Nathan B. Morris

Ice Slurry Ingestion Leads to a Lower Net Heat Loss during Exercise in the Heat.

PURPOSE To compare the reductions in evaporative heat loss from the skin (Esk) to internal heat loss (Hfluid) induced by ice slurry (ICE) ingestion relative to 37 °C fluid and the accompanying body temperature and local thermoeffector responses during exercise in warm, dry conditions (33.5 °C ± 1.4 °C; 23.7% ± 2.6% relative humidity [RH]). METHODS Nine men cycled at approximately 55% VO2peak for 75 min and ingested 3.2 mL · kg(-1) aliquots of 37 °C fluid or ICE after 15, 30, and 45 min of exercise. Metabolic heat production (M-W), rectal temperature (Tre), mean skin temperature (Tsk), whole-body sweat loss (WBSL), local sweat rate (LSR), and skin blood flow (SkBF) were measured throughout. Net heat loss (HLnet) and heat storage (S) were estimated using partitional calorimetry. RESULTS Relative to the 37 °C trial, M-W was similar (P = 0.81) with ICE ingestion; however, the 200 ± 20 kJ greater Hfluid (P < 0.001) with ICE ingestion was overcompensated by a 381 ± 199-kJ lower Esk (P < 0.001). Net heat loss (HLnet) was consequently 131 ± 120 kJ lower (P = 0.01) and S was greater (P = 0.05) with ICE ingestion compared with 37 °C fluid ingestion. Concurrently, LSR and WBSL were lower by 0.16 ± 0.14 mg · min(-1) · cm(-2) (P < 0.01) and 191 ± 122 g (P < 0.001), respectively, and SkBF tended to be lower (P = 0.06) by 5.4%maxAU ± 13.4%maxAU in the ICE trial. Changes in Tre and Tsk were similar throughout exercise with ICE compared to 37 °C fluid ingestion. CONCLUSIONS Relative to 37 °C, ICE ingestion caused disproportionately greater reductions in Esk relative to Hfluid, resulting in a lower HLnet and greater S. Mechanistically, LSR and possibly SkBF were suppressed independently of Tre or Tsk, reaffirming the concept of human abdominal thermoreception. From a heat balance perspective, recommendations for ICE ingestion during exercise in warm, dry conditions should be reconsidered.

A comparison between the technical absorbent and ventilated capsule methods for measuring local sweat rate

This study assessed the accuracy of the technical absorbent (TA) method for measuring local sweat rate (LSR) relative to the well-established ventilated capsule (VC) method during steady-state and nonsteady-state sweating using large and small sample surface areas on the forearm and midback. Forty participants (38 males and two females) cycled at 60% peak oxygen consumption for 75 min in either a temperate [22.3 ± 0.9°C, 32 ± 17% relative humidity (RH)] or warm (32.5 ± 0.8°C, 29 ± 7% RH) environment. Simultaneous bilateral comparisons of 5-min LSR measurements using the TA and VC methods were performed for the back and forearm after 10, 30, 50, and 70 min. LSR values, measured using the TA method, were highly correlated with the VC method at all time points, irrespective of sample surface area and body region (all P < 0.001). On average, ≈ 79% of the variability observed in LSR measured with the VC method was described by the TA method. The mean difference in absolute LSR using the TA method (TA-VC with 95% confidence intervals) was -0.23 [-0.30,-0.16], -0.11 [-0.21,0.00], -0.03 [-0.14,+0.08], and +0.02 [-0.07,+0.11] mg · cm(-2) · min(-1) after 10, 30, 50, and 70 min of exercise, respectively. Duplicate LSR measurements within each method during steady-state sweating were highly correlated (TA: r = 0.96, P < 0.001, n = 20; VC: r = 0.97, P < 0.001, n = 20) with a mean bias of +0.07 ± 0.14 and +0.01 ± 0.10 mg · cm(-2) · min(-1) for TA and VC methods, respectively. The mean smallest detectable difference in LSR was 0.12 and 0.05 mg · min(-1) · cm(-2) for TA and VC methods, respectively. These data support the TA method as a reliable alternative for measuring the rate of sweat appearance on the skin surface.

