Body Sweat Loss Research Articles

Does diphenhydramine exert a sex-dependent effect on sudomotor responses during exertional heat stress?

Introduction: The first-generation H1 antihistamine diphenhydramine hydrochloride (DPH) is a potent antimuscarinic agent which may compromise sweat output during heat stress. DPH consumption prior to exertional heat stress could further suppress the sweating response in females due to their lower postjunctional cholinergic sensitivity of the sweat glands relative to males. Objective and Hypothesis: The purpose of the present study was to test the hypothesis that DPH would reduce sweating to a greater extent in females relative to males due to differences in cholinergic sensitivity at the sweat gland. Methods: Equal groups (10 per group) of males (24±3 y; 82.6±10.1 kg; 182±5 cm) and females (22±3 y, 67.7±7.8 kg, 171±6 cm) completed two experimental trials presented in a randomized double-blind fashion. Upon arrival to lab, participants consumed either i) a placebo (PLA) pill or ii) 50 mg DPH and then rested in a room maintained at ~29.5°C & 35% RH for 2 hours before conducting a 60-minute treadmill march at a fixed rate of heat production relative to body surface area (~245 W/m2). Rectal temperature (Trec), local sweat rate of the forearm (LSR), and whole body sweat loss (WBSL) were measured. Sex-segregated analysis was conducted for each outcome variable as the difference between PLA and DPH. Results: Baseline Trec did not differ within sex between PLA and DPH (males: -0.10±0.26°C; females: -0.05±0.33°C, P=0.70), however a higher absolute baseline Trec was observed in females compared to males (P=0.006). In comparison to PLA, DPH did not exert a sex-dependent effect on ΔTrec following 60-min of exercise (males: 0.04±0.11°C; females: 0.02±0.37°C, P=0.90). No sex-dependent effect on the onset of sweating was observed between DPH and PLA (males: -0.4±2.6 min; females: 0.6±2.4 min, P=0.34). DPH did not exert a sex-dependent effect on LSR at steady-state (males: 0.03±0.25 mg/cm2/min; females: -0.01±0.21 mg/cm2/min, P=0.71). Lastly, no sex-dependent effect on WBSL was observed between PLA and DPH (males: 14±34 g; females: 6±43 g, P=0.98). Conclusion: Current evidence suggests that sex does not independently modulate the sweating response during exertional heat stress following the consumption of 50 mg of DPH. This research is supported by Dr. Ravanelli’s Research Seed Grant from the Centre for Research in Occupational Safety & Health, and Dr. Ravanelli’s Natural Sciences and Engineering Research Council of Canada Discovery Grant (PIN#2022-05096). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Multispectral sensor fusion in SmartWatch for in situ continuous monitoring of human skin hydration and body sweat loss

Post-pandemic health operations have become a near-term reality, discussions around wearables are on the rise. How do wearable health solutions effectively deploy and use this opportunity to fill the gap between wellness and healthcare? In this paper, we will talk about wearable healthcare diagnosis, with a particular focus on monitoring skin hydration using optical multi-wavelength sensor fusion. Continuous monitoring of human skin hydration is a task of paramount importance for maintaining water loss dynamics for fitness lovers as well as for skin beauty, integrity and the health of the entire body. Preserving the appropriate levels of hydration ensures consistency of weight, positively affects psychological state, and proven to result in a decrease in blood pressure as well as the levels of “bad” cholesterol while slowing down the aging processes. Traditional methods for determining the state of water content in the skin do not allow continuous and non-invasive monitoring, which is required for variety of consumer, clinical and cosmetic applications. We present novel sensing technology and a pipeline for capturing, modeling and analysis of the skin hydration phenomena and associated changes therein. By expanding sensing capabilities built into the SmartWatch sensor and combining them with advanced modeling and Machine Learning (ML) algorithms, we identified several important characteristics of photoplethysmography (PPG) signal and spectral sensitivity corresponding to dynamics of skin water content. In a hardware aspect, we newly propose the expansion of SmartWatch capabilities with InfraRed light sources equipped with wavelengths of 970 nm and 1450 nm. Evaluation of the accuracy and characteristics of PPG sensors has been performed with biomedical optics-based simulation framework using Monte Carlo simulations. We performed rigorous validation of the developed technology using experimental and clinical studies. The developed pipeline serves as a tool in the ongoing studies of the next generation of optical sensing technology.

