Rate Of Metabolic Heat Production Research Articles

Reduced Maximum Core Temperature on a Second, but not Third, Consecutive Workday in the Heat in Both Males and Females

Two consecutive workdays in the heat can lead to exacerbated core temperature (Tc) responses on the second day, potentially causing excessive hyperthermia. To our knowledge, no studies have investigated Tc responses across three consecutive workdays or compared responses between males and females. We hypothesized that maximum Tc would be greater on the second workday compared to the first and third day in both males and females. Fifteen participants (7 females, Age: 27±6 y, Body mass: 75.0±14.4 kg, Body fat: 16.7±9.3%) completed three consecutive days of 4 h of heavy intensity work to a 45:15 work:rest ratio in a hot environment (36.3±0.8°C, 21±5% relative humidity), abiding by national worker recommendations. Equal rest was provided between days. Tc, metabolic heat production, skin temperature (Tsk), and sweat rate were measured during work. Repeated measures analysis of variance and t-tests were used to compare responses. Maximum Tc on Day 1 (38.16±0.36°C) was greater than Day 2 (38.03±0.24°C, p=0.03), with Day 3 (38.08±0.38°C) not different from Day 1 (p=0.433) or Day 2 (p=0.70). Maximum Tc was not different between sexes on any day (all p>0.05). Resting Tc was greater on Day 1 (37.10±0.30°C) compared to Day 2 (36.96±0.32°C, p=0.02) but was not different than Day 3 (37.00±0.38°C, p=0.06). Resting Tc was not different between sexes on any day (all p>0.05). Males had a lower resting Tc on Day 2 compared to Day 1 (p=0.01) while females did not differ between these days (p=0.57). Metabolic heat production was greater on Day 1 (395±10 W) compared to Day 2 (384±12 W, p=0.01), with Day 3 (390±22 W) similar to both Day 1 (p=0.33) and Day 2 (p=0.24). There were no differences in metabolic heat production between sexes on any day (all p>0.05). Metabolic heat production was not different in males among days (all p>0.05), while females produced less metabolic heat on Day 2 (382±10) compared to Day 1 (398±8 W, p=0.02) despite completing identical work. Metabolic heat production in females on Day 2 was not different from Day 3 (394±25 W, p=0.13). Sweat rate did not differ across days (p=0.90) or between sexes (p=0.38). Resting Tsk and maximum Tsk were not different across days (p=0.28, p=0.97) or between sexes (p=0.98, p=0.19), respectively. Contrasting our hypothesis, participants experienced a lower maximum Tc on Day 2 compared to Day 1, indicating a somewhat protective effect during consecutive days of heat stress. Both males and females abiding by national worker recommendations for hot environments may see a reduced Tc on the second, but not third, day of consecutive workdays. This project was funded by the National Institute for Occupational Safety and Health (1K01OH012016-01A1). 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.

Biological sex does not independently influence time-dependent changes in core temperature and sweating of children exercising in uncompensable heat stress.

To investigate the influence of biological sex, independent of differences in aerobic fitness and body fatness, on the change in gastro-intestinal temperature (∆Tgi) and whole-body sweat rate (WBSR) of children exercising under uncompensable heat stress. Seventeen boys (mean±SD; 13.7±1.3 years) and 18 girls (13.7±1.4) years) walked for 45 min at a fixed rate of metabolic heat production per kg body mass (8 W·kg-1) in 40°C and 30% relative humidity. Sex and V̇O2peak were entered into a Bayesian hierarchical general additive model (HGAM) for Tgi. Sex, V̇O2peak and the evaporative requirement for heat balance (Ereq) were entered into a Bayesian hierarchical linear regression for WBSR. For 26 (12 M, 14 F) of the 35 children with measured body composition, body fat percentage was entered in a separate HGAM and hierarchical linear regression for Tgi and WBSR respectively. Conditional on sex-specific mean V̇O2peak, ∆Tgi was 1.00°C [90% credible intervals: 0.84, 1.16] for boys and 1.17°C [1.01, 1.33] for girls, with a difference of 0.17°C [-0.39, 0.06]. When sex differences in V̇O2peak were accounted for, the difference in ∆Tgi between boys and girls was 0.01°C [-0.25, 0.22]. The difference in WBSR between boys and girls was 0.03 L·h-1 [-0.02, 0.07], when isolated from differences in Ereq. The difference in ∆Tgi between boys and girls was -0.10°C [-0.38, 0.17] when sex differences in body fat (%) were accounted for. Biological sex did not independently influence the ∆Tgi and WBSR of children exercising under uncompensable heat stress.