Running economy, not aerobic fitness, independently alters thermoregulatory responses during treadmill running

We sought to determine the independent influence of running economy (RE) and aerobic fitness [maximum oxygen consumption (V̇O 2max)] on thermoregulatory responses during treadmill running by conducting two studies. In study 1, seven high (HI-FIT: 61 ± 5 ml O2 · kg(-1) · min(-1)) and seven low (LO-FIT: 45 ± 4 ml O2 · kg(-1) · min(-1)) V̇O 2max males matched for physical characteristics and RE (HI-FIT: 200 ± 21; LO-FIT: 200 ± 18 ml O2 · kg(-1) · km(-1)) ran for 60 min at 1) 60%V̇O 2max and 2) a fixed metabolic heat production (Hprod) of 640 W. In study 2, seven high (HI-ECO: 189 ± 15.3 ml O2 · kg(-1) · km(-1)) and seven low (LO-ECO: 222 ± 10 ml O2 · kg(-1) · km(-1)) RE males matched for physical characteristics and V̇O 2max (HI-ECO: 60 ± 3; LO-ECO: 61 ± 7 ml O2 · kg(-1) · min(-1)) ran for 60 min at a fixed 1) speed of 10.5 km/h and 2) Hprod of 640 W. Environmental conditions were 25.4 ± 0.8°C, 37 ± 12% RH. In study 1, at Hprod of 640 W, similar changes in esophageal temperature (ΔTes; HI-FIT: 0.63 ± 0.20; LO-FIT: 0.63 ± 0.22°C; P = 0.986) and whole body sweat losses (WBSL; HI-FIT: 498 ± 66; LO-FIT: 497 ± 149 g; P = 0.984) occurred despite different relative intensities (HI-FIT: 55 ± 6; LO-FIT: 39 ± 2% V̇O 2max; P < 0.001). At 60% V̇O 2max, ΔTes (P = 0.029) and WBSL (P = 0.003) were greater in HI-FIT (1.14 ± 0.32°C; 858 ± 130 g) compared with LO-FIT (0.73 ± 0.34°C; 609 ± 123 g), as was Hprod (HI-FIT: 12.6 ± 0.9; LO-FIT: 9.4 ± 1.0 W/kg; P < 0.001) and the evaporative heat balance requirement (Ereq; HI-FIT: 691 ± 74; LO-FIT: 523 ± 65 W; P < 0.001). Similar sweating onset ΔTes and thermosensitivities occurred between V̇O 2max groups. In study 2, at 10.5 km/h, ΔTes (1.16 ± 0.31 vs. 0.78 ± 0.28°C; P = 0.017) and WBSL (835 ± 73 vs. 667 ± 139 g; P = 0.015) were greater in LO-ECO, as was Hprod (13.5 ± 0.6 vs. 11.3 ± 0.8 W/kg; P < 0.001) and Ereq (741 ± 89 vs. 532 ± 130 W; P = 0.007). At Hprod of 640 W, ΔTes (P = 0.910) and WBSL (P = 0.710) were similar between HI-ECO (0.55 ± 0.31°C; 501 ± 88 g) and LO-ECO (0.57 ± 0.16°C; 483 ± 88 g), but running speed was different (HI-ECO: 8.2 ± 0.6; LO-ECO: 7.2 ± 0.4 km/h; P = 0.025). In conclusion, thermoregulatory responses during treadmill running are not altered by V̇O 2max, but by RE because of differences in Hprod and Ereq.