Thermoregulatory and Renal Responses to Simulated Heatwaves in Younger and Older Adults

The frequency, intensity, and duration of heat waves are projected to increase in the coming decades. Older adults have impaired heat-dissipating capabilities and are at a greater risk for heat-related morbidity and mortality. Many hospitalizations and deaths in this population during heat waves are due to renal complications. Indeed, age-related reductions in kidney function, altered fluid regulation, and impaired thermoregulation, may place the elderly at an increased risk for heat-related renal complications. Purpose: Therefore, we tested the hypothesis that aging augments heatwave-induced renal stress. Methods: We assessed core temperature, mean skin temperature, whole-body sweat loss, and estimated glomerular filtration rate (eGFR) via serum creatinine in healthy older (8 Female/7 Male; 70±4 years) and younger (6 Female/7 Male; 29±4 years) adults exposed to three hours of 1) very hot and dry (DRY: 47ºC, 10% relative humidity) and 2) hot and humid (HUMID: 41ºC, 40% relative humidity) heatwave conditions on different days (random order). To simulate activities of daily living, participants performed seven 5-minute periods of light exercise (~3 METS; metabolic heat production: Old: 2.98±0.42; Young: 2.96±0.34 W/kg) dispersed across the heat exposure. Additionally, participants consumed 3 mL/kg/hr of tap-temperature water to maintain euhydration. We analyzed data from each heatwave condition independently using linear mixed effect models with main effects of time (pre and post) and group (Older and Younger) or unpaired t-tests, as appropriate. Repeated measures correlation was used to determine any relation between the change (Δ) in core temperature and Δ eGFR across both heatwave conditions. Results: Following exposure to DRY, older adults had greater increases in core temperature (Older: Δ 1.46±0.42, Younger: Δ 0.74±0.28ºC, interaction p<0.01), but no difference in mean skin temperature increases (Older: Δ 2.09±1.31, Younger: Δ 1.73±0.48ºC, interaction p=0.404) or whole body sweat loss (Older: 1479±391, Younger: 1556±542 mL, p=0.661). In addition, following DRY, eGFR decreased in older but increased in younger adults (Older: Δ -4.3±5.9; Younger: Δ 4.12±4.75 mL/min/1.73 m2, interaction p<0.01). Following exposure to HUMID, older adults had greater increases in core temperature (Older: Δ 1.09±0.29, Younger: Δ 0.55±0.28ºC, interaction p<0.01) and mean skin temperature (Older: Δ 2.43±1.05, Younger: Δ1.37±1.13ºC, interaction p=0.018), but no difference in whole body sweat loss (Older: 979±304, Younger: 1011 ± 269 mL, p=0.774). eGFR increased in younger, but not older, adults (Older: Δ 0.30±4.01, Younger: Δ 6.16±3.37 mL/min/1.73 m2, interaction p<0.01) following HUMID. There was a significant negative relation between Δ core temperature and Δ eGFR (r=-0.48, p<0.01). Conclusion: These preliminary data suggest that older individuals exhibit greater increases in core temperature during heatwave exposures that may contribute to reducedrenal function. Supported by NIH R01AG069005 (CGC), F32HL154559 & K01HL160772 (JCW), and F32HL154565 (LNB). This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Perceptual responses of (sports-)clothing-body interaction simulating pre- and post-purchase experience

The appreciation of textile products highly depends on a satisfactory ‘feel’ in fabric-skin contact. The question arising is whether the haptic interpretation of a garment (by hand) is comparable to a feeling produced when it is donned or used in its intended application. Sports T-shirts made from three different fiber types (CO, PES I, PES II) were studied in a pre- and post-purchase scenario by exposing 20 female participants to a hand, a donning (pre-purchase) and running evaluation (post-purchase) in 22 °C and 50% relative humidity (RH). Objective measurements such as skin temperatures, heart rate, body sweat loss, and sweat absorption of the garments were recorded. Subjective data was collected during the fabric hand and the donning evaluation as well as within the running protocol after 5 min, 20 min, and 5 min of cool down. Perceptual responses to 12 hand-/skin-feel descriptors (e.g., rough, smooth) were rated on a scale from 0 (not at all) to 10 (completely) and a feeling of discomfort was given. No significant differences between a hand and a donning evaluation were found in the rating of the sensations. The hand evaluation provided sufficient information for a comfort response to garment wear. The pre- and post-purchase comparison found a significantly lower perception of the feeling of roughness whilst running with the CO shirt and smoothness during running in PES II. The stickiness and comfort perception increased significantly in the post-purchase wear trial. Hence, moisture on the skin provoked through running influences comfort characteristics as well as the perception on haptic cues in t-shirts. Especially surface related haptic characteristics e.g., roughness and smoothness, are reduced.

Effects of Beta‐Blockers on Thermal and Cardiovascular Strain of Adults With Coronary Artery Disease During Extreme Heat Events