Estimation of Heat Production Rate using Thermal Data During Exercise in Indoor Environments: A Study of Heat Storage Rate in Male Athletes.

The increasing preference for indoor exercise spaces highlights the relationship between indoor thermal environments and physiological responses, particularly concerning thermal comfort during physical activity. Determining the metabolic heat production rate during exercise is essential for optimizing the thermal comfort, well-being, and performance of individuals engaged in physical activities. This value can be determined during the activity using several methods, including direct calorimetry measurement, indirect calorimetry that uses analysis of respiratory gases, or approximations using collected data such as speed, body mass, and heart rate. The study aimed to calculate the metabolic heat production rate by infrared thermal evaluation (ITE) based on the body's thermal balance approach and compare it with the values determined by indirect calorimetry (IC). Fourteen participants volunteered for the study, using a cycling ergometer in a controlled climatic chamber. After the familiarization sessions, maximal O2 intake levels (VO2max) were determined through maximal graded exercise tests. Subsequently, constant work rate exercise tests were performed at 60% of VO2max for 20min. The metabolic heat production rates were calculated by IC and ITE for each athlete individually. Respiratory gases were used to determine IC, while body skin and core temperatures, along with physical environmental data, were applied to calculate ITE using the human body thermal balance approximation of ASHRAE. According to the results, heat storage rates were misleading among the body's heat transfer modes, particularly during the first 8min of the exercise. ITE showed a moderate level of correlation with IC (r: 0.03-0.86) with a higher level of dispersion relative to the mean (CV%: 12-84%). Therefore, a new equation (ITEnew) for the heat storage rates was proposed using the experimental data from this study. The results showed that ITEnew provided more precise estimations for the entire exercise period (p > 0.05). Correlations between ITEnew and IC values were consistently strong throughout the exercise period (r: 0.62-0.85). It can be suggested that ITEnew values can predict IC during the constant work rate steady-state exercise.

Phenotypic flexibility in metabolic adjustments and digestive function in white-shouldered starlings: responses to short-term temperature acclimation.

Changing the intrinsic rate of metabolic heat production is the main adaptive strategy for small birds to cope with different ambient temperatures. In this study, we tested the hypothesis that the small passerine the white-shouldered starling (Sturnus sinensis) can modulate basal metabolism under temperature acclimation by changing the morphological, physiological and biochemical state of its tissues and organs. We measured the effects of temperature on body mass, basal metabolic rate (BMR), wet mass of various internal organs, state 4 respiration (S4R) and cytochrome c oxidase (CCO) activity in the pectoral muscle and organs, metabolites in the pectoral muscle, energy intake, histological dynamics and the activity of duodenal digestive enzymes. Warm acclimation decreased BMR to a greater extent than cold acclimation. At the organ level, birds in the cold-acclimated group had significantly heavier intestines but significantly lighter pectoral muscles. At the cellular level, birds in the cold-acclimated group showed significantly higher S4R in the liver and heart and CCO activity in the liver and kidney at both the mass-specific and whole-organ levels. A metabolomic analysis of the pectoral tissue revealed significantly higher lipid decomposition, amino acid degradation, ATP hydrolysis, and GTP and biotin synthesis in cold-acclimated birds. Acclimation to cold significantly increased the gross energy intake (GEI), feces energy (FE) and digestive energy intake (DEI) but significantly decreased the digestive efficiency of these birds. Furthermore, cold-acclimated birds had a higher maltase activity and longer villi in the duodenum. Taken together, these data show that white-shouldered starlings exhibit high phenotypic flexibility in metabolic adjustments and digestive function under temperature acclimation, consistent with the notion that small birds cope with the energy challenges presented by a cold environment by modulating tissue function in a way that would affect BMR.