Acute acetaminophen ingestion does not alter core temperature or sweating during exercise in hot-humid conditions

Acute acetaminophen (ACT) ingestion has been reported to reduce thermal strain during cycling in the heat. In this study, nine active participants ingested 20 mg of ACT per kg of total body mass (ACT) or a placebo (PLA), 60 min prior to cycling at a fixed rate of metabolic heat production (ACT: 8.3 ± 0.3 W/kg; PLA: 8.5 ± 0.5 W/kg), which was equivalent to 55 ± 6% VO2max, for 60 min at 34.5 ± 0.1 °C, 52 ± 1% relative humidity. Resting rectal temperature (Tre; ACT: 36.70 ± 0.17 °C; PLA: 36.80 ± 0.16 °C, P = 0.24), esophageal temperature (Tes; ACT: 36.54 ± 0.22 °C; PLA: 36.61 ± 0.17 °C, P = 0.50) and mean skin temperature (Tsk; ACT: 34.00 ± 0.14 °C; PLA: 33.96 ± 0.20 °C, P = 0.70) were all similar among conditions. At end‐exercise, no differences in ΔTre (ACT: 1.12 ± 0.15 °C; PLA: 1.11 ± 0.21 °C, P = 0.92), ΔTes (ACT: 0.90 ± 0.28 °C; PLA: 0.88 ± 0.23 °C, P = 0.84), ΔTsk (ACT: 0.80 ± 0.39 °C; PLA: 0.70 ± 0.46 °C, P = 0.63), mean local sweat rate (ACT: 1.02 ± 0.15 mg/cm2/min; PLA: 1.02 ± 0.13 mg/cm2/min, P = 0.98) and whole‐body sweat loss (ACT: 663 ± 83 g; PLA: 663 ± 77 g, P = 0.995) were evident. Furthermore, ratings of perceived exertion and thermal sensation and thermal comfort were not different between ACT and PLA conditions. In conclusion, ACT ingested 60 min prior to moderate intensity exercise in hot–humid conditions does not alter physiologic thermoregulatory control nor perceived strain.

Evidence of viscerally-mediated cold-defence thermoeffector responses in man.

Visceral thermoreceptors that modify thermoregulatory responses are widely accepted in animal but not human thermoregulation models. Recently, we have provided evidence of viscerally‐mediated sweating alterations in humans during exercise brought about by warm and cool fluid ingestion. In the present study, we characterize the modification of shivering and whole‐body thermal sensation during cold stress following the administration of a graded thermal stimuli delivered to the stomach via fluid ingestion at 52, 37, 22 and 7°C. Despite no differences in core and skin temperature, fluid ingestion at 52°C rapidly decreased shivering and sensations of cold compared to 37°C, whereas fluid ingestion at 22 and 7°C led to equivalent increases in these responses. Warm and cold fluid ingestion independently modifies cold defence thermoeffector responses, supporting the presence of visceral thermoreceptors in humans. However, the cold‐defence thermoeffector response patterns differed from previously identified hot‐defence thermoeffectors.

Warm hands, cold heart: progressive whole-body cooling increases warm thermosensitivity of human hands and feet in a dose-dependent fashion

What is the central question of this study? Investigations on inhibitory/facilitatory modulation of vision, touch and pain show that conditioning stimuli outside the receptive field of testing stimuli modulate the central processing of visual, touch and painful stimuli. We asked whether contextual modulation also exists in human temperature integration. What is the main finding and its importance? Progressive decreases in whole‐body mean skin temperature (the conditioning stimulus) significantly increased local thermosensitivity to skin warming but not cooling (the testing stimuli) in a dose‐dependent fashion. In resembling the central mechanisms underlying endogenous analgesia, our findings point to the existence of an endogenous thermosensory system in humans that could modulate local skin thermal sensitivity to facilitate thermal behaviour.

To drink or to pour: How should athletes use water to cool themselves?

