cm vertically stacked mechanical fans were placed 1.25 m in front of the participants and were set to high, which yielded an air speed velocity of 2.25 m·s−1. The participants were given three serial aliquots of water (3.2 mg·kg−1) to ingest at 15-min intervals for the first 45 min of the session (i.e., after 15, 30, and 45 min of exercise). The temperature of the ingested water was 37°C or a 1:2 mixture of shaved ice and 1.5°C water, depending on the session. Before the onset of exercise, there was a 30-min baseline period within the climate chamber during which the participants were instrumented and resting measurements were obtained. Fully instrumented body mass measurements were taken 5 min before the beginning of exercise and once again immediately upon completion of exercise, on a platform scale with an accuracy of ±2 g (Combics 2; Sartorius, Mississauga, ON, Canada). The participants also gave urine samples upon arrival to the laboratory, which were analyzed for urine specific gravity (USG) using a refractometer (Reichert TS 400; Reichert Analytical Instruments, Depew, NY, USA). A preexercise cutoff USG value of <1.020 was used to ensure that all participants were adequately hydrated before starting each experimental trial (16).
Rectal temperature (Tre) was measured with general-purpose pediatric thermistor probe (TM400, Covidien, Mansfield, MA, USA) inserted to a depth of 15 cm past the anal sphincter. Skin temperature (Tsk) was measured using thermistors integrated into heat flow sensors (2252 Ohms, Concept Engineering, Old Saybrook, CT, USA) secured to shaved and cleaned skin with double-sided adhesive discs and surgical tape (Transpore, 3M, London, ON, Canada) at four sites. Mean Tsk was expressed as an average of the four sites using the weighting of 30% chest, 30% arm, 20% thigh, and 20% calf (30). Mean body temperature (Tb) was estimated using a weighting of 0.9 Tre and 0.1 Tsk (13,26). Thermometric measurements were displayed in real time using LabView (version 7.0; National Instruments, Austin, TX, USA) at a sampling rate of 5 s.
Local sweat rates (LSRs) were measured using three 4-cm2 ventilated capsules secured to the skin with surgical tape on the forearm, upper back, and forehead. Influent anhydrous air was measured individually for each capsule and flowed through the capsules at a rate of 1.00 L·min−1 (Omega FMA-A2307, Omega Engineering, Stamford, CT, USA), and the effluent air was directed to factory-calibrated capacitance hygrometers (HMT333, Vaisala, Vantaa, Finland), which measured temperature and humidity every 5 s. Local sweat rate values were calculated using the flow rate and the difference in water vapor content between influent and effluent air. These values were normalized to the surface area of skin underneath the capsule and expressed in milligram per minute per square centimeter. As the change in LSR does not differ between the selected measurement sites after fluid ingestion (25), LSR data were analyzed and expressed as an unweighted mean of these three sites. Whole-body sweat losses (WBSLs) were estimated from pre-exercise and postexercise body mass measurements, corrected for metabolic, saliva, and water vapor losses via respiration (24) and for weight gain through fluid ingestion.
Local SkBF was measured using laser-Doppler velocimetry (PeriFlux System 5000; Perimed AB, Stockholm, Sweden) on the upper back by affixing an integrated probe to the skin above the trapezius midway between the C-7 vertebrae and the acromion process, near the upper back ventilated sweat capsule, as has been previously done (12). Before exercise, 30 min of baseline SkBF data were collected. After exercise, the participant remained seated while a local heater encompassing the laser-Doppler probe was heated to 42°C, and maximum SkBF was determined when SkBF arbitrary unit (AU) values reached a plateau (∼45 min after exercise). Values were expressed as percentage of maximum arbitrary unit values (%maxAU) (7).
Heart rate (HR) was recorded at 5-s intervals throughout the trial using a Polar RS400X coded transmitter and stored with a Polar Advantage interface and Polar Precision Performance software (Polar Electro Oy, Kempele, Finland).
The temperature of the water to be ingested was carefully maintained until 2 min before each time of ingestion for the fluid to be prepared. In the ice slurry trials, cold water was poured into an insulated thermos 2 h before the onset of exercise and then kept in a refrigerator until needed. Additionally, a commercially available ice shaver (Hamilton Beach, Richmond, VA, USA) was used to create ice flakes, which were stored in a freezer until used. Two minutes before ingestion, the ice flakes and 1.5°C water were mixed in a 1:2 ice-to-water ratio to create ice slurry. This timing was used to accurately assess the precise mass and temperature of the ice and water and to minimize the heat lost from either the ice or the water to the surrounding environment. A hydrostatic controlled water bath (Polyscience – DA05A, Niles, IL, USA) was used to warm the water for the 37°C trials. The temperature of the water was measured just before ingestion using a factory-calibrated glass thermometer (Durac Plus, Blue Spirit, precision thermometer, Cole-Parmer) with a certified range between −1°C and +51°C with an accuracy of ±0.1°C. Fluid and ice temperatures were measured immediately before ingestion, and these precise values were used for internal heat loss calculations (see equations 2 and 3).