Influence of Biological Sex and Fitness on Core Temperature Change and Sweating in Children Exercising in Warm Conditions.

This study aimed to investigate the associations of biological sex and aerobic fitness (i.e., V̇O 2peak ) on the change in gastrointestinal temperature (∆ Tgi ) and whole-body sweat rate (WBSR) of children exercising in warm conditions. Thirty-eight children (17 boys, mean ± SD = 13.7 ± 1.2 yr; 21 girls, 13.6 ± 1.8 yr) walked for 45 min at a fixed rate of metabolic heat production (8 W·kg -1 ) in 30°C and 40% relative humidity. Biological sex and relative V̇O 2peak were entered as predictors into a Bayesian hierarchical generalized additive model for Tgi . For a subsample of 13 girls with measured body composition, body fat percent was entered into a separate hierarchical generalized additive model for Tgi . Sex, V̇O 2peak , and the evaporative requirement for heat balance ( Ereq ) were entered into a Bayesian hierarchical linear regression for WBSR. The mean ∆ Tgi for boys was 0.71°C (90% credible interval = 0.60-0.82) and for girls 0.78°C (0.68-0.88). A predicted 20 mL·kg -1 ·min -1 higher V̇O 2peak resulted in a 0.19°C (-0.03 to 0.43) and 0.24°C (0.07-0.40) lower ∆ Tgi in boys and girls, respectively. A predicted ~13% lower body fat in the subsample of girls resulted in a 0.15°C (-0.12 to 0.45) lower ∆ Tgi . When Ereq was standardized to the grand mean, the difference in WBSR between boys and girls was -0.00 L·h -1 (-0.06 to 0.06), and a 20-mL·kg -1 ·min -1 higher predicted V̇O 2peak resulted in a mean difference in WBSR of -0.07 L·h -1 (-0.15 to 0.00). Biological sex did not independently influence ∆ Tgi and WBSR in children. However, a higher predicted V̇O 2peak resulted in a lower ∆ Tgi of children, which was not associated with a greater WBSR, but may be related to differences in body fat percent between high and low fitness individuals.

Biophysical, thermo-physiological and perceptual determinants of cool-seeking behaviour during exercise in younger and older women.

Women continue to be under-represented in thermoregulatory research despite their undergoing unique physiological changes across the lifespan. This study investigated the biophysical, thermo-physiological, and perceptual determinants of cool-seeking behaviour during exercise in younger and older women. Eleven younger (25±5years; 1.7±0.1m; 63.1±5.2kg) and 11 older women (53±6years; 1.7±0.1m; 65.4±13.9kg) performed a 40-min incremental cycling test in a thermoneutral environment (22±1.7°C; 36±4% relative humidity). Throughout the test, participants freely adjusted the temperature of a cooling probe applied to their wrists to offset their thermal discomfort. We continuously recorded the probe-wrist interface temperature to quantify participants' cool-seeking behaviour. We also measured changes in participants' rate of metabolic heat production, core and mean skin temperatures, and skin wetness. Finally, we body-mapped participants' skin heat, cold and wetness sensitivity. Our results indicated that: (1) older and younger women exhibited similar onset and magnitude of cool-seeking behaviour, despite older women presented reduced autonomic heat-dissipation responses (i.e., whole-body sweat losses); (2) older women's thermal behaviour was less determined by changes in core temperature (this being a key driver in younger women), and more by changes in multiple thermo-physiological and biophysical parameters (i.e., physical skin wetness, temperature and heat production); (3) older women did not present lower regional skin thermal and wetness sensitivity than younger women. We conclude that predictions of female cool-seeking behaviours based on thermo-physiological variables should consider the effects of ageing. These findings are relevant for the design of wearable cooling systems and sports garments that meet the thermal needs of women across the lifespan.