IntroductionExtreme heat is associated with a greater risk of adverse cardiovascular events, especially in people with cardiovascular disease. Many patients with cardiovascular disease are prescribed beta‐blockers. Some research suggests that in young healthy adults, cardio‐selective beta‐blockers may alter heat loss capacity. However, the effects of beta‐blockers on temperature regulation in older adults during an extreme heat event are unknown. The objective of this study was to compare thermal and cardiovascular responses during simulated extreme heat events between older adults with coronary artery disease (CAD) who use and do not use beta‐blockers. It was hypothesised that the increase in heart rate and skin blood flow would be smaller in CAD patients who use beta‐blockers compared to those who do not use beta‐blockers.Method25 older adults (23 males/2 females) with stable CAD, 18 taking cardio‐selective beta‐blockers (BB: 66±8 y; 85.7±13.3 kg; 28.0±4.6 kg/m2) and 7 not taking beta‐blockers (NBB: 66±7 y; 77.1±14.6 kg; 26.1±3.9 kg/m2), were exposed to two 3‐hour simulated extreme heat events; one hot and humid (38°C, 60% relative humidity) and one hot and dry (45°C, 10% relative humidity). Rectal and mean skin temperatures, heart rate, and skin blood flow were continuously recorded. Whole body sweat losses were calculated from changes in nude body weight.ResultsAt the end of the 3‐h hot and humid heat exposure, relative to pre‐exposure baseline the change in heart rate (BB: +7±5 bpm; NBB: +11±9 bpm, p=0.20), rectal (BB: +0.57±0.27°C; NBB: +0.60±0.31°C, p=0.76) and mean skin (BB: +5.1±1.2°C; NBB: +5.1±1.1°C, P=0.91) temperatures, and whole body sweat losses (BB: 771±480 g; NBB: 798±172 g, p=0.88) did not differ between groups. The change in skin blood flow from baseline was greater in the NBB (+32±23 arbitrary units) compared to the BB (+9±7 arbitrary units; p=0.01) group. At the end of the 3‐h hot and dry heat exposure, the increase in heart rate (BB: +9±8 bpm; NBB: +9±5 bpm, p=0.93), rectal (BB: +0.61±0.40°C; NBB: +0.62±0.22°C, p=0.91) and mean skin (BB: +5.9±0.9°C; NBB: +5.7±1.3°C; p=0.79) temperatures, whole body sweat losses (BB: 1252±174 g; NBB: 1207±169, p=0.56) and skin blood flow (BB: +26±22 arbitrary units; NBB: +26±39 arbitrary units; p=0.98) from baseline did not differ between groups.ConclusionOur preliminary evidence indicates that cardio‐selective beta‐blocker use does not alter thermal or cardiovascular strain in adults with stable coronary artery disease during simulated hot and humid or hot and dry extreme heat events.

Comparison of the effect of post-exercise cooling with ice slurry ingestion between males and females.

This study aimed to compare the effects of ice slurry ingestion on post-exercise cooling in males and females. Twenty-four healthy adults (male n=12; body weight [BW], 65.8±10.3kg; female, n=12; BW, 58.2±10.0kg) participated in this study. Participants ingested 7.5g/kg of either ice slurry at -1°C (ICE) or control fluid at 20°C (CON) during recovery after cycling at 55% VO2max until the rectal temperature reached 38.5°C or exhaustion in a hot environment (controlled at 38°C, 40% relative humidity). Rectal (Tre) and skin (Tsk) temperature, ratings of thermal sensation (TS), thermal comfort (TC), heart rate (HR), mean arterial pressure (MAP), and whole body sweat loss (WBSL) were measured 60min after exercise. Ice slurry ingestion reduced Tre and TS and improved post-exercise hypotension only in females (p<0.05). In comparison, males did not receive cooling effect from post- exercise cooling with ice slurry ingestion. WBSL tended to be lower in ICE than CON in males (ICE, 454.3±172.3g; CON, 539.7±157.2g; p=0.065). In conclusion, sex differences were observed in the effects of post-exercise cooling with ice slurry ingestion.

The Impact Of Short-Term Hot Water Immersion On Heat Acclimation And Thermotolerance

BACKGROUND: Environmental heat stress increases physiological strain during exercise in non-acclimated individuals. Heat acclimation (HA) protocols are often used as countermeasures to preserve physiological function during exercise in the heat. Passive heat strategies could be a potential method of HA that reduces excess physical exertion prior to activity or relocation. PURPOSE: To determine the effect of hot water immersion (HWI) on heat acclimation and thermotolerance. METHODS: 6 males (Age: 23.8 ± 1.5; VO2max: 45.0 ± 7.5 mL/kg/min.) participated in a crossover, counterbalanced study with a four-week washout between conditions. Heat stress tests (HST) were performed PRE and POST acclimation sessions and consisted of 45 min of cycling at 50% of VO2max in 40 °C, 40% RH. Acclimation sessions were either three consecutive bouts of HWI or traditional heat-exercise training (TRAD). HWI sessions consisted of 40 min of submersion at 40 °C. TRAD sessions consisted of 40 min of cycling at 50% VO2max in 40 °C, 40% RH. Core body temperature (Tcore), heart rate (HR), rate of perceived exertion (RPE), and thermal sensation (TSS) were recorded during HSTs. Blood was drawn PRE and POST HST to determine change in plasma volume. Nude body mass was recorded before and after HSTs to calculate whole body sweat loss (WBSL). Tcore and HR were used to calculate physiological strain index (PSI). RESULTS: HWI decreased average HR (PRE: 158 ± 7; POST: 149 ± 7; p = < 0.05), peak HR (176 ± 7; 164 ± 5; p = < 0.05), and end exercise RPE (15.3 ± 1.8; 13.2 ± 1.9; p = < 0.05). HWI had no significant effect on resting TCore (37.2 ± 0.4; 36.8 ± 0.3; p = 0.66), end exercise TCore (38.7 ± 0.4; 38.4 ± 0.3; p = 0.20), PSI (7.8 ± 1.1; 7.0 ± 0.8; p = 0.56), or TSS (10.8 ± 1.0; 9.5 ± 1.2; p = 0.09). TRAD resulted in no significant changes in average HR (PRE: 153 ± 11; POST: 149 ± 11; p = 0.82), peak HR (170 ± 9; 165 ± 10; p = 0.51), end exercise RPE (15.1 ± 1.2; 13.3 ± 2.0; p = 0.87), PSI (9.3 ± 2.1; 8.6 ± 1.4; p = 0.73), or TSS (10.5 ± 0.8; 10.7 ± 1.5; p = 0.77). Plasma volume expansion (%) was observed in both groups (HWI: 6.6 ± 6.4; TRAD: 4.3 ± 2.0; p = 0.41). CONCLUSION: Three consecutive HWI sessions are effective in lowering HR during submaximal exercise in the heat. Compared to heat-exercise exposures, HWI is a method in which to more rapidly elicit a HA phenotype.