Heat balance calculations.
Metabolic energy expenditure (M)
Metabolic energy expenditure was calculated from minute-average values for oxygen consumption (V˙O2) in liters per minute and the respiratory exchange ratio (RER) (27):
where ec is the caloric equivalent per liter of oxygen for the oxidation of carbohydrates (21.13 kJ) and ef is the caloric equivalent per liter of oxygen for the oxidation of fat (19.62 kJ). Subsequently, when the amount of mechanical work performed (W) is subtracted from M, metabolic heat production (M-W) is determined (29).
Heat exchange with ingested fluid (Hfluid)
Heat exchange with ingested fluid (Hfluid) was determined for the 37°C trial as:
where Tfluid is the ingested fluid temperature, Cp(fluid) is the specific heat capacity of water (4.184 J·g−1·°C−1), and massfluid is the mass of the ingested fluid in kilogram. Alternatively, Hfluid in the ice slurry trials was determined as:
where 0 is the temperature at which ice melts (0°C), Tice is the temperature of the ice, Cp(ice) is the specific heat capacity of ice (2.108 J·g−1·°C−1), massice is the mass of the ice in kilogram, and Hice is the enthalpy of the fusion of ice or the amount of energy lost when ice melts (334 J·g−1).
Evaporative heat loss
The estimated potential for evaporative heat loss (Esk) was determined as:
where BMpre and BMpost are the masses of the participant before and after exercise, respectively; masssaliva is the saliva losses collected from the mouthpiece throughout the trial; me is the mass of evaporative water loss from respiration; Hsweat is the enthalpy of sweat, assumed to be equal to the enthalpy of the evaporation of water (2427 J·g−1) (10,16); and mr is metabolic mass loss (i.e., carbon dioxide produced as a metabolic byproduct, which is subsequently exhaled) calculated (18) as:
where t is time in seconds. Subsequently, the net heat loss (HLnet), was calculated by summing all available avenues for heat loss, defined as:
where C and R are convective and radiative heat loss (determined using air speed, ambient temperature and Tsk), respectively, and Cres and Eres are convective and evaporative heat loss due to respiration (10,17,27), respectively. Similarly, total body heat storage (S) was calculated by summing the different factors contributing to heat exchange as such (11):
Finally, the evaporative requirement for heat balance (Ereq) was calculated using:
Whereas the primary outcome variables of interest have been fully defined above, the complete breakdown and explanation of all the variables pertaining to these calculations can be found in previous publications from our laboratory (1,8), as well as original sources (11), reviews (10,17,27), and textbooks (18,29).
All data are expressed as means ± SD. The difference between change in evaporative heat loss at the skin (ΔEsk) and evaporative requirement for heat balance (ΔEreq) in the ice slurry trial relative to 37°C trial was compared using a two-tailed paired Student t-test. To assess the LSR, SkBF, HR, Tre, Tsk, and Tb, seven 1-min averages from six time points (7 min pre-exercise and minutes 9–15, 17–23, 32–38, 47–53, and 69–75 of exercise), corresponding with the 7 min before the start of exercise, the 7 min before the first ingestion during exercise, the 7 min after each fluid ingestion during exercise, and the final 7 min of exercise were analyzed. Thermometry, SkBF and LSR data were analyzed using a two-way repeated-measures ANOVA with the repeated factors of exercise time (six levels: baseline, 9–15, 17–23, 32–38, 47–53, and 69–75 min) and fluid (two levels: 37°C and ice slurry [ICE]). The effect size of each ANOVA was calculated and reported as an eta-squared value (η2), where 0.01 is a small effect size, 0.09 is a medium effect size, and 0.25 is a large effect size (6). All heat balance parameters (M-W, C + R, Cres + Eres, Esk, Hfluid, HLnet, and S) and WBSL data were analyzed using two-tailed paired Student t-tests.