Gender and age effects on metabolic rates of office work, housework, and different walking speeds

Metabolic rate is an essential factor in determining human thermal comfort. Existing metabolic rates for common activities in the current standards are based on mean adult values and do not include values for the middle-aged and the elderly. This study examines the metabolic rate of 151 Chinese male and female subjects aged 20 ∼ 69 during eighteen activities (reclining, 7 types of office work including seated and standing, 5 types of housework, and 5 walking speeds) using an indirect calorimeter in lab studies. The results show that women exhibited lower metabolic heat production rates (6–9% for sedentary and standing activities, 9–13% for household activities, and 4–9% for walking activities) than men across all activities, regardless of age. No age-related declines in metabolic heat production during lower activities (reclining, sedentary, standing, and household). Rather, metabolic heat production rates increased at a rate of 5–7 W/m2 per decade with age, which is contrary to popular beliefs. Women demonstrated a distinct pattern of changes compared to men, with significant declines observed in sitting and standing activities, at a rate of 2–3 W/m2 per decade. The ISO and ASHRAE standards tended to overestimate the metabolic rate for standing position activities and significantly underestimate the metabolic rate during walking. Some simple household activities, such as folding cloth, wiping windows, and vacuuming, had a moderate metabolic rate level, significantly higher than standing quiet. These findings could inform the development of interventions to promote healthy ageing and the design of more effective strategies for managing thermal comfort in different populations. It is suggested that the metabolic rate reference table or prediction method in international standards needs modification when applied to the Chinese population.

Continuous Monitoring of Entropy Production and Entropy Flow in Humans Exercising under Heat Stress.

Complex living systems, such as the human organism, are characterized by their self-organized and dissipative behaviors, where irreversible processes continuously produce entropy internally and export it to the environment; however, a means by which to measure human entropy production and entropy flow over time is not well-studied. In this article, we leverage prior experimental data to introduce an experimental approach for the continuous measurement of external entropy flow (released to the environment) and internal entropy production (within the body), using direct and indirect calorimetry, respectively, for humans exercising under heat stress. Direct calorimetry, performed with a whole-body modified Snellen calorimeter, was used to measure the external heat dissipation from the change in temperature and relative humidity between the air outflow and inflow, from which was derived the rates of entropy flow of the body. Indirect calorimetry, which measures oxygen consumption and carbon dioxide production from inspired and expired gases, was used to monitor internal entropy production. A two-compartment entropy flow model was used to calculate the rates of internal entropy production and external entropy flow for 11 middle-aged men during a schedule of alternating exercise and resting bouts at a fixed metabolic heat production rate. We measured a resting internal entropy production rate of (0.18 ± 0.01) W/(K·m2) during heat stress only, which is in agreement with published measurements. This research introduces an approach for the real-time monitoring of entropy production and entropy flow in humans, and aims for an improved understanding of human health and illness based on non-equilibrium thermodynamics.

Core temperature responses in well-healed burn survivors during exercise heat stress: effect of cooling modalities