Update: Efficacy of Military Fluid Intake Guidance.

Fluid intake during military training is prescribed based on the interactions among work rates, environmental conditions, and uniform configurations. The efficacy of this guidance has not been empirically assessed in over a decade. To determine the acceptability of the fluid intake guidance, sweat losses were measured in a variety of conditions with modern uniform/body armor configurations and were then compared to prescribed fluid intakes for each condition (workload, environment, clothing). Whole body sweat losses of 324 Soldiers and 14 model simulations were measured under a variety of work intensities ((Watts) easy, moderate, hard), work durations (2-25 h), environmental conditions (White-Black flag), and uniform configurations (including Army Combat Uniform and body armor). Whole body sweat losses were then calculated relative to 4 h drinking guidance and in accordance with TB MED 507 recommended work/rest ratios. The differences between the prescribed fluid intake and sweat loss were calculated and expressed as a percent loss or gain of body weight. Values within a threshold of ±2% body water flux (BWF) were deemed an acceptable conservative starting point for performance and health concerns. Values within ±2% BWF numbered 309/338; 25 of 338 observations exceeded the +2% BWF while 4 of 338 observations exceeded the -2% BWF. When total fluid restriction was simulated, all experienced body weight loss with 151 of 338 observations exceeding the -2% BWF. When calculated using actual measured sweating rates from the laboratory and model simulations, current fluid intake guidance appears to predict with 91.4% accuracy the volume of fluid required to maintain a proper euhydrated state (±2%) during 4 h of exercise. Simulations of total fluid restriction support the necessity for fluid intake guidance so that the Warfighter's performance does not degrade. It is recommended that the current military fluid intake guidance focuses on methods for accurately tracking fluid intakes.

Circadian Rhythm Does Not Alter the Sweating Response to Exercise in Thermoneutral and Warm Ambient Temperatures

The present study sought to determine whether the diurnal fluctuation in core temperature alters the sweating response to exercise in thermoneutral (23°C) and warm (33°C) environments. Six healthy males exercised on a semi‐recumbent ergometer at a fixed evaporative requirement for heat balance (200W/m2) for 60 min at two different times of day and two ambient temperatures matched for absolute humidity (1.3 kPa); i) 0800h/23°C, ii) 0800h/33°C, iii) 1600h/23°C, and iv) 1600h/33°C. Esophageal temperature (Tes), mean body temperature (Tb), local sweat rate (LSR) measured at the arm and upper back, and whole body sweat loss (WBSL) were measured. Resting absolute Tes was higher at 1600h compared to 0800h at 23°C (0800h: 36.7±0.2°C, 1600h: 36.9±0.2°C; P=0.04) and 33°C (0800h: 36.6±0.2°C, 1600h 36.9±0.2°C; P=0.005). Similarly, end‐exercise (steady‐state) absolute Tes was higher at 1600h relative to 0800h at 23°C (0800h: 37.4±0.2°C, 1600h: 37.6±0.2°C; P=0.04) and 33°C (0800h: 37.0±0.2°C, 1600h 37.2±0.2°C; P=0.01). However the change in Tes was similar irrespective of the time of day at both 23°C (0800h: 0.7±0.1°C, 1600h: 0.7±0.2°C; P=0.37) and 33°C (0800h: 0.4±0.1°C, 1600h: 0.3±0.1°C; P=0.35). Further, mean steady‐state LSR of arm and back after 60 min of exercise was similar irrespective of time at both 23°C (0800h: 0.66±0.06 mg/cm2/min, 1600h: 0.65±0.13 mg/cm2/min; P=0.74) and 33°C (0800h: 0.62±0.11 mg/cm2/min, 1600 h: 0.58±0.11 mg/cm2/min, P=0.50). Likewise, WBSL at steady‐state (46 to 60 min) was similar between morning and afternoon trials at 23°C (0800h: 156±28 g, 1600h: 162±36 g; P=0.38) and 33°C (0800h: 147±29 g, 1600h: 158±34 g, P=0.55). While sweating commenced prior to exercise in 33°C, the onset threshold for sweating occurred at a higher Tb at 1600h compared to 800h at 23°C (0800h: 35.6±0.1°C, 1600h 35.9±0.2°C; P=0.03), however the change in Tb was similar at 23°C regardless of time of day (0800h: 0.14±0.07°C, 1600h 0.17±0.12°C; P=0.47). Lastly, sudomotor thermosensitivity (LSR‐Tb) was similar at both 23°C (0800h: 0.80±0.26 mg/cm2/min/°C, 1600h: 0.82±0.30 mg/cm2/min/°C; P=0.80) and 33°C (0800h: 0.81±0.18 mg/cm2/min, 1600h: 0.90±0.40 mg/cm2/min, P=0.87). Taken together, preliminary data suggests the difference in absolute core temperature (~0.3°C) associated with diurnal fluctuation does not alter the sweating response at a fixed evaporative requirement for heat balance in both thermoneutral and warm conditions.Support or Funding InformationN.R. was supported by an NSERC Postgraduate Scholarship‐Doctoral and a University of Ottawa Excellence Scholarship.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The Sweating and Core Temperature Response to Compensable and Uncompensable Heat Stress Following Heat Acclimation