It’s almost that time again. With the 2016 summer Olympics in Rio just around the corner, the season of dreams is upon us. It’s the time when we watch seemingly real life super heroes push themselves to the limit, while bringing back early memories of when we wanted to be those athletes on TV that children look up to. At the Rio games, not only will these super humans compete against each other, they must also contend with the hot and balmy conditions. Some of us, during sporting activities, may be familiar with battling against our own minds and dealing with the oppressive force of the heat, when we would happily accept any measure that alleviates the discomfort of our exertions. With respect to endurance sports in the heat, this relief can come with sipping cool water, spraying our face with a cool mist, or wrapping an ice towel around our necks. The cover photo of this edition of Temperature illustrates a scenario of two elite triathletes, Andrea Hewitt (on the left) and Rachel Klamer (on the right), dousing themselves with water from their bottles in order to cope with both the internal heat they are producing through muscular contractions, and the external heat from the surrounding environment. We sometimes see athletes self-dousing with water, or have done it ourselves, in order to attain an immediate relief from the heat rather than taking the time to drink. But is this a smart move? Shouldn’t we just drink the water instead, or even better – drink iced water? To answer this question, we must consider how much heat we can lose to water in its various forms. The most straightforward way is to do so via conduction following the ingestion of cold water. The amount of heat lost is determined by the temperature difference between the ingested water and the body core, the volume of water drunk, and the specific heat capacity of water, i.e. the amount of heat energy needed to warm up 1 g of water by 1 C, which is 4.184 J/g/ C. We can dramatically increase the amount of heat lost to water by adding ice into the mix, as the amount of heat required to melt ice, known as the latent heat of fusion, is much greater than the specific heat capacity of water at 334 J/g. It is this much greater potential for heat loss that has led to the recent trend of athletes consuming ice slurry drinks, a mixture of shredded ice and water, before or during their athletic activities. Despite this improved potential for heat dissipation, melting ice is still a far cry from the amount of heat we can lose through the evaporation of water, as just one gram of evaporated water results in the liberation of a massive 2430 J of latent heat energy. To put these different cooling strategies into context, we can directly compare heat loss potential with a fixed volume of water (Fig. 1). Assuming a core body temperature of 38 C, drinking one glass (250 ml) of 1 C water would result in a net heat loss of 39 kJ. Whereas if the contents of that glass were changed to half-water and halfice, the potential for heat loss would more than double to 81 kJ. However, if we could somehow spread that 250 ml of water across our skin surface so that it all evaporated, the resultant heat loss would be a whopping 607 kJ. One caveat is that, while it is relatively easy to ingest water without spilling it, ensuring 250 ml of water is distributed across the skin in a way that it all evaporates is much more difficult. However, it is important to keep in mind that if just 15% of that water evaporates from the skin, the heat loss would still be greater than ingesting the entire 250 ml ice slurry. Another consideration is that the exercise modality and environment in which we perform exercise may alter the effectiveness of dousing ourselves with water, based on how likely the water is to evaporate. For example, dry air and high wind speeds greatly favor evaporation, so cycling in the desert may be an ideal situation for selfdousing with water, as most of it is likely to evaporate. Conversely, high levels of ambient humidity and low air

Dissociating biophysical and training-related determinants of core temperature.