Introduction: Every year, ~150,000 patients are treated in the United States with burn-related injuries. Previously, our group has shown that ~75% of burn survivors have a maximal aerobic capacity in the lowest 20th percentile rankings, relative to sex and age-adjusted non-burned individuals. This level of impairment is strongly associated with cardiovascular-specific, as well as all cause, morbidity, and mortality. Moreover, burn injuries that require grafting impair thermoregulation, and the subsequent increases in skin and core temperature may dissuade these individuals from engaging in exercise and/or physical activity. Low-energy cooling strategies have potential to attenuate increases in core temperature in burn survivors, which may promote regular physical activity. Thus, the purpose of this study was to evaluate the efficacy of low-energy cooling modalities on thermoregulatory responses during physical activity in the heat. Methods: Twenty-four adults with NO-BURN (n = 9), MODERATE burns (20-40% body surface area burned, n = 7), or LARGE burns (> 40% body surface area burned, n = 8) walked for one hour at a moderate intensity (rate of metabolic heat production of 4.5 W/kg body mass) in an environmental chamber (39°C, 40% RH). Burn survivors were at least 2 years post-injury. Participants performed this activity four times, with the following modalities randomly applied throughout each trial on separate days: no cooling (i.e., control), fan at 4 m/s (FAN), water spray (WS, minimum 10-50 g every 5 min, scaled to burn area size), or WS+FAN. Core temperature (Tcore) was recorded throughout each trial via a telemetric pill. Final Tcore values for each cooling modality were compared against the no cooling condition. Results: For the LARGE burn group, WS reduced end-exercise Tcore by 0.37°C (range 0.09 to 0.66°C) compared with no cooling ( p = 0.01). FAN and WS+FAN did not impact end-exercise Tcore for the LARGE burn group ( p ≥ 0.48). No cooling modality affected Tcore for the MODERATE burn group ( p ≥ 0.12) or the NO-BURN group (p ≥ 0.55). Conclusion: At the assessed workload and environmental condition, the efficacy of the applied cooling modalities on Tcore responses for burn survivors appears to be dependent on both the selected modality and the size of the burn surface area. Namely, WS attenuated the increases in Tcore in participants with LARGE burn injuries, but FAN and WS+FAN did not attenuate Tcore responses in any group. Supported by NIHGrant R01GM068865 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.

Fluid balance during physical work in the heat is not modified by the menstrual cycle when fluids are freely available.

We tested the hypothesis that women may be more at risk of becoming dehydrated during physical work in the heat in the early follicular phase (EF), compared with the late follicular (LF) and mid-luteal (ML) phases of the menstrual cycle when allowed free access to drink. Twelve healthy, eumenorrheic, unacclimated women (26 ± 5 yr) completed three trials (EF, LF, and ML phases) involving 4 h of exposure to 33.8 ± 0.8 °C, 54 ± 1% relative humidity. Each hour, participants walked on a treadmill for 30 min at a rate of metabolic heat production of 338 ± 9 W. Participants drank a cool, flavor-preferred non-caloric sport drink ad libitum. Nude body weight was measured pre- and post-exposure, and percent changes in body weight loss were interpreted as an index of changes in total body water. Total fluid intake and urine output were measured and sweat rate was estimated from changes in body mass corrected for fluid intake and urine output. Fluid intake was not different between phases (EF: 1,609 ± 919 mL; LF: 1,902 ± 799 mL; ML: 1,913 ± 671; P = 0.202). Total urine output (P = 0.543) nor sweat rate (P = 0.907) differed between phases. Percent changes in body mass were not different between phases (EF: -0.5 ± 0.9%; LF: -0.3 ± 0.9%; ML: -0.3 ± 0.7%; P = 0.417). This study demonstrates that the normal hormonal fluctuations that occur throughout the menstrual cycle do not alter fluid balance during physical work in the heat.NEW & NOTEWORTHY The effect of the menstrual cycle on fluid balance during physical work in the heat when fluids are freely available is unknown. This study demonstrates that fluid balance is not modified in women across three distinct phases of the menstrual cycle during physical work in the heat These results indicate that when women have free access to cool fluid during physical work in the heat, they respond similarly across all three phases to maintain fluid homeostasis across the menstrual cycle.

An Overview of Exertional Heat Illness in Thoroughbred Racehorses: Pathophysiology, Diagnosis, and Treatment Rationale.

Exertional heat illness (EHI) is a complex medical disease. The thoroughbred (TB) racehorse is at considerable risk because of the intensity of its exercise activity and its high rate of metabolic heat production. The pathophysiology of EHI can combine aspects of both the heat toxicity pathway and the heat sepsis or endotoxemic pathway. Treatment regimes depend upon the detection of earliest clinical signs, rapid assessment, aggressive cooling and judicious use of ancillary medications. Ice-cold water provides the most rapid cooling, consistent with the need to lower core body temperature before tissue damage occurs. Research into EHI/HS by inducing the condition experimentally is ethically unjustifiable. Consequently, leading researchers in the human field have conceded that "most of our knowledge has been gained from anecdotal incidents, gathered from military personnel and athletes who have collapsed during or following physical activity, and that retrospective and case studies have provided important evidence regarding recognition and treatment of EHI". The authors' review into EHI shares that perspective, and the recommendations made herein are based on observations of heat-affected racehorses at the racetrack and their response, or lack of response, to treatment. From 2014 to 2018, 73 race meetings were attended, and of the 4809 individual starters, signs of EHI were recorded in 457. That observational study formed the basis for a series of articles which have been published under the title, 'EHI in Thoroughbred racehorses in eastern Australia', and forms the background for this review.