While a greater maximal sweat rate following heat acclimation has been suggested to mitigate the rise in core temperature during uncompensable heat stress, it remains unclear whether heat acclimation will alter the core temperature response to compensable heat stress wherein heat balance is attainable. Thus, the primary aim of the present study was to evaluate the influence of complete heat acclimation on sweating and core temperature response to exercise in a compensable and uncompensable environment. A total of 8 (6 males, 2 females) unacclimated individuals were recruited. On separate days, participants exercised at a fixed rate of heat production (450 W) for 45‐mins of compensable heat stress (CHS; 37°C, 30% RH) and 60‐min of uncompressible heat stress (UCHS; 37°C, 60% RH) before and after heat acclimation. Core temperature [esophageal (Tes) and rectal (Tre)], local sweat rate of the arm (LSRarm) and back (LSRback), and whole body sweat loss (WBSL) were measured during experimental trials. Heat acclimation included an 8‐wk aerobic training intervention followed by 10 consecutive days of up to 90 minutes of treadmill walking in hot and humid conditions (38°C, 65% RH). Prior to CHS or UCHS, resting absolute Tes and Tre were lower following heat acclimation (Tes: 36.6±0.2°C; Tre: 36.8±0.1°C) relative to unacclimated (Tes: 36.9±0.2°C, P=0.002; Tre: 37.1±0.2°C, P=0.003). During CHS, the change in Tes and Tre were similar following heat acclimation (ΔTes: 0.4±0.1°C; ΔTre: 0.7±0.1°C) compared to unacclimated (ΔTes: 0.4±0.2°C, P=0.72; ΔTre: 0.7±0.1°C, P=0.61). Cumulative WBSL during CHS was marginally greater with acclimation (557±40 g) relative to unacclimated (494±59 g, P=0.01). Despite no difference in LSRback following 45‐min of CHS with or without heat acclimation (P=0.94), stead‐state LSRarm was slightly higher with acclimation (0.75±0.16 mg/cm2/min) compared to unacclimated (0.61±0.15 mg/cm2/min, P&lt;0.001). In contrast, the change in Tes and Tre during UCHS was significantly smaller following heat acclimation (ΔTes: 0.7±0.2°C; ΔTre: 0.9±0.2°C) relative to an unacclimated state (ΔTes: 1.1±0.3°C, P=0.04; ΔTre: 1.1±0.2°C, P&lt;0.001). WBSL was far greater during UCHS post‐heat acclimation (913±126 g) in comparison to prior to acclimation (671±83 g, P&lt;0.001). Further, end‐exercise LSRback and LSRarm were higher post‐heat acclimation (LSRback: 1.48±0.28 mg/cm2/min; LSRarm: 1.20±0.33 mg/cm2/min) relative to pre‐acclimation (LSRback: 1.21±0.26, P&lt;0.001; LSRarm: 0.91±0.26 mg/cm2/min, P&lt;0.001) during UCHS. Taken together, the core temperature response to compensable heat stress is similar irrespective of acclimation status, despite marginal greater whole‐body sweat rates. However, the large increases local and whole body sweat rate following heat acclimation clearly mitigated the rise in core temperature during uncompensable heat stress.Support or Funding InformationThis research was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada (#386143‐2010, held by O.J.). N.R. was supported by an NSERC Postgraduate Scholarship‐Doctoral and a University of Ottawa Excellence Scholarship.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Relationship between novel design modifications and heat stress relief in structural firefighters’ protective clothing

The purpose of this study was to investigate design modifications in structural firefighter turnout suits for their ability to reduce heat stress during firefighting activities. A secondary aim of this research established a benchmark for the manikin heat loss value necessary to achieve significant improvements in physiological comfort. Eight professional firefighters participated in five simulated exercise sessions wearing a control turnout suit and one of four turnout prototypes: Single Layer, Vented, Stretch, and Revolutionary. Physiological responses (internal core body temperature, skin temperature, physiological strain, heart rate, and sweat loss) were measured when wearing each turnout suit prototype. Results demonstrated a significant increase in work time and significant reductions in heat stress (core temperature, skin temperature, and physiological strain) when participants wore the Single Layer, Vented, and Revolutionary prototypes. An estimated garment heat loss value of 150 W/m2 was determined in order to achieve a significant reduction in heat stress.