To further assess the change in LSR and SkBF after each fluid ingestion, the data for the 14 min after ingestion were subtracted from the 1-min average before each ingestion and was calculated separately for the ICE and 37°C fluid trials; and the values from the 37°C trial were subsequently subtracted from the values from the ICE trials, thus isolating the independent influence of fluid temperature on changes in LSR and SkBF as has been done previously (25). These data were assessed using a two-way repeated-measures ANOVA using the independent variables of postingestion time (0–14 min) and fluid.
When significant main effects or interactions were found, independent differences were assessed using a two-tailed paired Student t-tests while maintaining a fixed probability (5%) of making a type I error using a Holm–Bonferroni correction. The effect size of each t-test was calculated and reported as Cohen d, where 0.20 is a small effect size, 0.50 is a medium effect size, and 0.80 is a large effect size (6). All statistical analyses were performed with GraphPad Prism (version 6.0, GraphPad Software, La Jolla, CA).
Urine specific gravity
By design, USG was lower than the cutoff for euhydration (<1.020) and did not differ between trials (37°C, 1.016 ± 0.006; ICE, 1.013 ± 0.007; P = 0.29; d = 0.46).
Whole-body sweat loss
Whole-body sweat loss was 191 ± 122 g lower (P < 0.001; d = 2.18) in the ICE compared to the 37°C trials (Fig. 1).
Heat balance parameters
Metabolic heat production (M-W) (P = 0.81; d = 0.08), dry heat loss (C + R) (P = 0.35; d = 0.33), and respiratory heat loss (Cres + Eres) (P = 0.35; d = 0.33) were all similar between trials (Table 1). Hfluid was 200 ± 20 kJ greater in the ICE relative to the 37°C trial, whereas the estimated potential for evaporative heat loss (Esk) was 381 ± 199 kJ lower with ICE ingestion compared to 37°C fluid ingestion (P < 0.001; d = 1.91).
Changes in Ereq relative to changes in Esk and HLnet
The reduction in Esk was greater than the reduction in evaporation required for heat balance (Ereq) in the ICE trial (P = 0.05; d = 0.82) (Fig. 2); and consequently, net heat loss (HLnet) was 131 ± 120 kJ lower (P = 0.01; d = 1.09) and heat storage (S) was 148 ± 105 kJ greater in the ICE compared to 37°C trial (P = 0.05; d = 0.82) (Table 1).
After 75 min of exercise, the change in Tre from baseline was similar (P = 0.92; η2 < 0.001) after the ingestion of 37°C fluid (0.86°C ± 0.23°C) and ICE (0.93°C ± 0.19°C) (Fig. 3A). Moreover, skin temperature (Tsk) was not different between trials throughout (P = 0.71; η2 < 0.01) and at end-exercise, ΔTsk was 0.36°C ± 0.38°C and 0.31°C ± 0.32 °C in the 37°C and ICE trials, respectively (Fig. 3B). Finally, the change in mean body temperature (Tb) was similar (P = 0.90; η2 < 0.001) throughout, and after 75 min of exercise was 0.81°C ± 0.22°C and 0.87°C ± 0.19°C for the 37°C and ICE trials, respectively (Fig. 3C).
Local sweat rate
Ingestion of the different fluids resulted in a difference in LSR (P < 0.001; η2 = 0.07), where relative to the 37°C trial, LSR was lower by 0.16 ± 0.14 mg·min−1·cm−2 in ICE trial (Fig. 4A). After standardizing LSR in the ICE trials relative to the 37°C fluid trial, a significant interaction between fluid and time (P < 0.001; η2 = 0.05) was observed with LSR in the ICE trial was lower than in the 37°C trial from 2 to 13 min after ingestion, by an average of 0.07 ± 0.04 mg·cm−2·min−1 (Fig. 4B).
Skin blood flow
There was a trend for a different SkBF between trials (P = 0.06; η2 = 0.02), with mean SkBF 5.4% ± 13.4%maxAU lower in the ICE trial relative to the 37°C trial (Fig. 4C). After standardizing SkBF in the ICE trials relative to the 37°C fluid trial, no differences (P = 0.85, η2 = 0.02) were found (Fig. 4D).
Heart rate was not different between the 37°C (mean, 119 ± 22 bpm) and ICE (mean, 122 ± 20 bpm) trials (P = 0.47; η2 < 0.001).