In a recent review, Dr. Mora-Rodriguez (5) concluded that core temperature is predicted by the percentage of peak oxygen uptake (%V̇O2peak) in physiologically compensable conditions and absolute heat production in uncompensable conditions (see Fig. 4 in (5)). Heat balance calculations (3) and recent evidence from our laboratory (4) suggest otherwise. High (HI) and low (LO) V̇O2peak groups matched for mass and body surface area (BSA), exercising at 540 W heat production in compensable conditions, showed similar changes in rectal temperature (Tre) and whole-body sweat losses despite vastly different relative intensities (39.7% vs 57.6% V̇O2peak) (4). Furthermore, absolute end-exercise Tre was È0.2-C lower in the HI group simply because of lower preexercise values. In contrast, exercise at 60% V̇O2peak (heat production, 844 vs 600W) yielded greater changes in Tre and absolute endexercise Tre values in the HI group, and whole-body sweat losses were greater in the HI group because of higher evaporative heat balance requirements (Ereq) (4). In compensable conditions, these findings suggest the following after eliminating differences in mass and BSA: (i) changes in Tre are determined by heat production, not %V̇O2peak; (ii) any differences in end-exercise absolute Tre between fitness groups only arise because of differences in preexercise Tre; and (iii) sweating is not altered by a high V̇O2peak. We further suggested that groups heterogeneous for body morphology may be compared for changes in Tre using a fixed heat production per unit mass (WIkg) in compensable environments. This approach explains the greater Tre changes in trained subjects at 40%V̇O2peak (8.2 vs 6.1 WIkg ) (6), with these greater changes compensated by different preexercise Tre values, leading to similar absolute end-exercise temperatures between training groups. By definition, uncompensable conditions arise when Ereq exceeds the maximum possible evaporation rate (Emax). Dr. Mora-Rodriguez suggests that Ereq 9 Emax at a similar %V̇O2peak in trained and untrained groups (see Fig. 4 in (5)). However, at a given %V̇O2peak, Ereq is lower in untrained individuals because of their lower heat production, and the primary reason that Ereq 9 Emax at the same V̇O2peak in the proposed model is the lower maximum skin wettedness (Umax) assigned to untrained individuals (Umax = 0.85). Although maximum sweat rate is probably different (1), such large Umax adjustments as a function of training status do not seem justified by the literature. A Umax of 0.85 and 1.00 were proposed originally for nonheat-acclimated and heat-acclimated individuals, respectively (2), but physical training only imparts partial acclimation (7). Even if Umax differences between training groups are as large as proposed, heat balance calculations (3) show the %V̇O2peak at which Ereq 9 Emax still should be greater in unfit/ untrained subjects with the same BSA/mass ratio. The %V̇O2peak at which Ereq 9 Emax declines with decreasing BSA/ mass ratio. Because the BSA/mass ratio of the author’s untrained group (6) was lower, it appears that a combination of different physical characteristics and assigned Umax values led to a conclusion with restricted validity. A more robust descriptor of the reported differences in Tre between training groups at high relative exercise intensities (6) may be the difference between Ereq and Emax expressed in WIkg .

Staying warm in the cold with a hot drink: The role of visceral thermoreceptors

Comment on: Morris NB, Bain AR, Cramer MN, Jay O. Evidence that transient changes in sudomotor output with cold and warm fluid ingestion are independently modulated by abdominal, but not oral thermoreceptors. J Appl Physiol (1985). 2014;116:1088-1095. PMID:24577060; doi:10.1152/japplphysiol.01059.2013. Morris NB, Filingeri D, Halaki M, Jay O. Evidence of viscerally-mediated cold-defence thermoeffector responses in man. J Physiol. 2017; 595(4):1201-1212. PMID:27929204; doi:10.1113/JP273052.

Does Cold Water or Ice Slurry Ingestion During Exercise Elicit a Net Body Cooling Effect in the Heat

Cold water or ice slurry ingestion during exercise seems to be an effective and practical means to improve endurance exercise performance in the heat. However, transient reductions in sweating appear to decrease the potential for evaporative heat loss from the skin by a magnitude that at least negates the additional internal heat loss as a cold ingested fluid warms up to equilibrate with body temperature; thus explaining equivalent core temperatures during exercise at a fixed heat production irrespective of the ingested fluid temperature. Internal heat transfer with cold fluid/ice is always 100% efficient; therefore, when a decrement occurs in the efficiency that sweat evaporates from the skin surface (i.e. sweating efficiency), a net cooling effect should begin to develop. Using established relationships between activity, climate and sweating efficiency, the boundary conditions beyond which cold ingested fluids are beneficial in terms of increasing net heat loss can be calculated. These conditions are warmer and more humid for cycling relative to running by virtue of the greater skin surface airflow, which promotes evaporation, for a given metabolic heat production and thus sweat rate. Within these boundary conditions, athletes should ingest fluids at the temperature they find most palatable, which likely varies from athlete to athlete, and therefore best maintain hydration status. The cooling benefits of cold fluid/ice ingestion during exercise are likely disproportionately greater for athletes with physiological disruptions to sweating, such as those with a spinal cord injury or burn injuries, as their capacity for skin surface evaporative heat loss is much lower; however, more research examining these groups is needed.