Decreased thermal sweating of central sudomotor mechanism in African and Korean men.

Tropical natives sweat less and preserve more body fluid than temperate natives, tolerating heat stress. However, the mechanisms involved in such sweating reduction have not been fully elucidated. We examined the sudomotor responses of men of African (n = 36) and Korean (n = 41) ancestry during hot water (43 °C) leg immersion (central sudomotor response). Correlations between mean body temperature, basal metabolic rate (BMR), and sweat rate were also examined. All procedures were done in an automated climate chamber. Local skin temperatures and BMR were measured and mean body temperature was calculated. Sweating activities which include evaporative loss rate, sweat onset time, sweat rate, sweat volume, and whole-body sweat loss volume were examined. In the heat load test, Africans showed lower mean body and local skin temperatures than Koreans before and after heating. Before and after heating, BMR declined significantly in Africans, while that of Koreans declined less. Local sweat onset time increased more in Africans than in Koreans. Local evaporative loss rate, local sweat volume, local sweat rate, and whole body sweat loss volume were reduced in Africans compared with Koreans. There were positive associations of mean body temperature and resting BMR with mean sweat rate. In conclusion, we observed a larger reduction of sudomotor activity in tropical Africans than in temperate Koreans, which was associated with their lower mean body temperature and lower resting BMR.

Commentary on Myths and Methodologies: Making sense of exercise mass and water balance.

The article by Drs Cheuvront & Montain (2017) kicks off a Mini Review Series on Myths and Methodologies, which explores the underlying principles of a selected methodology, considers appropriate and inappropriate uses and outlines best practice in physiology for particular methods or equipment. Their review concisely summarizes water and body mass balances during exercise, with explanation as to why these differ and therefore when such differences might become problematic. The sources of water and mass exchanges are well contextualized. As water generated from oxidative metabolism offsets that lost from ventilation, body water loss approximates that lost from the skin (i.e. eqn 4; or sweat loss) if ingestion and excretion are accounted for. In turn, body water loss and sweat loss approximate body mass loss minus 48 g (mJ energy expended)−1 (∼eqn 9). This equation is applied to quantify dehydration (eqn 11), before other applications are addressed, including worked examples for a marathon run by two individuals of different fitness. The matter of chemiosmotically bound water being released from macronutrients during their metabolism is then addressed because of its relevance to practical applications of these equations and its potential to affect the validity of (some) consensus fluid replacement guidelines. The net effect of liberating this existing water is concluded to be small, e.g. <200 ml water exchanged between compartments for energy expenditures up to 2000 kcal (approximately a marathon). Thus, the efficacy of drinking during exercise is considered to be synonymous with percentage dehydration. This conclusion is appealing but belies physiological and methodological complexities that remain unresolved and are beyond the scope of this short review that otherwise does an excellent job of summarizing issues of water and mass balances. Physiologically or functionally, efficacious hydration behaviour in exercise concerns not only body water balance but also its composition and distribution. Whether these are impacted meaningfully by glycogen metabolism will depend on the mass of water liberated and its potassium content, both of which remain to be clarified and are likely to vary (e.g. Maughan et al. 2007; King et al. 2008). Cheuvront & Montain (2017) also point out that glycogen is not the only substrate oxidized or releasing water during exercise (Candlish, 1981). That is true, but glycogen would usually be most important not only because it binds the most water (one to four times its own mass) but also because it is the major substrate mass oxidized in most endurance and team sports, while also fuelling the glycolysis that predominates in team sports. Glycolysis permits the release of more water relative to energy expenditure and is coincident with high heat strain and thus rate of dehydration. Of course, if the liberated water is low in volume or high in potassium it will have little effect (systemically at least), as concluded by Cheuvront & Montain (2017), but more research is needed to elucidate its role and importance.

CORP: Improving the status quo for measuring whole body sweat losses.

The measurement of whole body sweat losses (WBSL) is important to the study of body heat balance, body water balance, establishing guidelines for water and electrolyte consumption, and the study of metabolism and health. In principal, WBSL is measured by an acute change in body mass (ΔBM) in response to a thermoregulatory sweating stimulus. In this Cores of Reproducibility in Physiology (CORP) review, we revisit several basic, but rarely discussed, assumptions important to WBSL research, including the common equivalences: mass = weight = water = sweat. Sources of large potential measurement errors are also discussed, as are best practices for avoiding them. The goal of this CORP review is to ultimately improve the accuracy, reproducibility, and application of WBSL research.