This is the first study to assess the influence of ICE ingestion on human heat balance using partitional calorimetry and on intertrial differences in dynamic SkBF and LSR responses. Contrary to our initial hypothesis, the large Hfluid after ICE ingestion was not paralleled by a smaller reduction in Esk; rather, the reduction in Esk with ICE ingestion was almost double Hfluid leading to lower net heat loss and greater whole-body heat storage compared to that observed with 37°C fluid ingestion. After slurry ingestion, LSR decreased suddenly and a trend for a lower SkBF was also observed. Concomitantly, similar Tre, Tsk, and Tb to the 37°C fluid trial were evident throughout. Collectively, these results suggest that from the perspective of maintaining heat balance during exercise in warm and dry conditions (33.5°C, 24% RH), ICE ingestion may be disadvantageous. Furthermore, the responsive capacity of previously proposed abdominal thermoreceptors (25) was sufficient to alter sweating to such an extent that an internal heat sink twice as large as was previously tested was negated.
In the present study, ICE ingestion during exercise, relative to 37°C fluid ingestion, led to a disproportionately greater reduction in Esk compared to the difference in Hfluid with ICE ingestion. In a recent publication investigating the effect of warm (50°C) and cold (1.5°C) water ingestion on heat balance during exercise using direct calorimetry (19), the authors observed no difference in body heat storage between trials indicating proportional changes in Esk with different levels of Hfluid. Using the partitional calorimetry method, we previously observed a similar finding with 1.5°C, 10°C, and 37°C fluid ingestion (1). Following on from these studies, it seems the ingestion of a substance that generates a sufficiently large heat sink suppresses sweating to a greater extent than needed to balance the heat lost internally. Therefore, contrary to the conclusions of earlier studies using thermometry (2,3,35), the present study provides evidence that ICE relative to thermoneutral fluid ingestion actually causes a lower net heat loss, and consequently greater net heat storage, during exercise.
Notwithstanding the differences in calorimetrically estimated net heat loss, no differences in core, skin, or mean body temperature were observed between the ICE and 37°C trials throughout exercise, despite large reductions in LSR and a tendency toward lower SkBF after ICE ingestion. These findings lend further support to our previous proposition that thermoreceptors in the abdomen elicit changes in thermoeffector responses independently from core and skin temperature (25). Together, these studies indicate a sizable capacity of internal thermoreceptors to detect intra-abdominal temperature changes and to initiate strong thermoeffector responses that equal or even surpass the thermal load administered internally. From a mechanistic point of view, these findings are important because they demonstrate that abdominal thermoreceptors in humans possess sufficient representation and integration within the central nervous system to elicit thermoeffector responses that modify skin surface heat loss to the environment by equal or greater amounts relative to the one applied internally.
Skin blood flow is considered to be important for thermoregulation, as it supplies fluid for sweat production and heats the skin to facilitate elevations in sensible and, most importantly, evaporative heat loss (29). Additionally, SkBF can influence LSR independently from core and skin temperature (40). Previous research using laser Doppler velocimetry reported no difference in SkBF with ICE relative to a thermoneutral fluid ingestion (2), whereas other research using venous occlusion plethysmography observed decreases in forearm blood flow from baseline levels with cold (0.5°C) water ingestion, regardless of ambient air temperature (39). In the present study, a trend (P = 0.06) for a lower SkBF after ICE ingestion was observed. However, when accounting for preingestion differences in SkBF, no difference was observed (P = 0.85). Indeed, by comparing panels B and D from Figure 4, LSR is clearly and consistently reduced in response to ICE ingestion, whereas SkBF is primarily reduced after the first ICE ingestion, which suggests SkBF did not independently modify LSR.
Previous performance studies using a self-selected exercise intensity demonstrated that ICE ingestion leads to an increased work rate and a subsequently greater core temperature (3,35), certainly owing to the greater metabolic heat production required for an increased effort or running pace. Additionally, whereas core and skin temperatures were not greater in the ICE trials despite a greater calculated heat storage, with prolonged exercise, as is common in cycling, higher body temperatures may emerge. Presently, new recommendations based on thermometric estimations of heat storage, the inaccuracies of which are well documented (14,15,38), advocate ice slurries as beneficial for both performance and thermoregulatory purposes and have postulated that the benefit of the heat sink afforded by ICE ingestion supersedes the need to maintain hydration status (2,34,35). The present findings seem to contradict this proposition for ice slurries ingested during exercise. Therefore, as ICE ingestion during self-paced exercise greatly increases exercise intensity and therefore metabolic heat production, while at the same time seems to negatively affect net heat balance, we suggest caution when recommending to forgo water ingestion in favor of ICE beverages during exercise. This caution may be particularly valid for novice/recreational athletes, as it is well established that these individuals are at greater risk of heat-related illness compared to well-trained athletes (5). Furthermore, although not measured formally, every participant in the present study reported a strong discomfort while ingesting the ICE beverages, which may lead to decreased fluid ingestion during a bout of exercise. As body heat storage is the same irrespective of ingested water temperature between 50°C and 1.5°C (1,19), we recommend water should be prepared at the temperature most palatable for the individual athlete, and therefore most likely to encourage fluid ingestion and thus best maintain hydration status.