Association between haemoglobin A1c and whole-body heat loss during exercise-heat stress in physically active men with type 2 diabetes.

What is the central question of this study? Is the impairment in heat dissipation during exercise observed in men with type 2 diabetes related to glycaemic control (indexed by glycated haemoglobin; haemoglobin A1c )? What is the main finding and its importance? No association was found between haemoglobin A1c (range: 5.1-9.1%) and whole-body heat loss in men with type 2 diabetes during exercise in the heat. However, individuals with elevated haemoglobin A1c exhibited higher body core temperature and heart rate responses. Thus, while haemoglobin A1c is not associated with heat loss per se, it may still have important implications for physiological strain during exercise. Type 2 diabetes is associated with a reduced capacity to dissipate heat. It is unknown whether this impairment is related to glycaemic control (indexed by glycated haemoglobin; haemoglobin A1c ) is unknown. We evaluated the association between haemoglobin A1c and whole-body heat loss (via direct calorimetry), body core temperature, and heart rate in 26 physically active men with type 2 diabetes (43-73 years; HbA1c 5.1-9.1%) during exercise at increasing rates of metabolic heat production (∼150, 200, 250Wm-2 ) in the heat (40°C, ∼17% relative humidity). Haemoglobin A1c was not associated with whole-body heat loss (P=0.617), nor the increase in core temperature from pre-exercise (P=0.347). However, absolute core temperature and heart rate were elevated ∼0.2°C (P=0.014) and ∼6beatsmin-1 (P=0.049), respectively, with every percentage point increase in haemoglobin A1c . Thus, while haemoglobin A1c does not appear to modify diabetes-related reductions in capacity for heat dissipation, it may still have important implications for physiological strain during exercise-heat stress.

Temperature Response of Metabolic Activity of an Antarctic Nematode.

Because of climate change, the McMurdo Dry Valleys of Antarctica (MCM) have experienced an increase in the frequency and magnitude of summer pulse warming and surface ice and snow melting events. In response to these environmental changes, some nematode species in the MCM have experienced steady population declines over the last three decades, but Plectus murrayi, a mesophilic nematode species, has responded with a steady increase in range and abundance. To determine how P. murrayi responds to increasing temperatures, we measured metabolic heat and CO2 production rates and calculated O2 consumption rates as a function of temperature at 5 °C intervals from 5 to 50 °C. Heat, CO2 production, and O2 consumption rates increase approximately exponentially up to 40 °C, a temperature never experienced in their polar habitat. Metabolic rates decline rapidly above 40 °C and are irreversibly lost at 50 °C due to thermal stress and mortality. Caenorhabditis elegans, a much more widespread nematode that is found in more temperate environments reaches peak metabolic heat rate at just 27 °C, above which it experiences high mortality due to thermal stress. At temperatures from 10 to 40 °C, P. murrayi produces about 6 times more CO2 than the O2 it consumes, a respiratory quotient indicative of either acetogenesis or de novo lipogenesis. No potential acetogenic microbes were identified in the P. murrayi microbiome, suggesting that P. murrayi is producing increased CO2 as a byproduct of de novo lipogenesis. This phenomenon, in conjunction with increased summer temperatures in their polar habitat, will likely lead to increased demand for carbon and subsequent increases in CO2 production, population abundance, and range expansion. If such changes are not concomitant with increased carbon inputs, we predict the MCM soil ecosystems will experience dramatic declines in functional and taxonomic diversity.