The Influence Of Aerobic Training On Maximum Skin Wettedness And Its Effects During Uncompensable Heat Stress

PURPOSE: The purpose of the present experiment was to quantify how maximum skin wettedness (ωmax) is altered by aerobic training, and compare it to what is achieved following heat acclimation (HA). METHODS: Eight sedentary individuals (6 males, 2 females) participated in an 8-week aerobic training regime followed by a 9-day heat acclimation (HA) protocol. Participants completed on separate days, i) a treadmill humidity ramp protocol trial to assess ωmax; and ii) a 60-min treadmill march (450 W of heat production) in an uncompensable environment: 38°C, 60% RH, on three separate occasions: pre-training (PRE-T), post-training (POST-T), and post-heat acclimation (POST-HA); The change in rectal (ΔTre), and mean skin temperature (Tsk) were recorded. Whole body sweat loss (WBSL) was calculated as the change in nude body mass and sweating efficiency (Seff) was derived by dividing the sweating required to achieve ωmax (with 100% evaporation) by the actual whole-body sweat rate between the 30th and 60th minute of exercise. RESULTS: Aerobic training increased aerobic capacity by ∼14% (PRE-T: 45.8±11.8 ml/kg/min; POST-T: 52.0±11.1 ml/kg/min, P<0.001). In the humidity ramp trial, ωmax was lower PRE-T (0.75±0.07) compared to POST-T (0.87±0.12, P=0.01) and POST-HA (1.00±0.00, P=0.001), and POST-T was lower than POST-HA (P=0.04). In the UC trial, ΔTre was greater PRE-T (1.13±0.16°C) compared to POST-T (0.96±0.13°C, P<0.001) and POST-HA (0.96±0.20°C, P<0.001). PRE-T Tsk was higher after 60-min (38.0±0.4°C) compared to POST-T (37.2±0.9°C, P<0.001) and POST-HA (37.1±0.4°C, P<0.001). WBSL was significantly greater POST-HA (913±126 g) compared to POST-T (794±78 g; P=0.002) and PRE-T (671±83 g, P<0.001), however Seff was similar throughout (PRE-T: 67±10%; POST-T: 68±11%; POST-HA: 66±8%). CONCLUSIONS: Aerobic training and HA independently increase ωmax without altering Seff. A graded reduction in thermal strain during uncompensable heat stress is observed from PRE-T to POST-T, and to POST-HA.

Assessment of the Efficacy of Military Training Fluid Intake Guidance in a Variety of Conditions

Fluid intake during military training is prescribed based on the interactions among environmental conditions, uniform configurations and work rates. The efficacy of this guidance has not been empirically assessed for work bouts lasting >4 hours. PURPOSE: To determine the acceptability of the fluid intake guidance, sweat losses were measured in a variety of conditions and modern uniform/body armor configurations and were then compared to prescribed fluid intakes for each condition (clothing, environment, workload, duration). METHODS: Whole body sweat losses of 141 soldiers were measured over a variety of environmental conditions (White-Black flag), uniform configurations (including Battle Dress Uniform and body armor), exercise intensities (easy, moderate, heavy), and work durations (2,4, and 8 hr). Using the prescribed fluid intake guidance for each condition, the differences between the prescribed fluid intake and the total observed sweat loss were calculated. Differences were then expressed as a percent loss or gain of body weight using the following equation: [% body water flux= ((drinking volume- sweating volume)/body weight) x 100]. Values within a threshold of ±2% body water flux (BWF) were deemed acceptable. This threshold was considered the starting point for performance and health concerns. To simulate a worst-case scenario, it was assumed no urine was produced throughout testing. RESULTS: During short work durations (2 and 4hr), 0 of 75 Soldiers exceeded the +2% BWF. During longer work durations (8hr), 50 of 66 Soldiers exceeded the +2% BWF. In all conditions, 50 of 141 Soldiers (35%) exceeded the +2% BWF. In no condition did a Soldier exceed the -2% BWF. CONCLUSION: Current fluid intake guidance appears to be sufficient (no over- or under-drinking ±2% BWF) during work durations lasting ≤4 hours. However, for conditions beyond published guidance (>4hr), recommended drinking rates over-prescribe water needs in worst-case scenarios where no urine was produced. It is recommended that military fluid intake guidance be re-evaluated to include longer work durations of 8 hours. The views expressed in this abstract are those of the authors and do not reflect the official policy of the Department of Army, Department of Defense, or the U.S. Government.

The effects of a systematic increase in relative humidity on thermoregulatory and circulatory responses during prolonged running exercise in the heat

ABSTRACTThis study examined the thermoregulatory and circulatory responses, and exercise performance of trained distance runners during exercise in the heat (31°C) at varying relative humidity (RH). In a randomized order, 11 trained male distance runners performed 5 60 min steady-state runs at a speed eliciting 70% of VO2max in RH of 23, 43, 52, 61 and 71%. This was followed immediately with an incremental exercise test to volitional exhaustion. Core (Tre) and mean skin temperature (T¯sk), cardiac output (Q), heart rate (HR), and stroke volume (SV) were recorded at regular intervals. A significant (P = 0.003) main effect was detected for RH on mean body temperature (Tb), with a significantly higher Tb detected during steady-state exercise in the 61 and 71% RH compared to that in the 23% RH. During the steady-state exercise, no differences were detected in whole body sweat loss (P = 0.183). However, a significant main effect of RH was observed for HR and SV (P = 0.001 and 0.006, respectively) but not Q (P = 0.156). The time to exhaustion of the incremental exercise test was significantly reduced at 61 and 71% RH compared with 23% RH (P = 0.045 and 0.005, respectively). Despite an increase in dry heat loss, a greater thermoregulatory and circulatory stress was evident during steady-state exercise at 61 and 71% RH. This ultimately limits the capacity to perform the subsequent incremental exercise to exhaustion. This study highlighted that in a warm environment, the range of the prescriptive zone progressively narrows as RH increases.