From a heat balance point of view, the potential benefit of ICE ingestion is likely dependent on timing and/or situation. Ice slurry and cold water ingestion before exercise effectively reduces core temperature (21,32), whereas during exercise, they do not reliably alter the core temperature response (2,20). This ineffectiveness is probably the result of rapid changes in the output of “primed” or already active sweat gland in response to an internal thermal load (26) as supported by the LSR traces of the present and previous investigations (25). Alternatively, cold fluid ingestion before the commencement of any thermoeffector response may allow core temperature to drift to the lowest point of the interthreshold zone, or the zone in which core temperature can change without activation of thermoeffector responses (22,23). Additionally, uncompensable heat stress, i.e., the point at which heat production exceeds the maximal rate at which heat can be lost to the environment (33), is another situation in which ICE ingestion could be particularly beneficial. Here, any reductions in sweating would likely not decrease evaporative heat loss, but a supraphysiological heat sink will still be provided; however, under self-paced exercise conditions, the balance between the additional heat produced and the added heat sink provided is unknown and may lead to greater heat storage. Future studies examining the influence of ingested fluid temperature on whole-body heat balance during steady-state and self-paced exercise under uncompensable heat stress conditions are therefore required.
Partitional calorimetry was used to estimate net heat loss in the present investigation, and like all calorimetric methods, it has some limitations. Specifically, since evaporation from the skin was estimated from body mass changes, any dripped sweat would have led to an overestimation of heat loss. However, in the ICE trials of the present study, any unbeknownst nonevaporated sweat losses would have led to an overestimation of Esk, and we observed a disproportionately lower, not higher, evaporative heat loss with ICE ingestion relative to internal heat exchange. Nonetheless, corroboration of the present findings by duplicating our study with direct calorimetry may be useful. Similarly, any melting of the ICE during preparation and/or delivery does not explain the difference in net heat loss in the ICE trial relative to 37°C in the present study. Additionally, whereas Tre and Tb were not different at any time point between trials, we were unable to measure hypothalamic temperature in the present investigation, and as such, differences in hypothalamic temperature, the temperature most responsible for dictating thermoeffector responses, could be different between trials but went undetected. However, this scenario seems unlikely due to the large differences in thermoeffector response with no difference in core temperature at any point in time.
Contrary to previous investigations that calculated heat storage using thermometry, we observed, using partitional calorimetry, that compared to 37°C fluid ingestion, ICE ingestion resulted in lower, rather than greater, net heat loss and subsequently a greater heat storage, during exercise in an approximately 34°C/20% RH environment. The difference in net heat loss and heat storage between trials occurred owing to a disproportionately large reduction in whole-body sweating, and therefore evaporative heat loss from the skin, relative to the amount of heat lost internally to the ingested ICE. No differences in rectal, skin, or mean body temperatures were detected at any time point between 37°C and ICE trials, whereas large reductions in LSR were observed in the ICE trial. These results add to previous findings that abdominal thermoreceptors can modify thermoreffector responses, even when the imposed heat sink is doubled and environmental temperatures are higher than those of previous findings. Taken together, these results indicate that ICE ingestion during exercise does not lead to a preferential thermal-homeostatic state but rather result in a lower net heat loss and thus a greater heat storage during steady-state exercise at a fixed metabolic heat production in hot and dry conditions. Following from these results, we suggest that, contrary to recent recommendations, water of any temperature, but not ICE beverages, should be consumed during athletic competition in hot-dry climates, especially for novice athletes. However, ice slurries consumed as a precoolant and during exercise in hot humid environments are probably beneficial.
This research was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada (#386143-2010, held by Ollie Jay). Funding for the metabolic cart and dew point sensor were provided by a Canadian Foundation for Innovation (CFI) Infrastructure grant (co-applicant: O. Jay). G. Coombs was supported by University of Ottawa Master’s Scholarships, and N. B. Morris was supported by an International Postgraduate Research Scholarship from the University of Sydney.
The authors thank the participants who volunteered for this study and M.N. Cramer and N.M. Ravanelli for their help during data collection.
The authors declare no conflict of interest. The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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