The Zero Heat Flux Method and Sweat Loss Modeling in Sports: Attempts of Next Generation Sports Information Systems

Today professional and amateur endurance athletes commonly use biofeedback systems to monitor their training and competition pace. Body core temperature and sweat loss are crucial factors influencing physical capabilities. Unfortunately, current gold standards are invasive and therefore impractical in active situations. Several non-intrusive technologies or models have been proposed, but partially are still unsuitable for active applications, not user friendly or lack knowledge on physiological relations. This paper is a first attempt for new information systems in sports and primarily assesses the Zero Heat Flux Method (ZHF) compared to aural temperatures during activity in two environments and four body sites. Furthermore, sweat loss has been analysed in various controlled experiments presented as simple modelling approach. Results showed that during activity in this set up, optimally 66% of differences in the zero heat flux method were within common core temperature deviations of ±0.5°C. Deviations varied highly across sites, the two conditions and individuals. The same is reflected from relations between body core temperature and sweat loss, since correlation coefficients increase with a more detailed subgrouping. In conclusion, it seems that slight modifications on the sensor assembly e.g. on insulation and size could partially solve remaining deviations. On contrast it's likely that skin wetness remains a problem. Due to manifold individual and environmental influences on sweat rate, insufficient technologies and impracticable models, it remains unclear how sweat loss could be reliably measured or predicted. From today's knowledge a further attempt could be using local sweat composition as predictor of blood sodium levels that potentially could describe hypo- and even hyper-hydration.

Determination of the maximum rate of eccrine sweat glands’ ion reabsorption using the galvanic skin conductance to local sweat rate relationship.

The purpose of the present study was to develop and describe a simple method to evaluate the rate of ion reabsorption of eccrine sweat glands in human using the measurement of galvanic skin conductance (GSC) and local sweating rate (SR). This purpose was investigated by comparing the SR threshold for increasing GSC with following two criteria of sweat ion reabsorption in earlier studies such as (1) the SR threshold for increasing sweat ion was at approximately 0.2–0.5 mg/cm2/min and (2) exercise heat acclimation improved the sweat ion reabsorption ability and would increase the criteria 1. Seven healthy non-heat-acclimated male subjects received passive heat treatment both before and after 7 days of cycling in hot conditions (50% maximum oxygen uptake, 60 min/day, ambient temperature 32 °C, and 50% relative humidity). Subjects became partially heat-acclimated, as evidenced by the decreased end-exercise heart rate (p < 0.01), rate of perceived exhaustion (p < 0.01), and oesophageal temperature (p = 0.07), without alterations in whole body sweat loss, from the first to the last day of training. As hypothesized, we confirmed that the SR threshold for increasing GSC was near the predicted SR during passive heating before exercise heat acclimation, and increased significantly after training (0.19 ± 0.09–0.32 ± 0.10 mg/cm2/min, p < 0.05). The reproducibility of sweat ion reabsorption by the eccrine glands in the present study suggests that the relationship between GSC and SR can serve as a new index for assessing the maximum rate of sweat ion reabsorption of eccrine sweat glands in humans.

Improved sweat gland function during active heating in tennis athletes

BackgroundRelatively few studies on the peripheral sweating mechanisms of trained tennis athletes have been conducted. The purpose of this study was to compare the sweating capacities of tennis athletes against untrained subjects (controls). MethodsThirty-five healthy male volunteers participated including 15 untrained subjects and 20 trained tennis athletes (nationally ranked). Active heat generation was performed for 30 min (running at 60%VO2max) in a climate chamber (temperature, 25.0°C ± 0.5°C; relative humidity, 60% ± 3%, termed active heating). Sweating data (local sweat onset time, local sweat volume, activated sweat glands, sweat output per gland, whole body sweat loss volume) were measured by the capacitance hygrometer-ventilated capsule method and starch-iodide paper. Mean body temperature was calculated from tympanic and skin temperatures. ResultsLocal sweat onset time was shorter for tennis athletes (p < 0.001). Local sweat volume, activated sweat glands of the torso and limbs, sweat output per gland, and whole body sweat loss volume were significantly higher for tennis athletes than control subjects after active heating (p < 0.001). Tympanic and mean body temperatures were lower among tennis athletes than controls (p < 0.05). ConclusionThese results indicate that tennis athletes had increased regulatory capacity of their sweat gland function.