Critical Environmental Limits for Young, Healthy Adults (PSU HEAT)

The average temperature of the Earth has been steadily increasing for decades and is expected to continue increasing throughout the 21st century. Along with the increase in average global temperatures, the frequency, duration, and severity of heat waves have increased. Thus, there is a critical need for research targeted at understanding the specific environmental conditions associated with increased risk of heat‐ and humidity‐related morbidity and mortality.PurposeTo establish critical environmental limits for healthy young adults at two metabolic rates that reflect activities of daily living and light activity. We hypothesized that critical environmental limits would be shifted toward lower combinations of temperature and humidity at higher compared to lower rates of metabolic heat production.MethodsTwenty‐five (12M/13F; 24 ± 4 yrs) subjects were exposed to progressive heat stress in an environmental chamber at two rates of metabolic heat production chosen to represent minimal activity (MinAct) or light ambulation (LightAmb). Progressive heat stress comprised either (1) constant dry‐bulb temperature (Tdb; 36, 38, or 40 °C) and increasing ambient water vapor pressure (Pa; Pcrit trials), or (2) constant Pa (12, 16, or 20 mmHg) and increasing Tdb (Tcrit trials). Chamber Tdb and Pa, and subject gastrointestinal temperature (Tgi), were continuously monitored throughout each trial. After a 30‐min equilibration period, progressive heat stress superseded until subject heat balance could no longer be maintained and a clear rise in Tgi was observed. Each subject was tested during MinAct and LightAmb in 2 or 3 randomized environmental conditions, for a total of 4–6 trials per participant.ResultsBy design, oxygen consumption (V̇O2: 0.46 ± 0.10 vs. 0.80 ± 0.16 L/min; 1.8 ± 0.3 vs. 3.2 ± 0.3 METs), metabolic heat production (82.9 ± 12.5 vs. 133.3 ± 14.8), and sweat rate (142.8 ± 80.2 vs. 230.8 ± 84.9) were lower for MinAct compared to LightAmb (all p < 0.0001). There were no differences in these variables (p ≥ 0.05) among environmental conditions for either MinAct or LightAmb intensities. Higher metabolic heat production (p < 0.001) during LightAmb compared to MinAct trials resulted in significantly lower critical environmental limits across all Pcrit and Tcrit conditions (all p < 0.001).ConclusionsThese data are the first to define critical environmental limits for the maintenance of heat balance in healthy young adults during minimal to light physical activity reflecting activities of daily living and light household tasks. By establishing critical environmental limits in a young, healthy population, the findings of this study will be used as a benchmark for future studies in aged and other vulnerable populations, and can be used to develop guidelines, policy decisions, and evidence‐based alert communications to minimize adverse impacts of extreme heat events.

Fluid Balance is Unlikely Modified by the Menstrual Cycle when Ad Libitum Drinking is Permitted During Physical Work in the Heat

PurposeFederal and local entities provide recommendations aiming to limit dehydration during occupational heat stress. These recommendations are largely founded on the premise that during physical work in the heat humans under drink, which increases the likelihood of dehydration. Nearly all these studies have been conducted in men. The female menstrual cycle is characterized by fluctuations in estrogen and progesterone, such that elevations in these ovarian sex hormones may modify fluid regulation by both reducing urine output and promoting fluid intake. However, it is unknown if body fluid balance (i.e., both fluid intake and output) during occupationally relevant heat stress scenarios is altered across the menstrual cycle. Therefore, we tested the hypothesis that during physical work in the heat women will be more dehydrated during the early follicular (EF) phase (when ovarian sex hormones are low), compared to the late follicular (LF) and mid‐luteal (ML) phases of the menstrual cycle (when ovarian sex hormones are elevated).MethodsEight healthy, eumenorrheic, unacclimated women (26 ± 5 y) completed three trials consisting of 4 h (half a workday) exposure to 33.8 ± 0.8°C, 54 ± 1% relative humidity (WBGT = 28.3 ± 0.2°C, the average heat stress encountered in outdoor occupations) during the EF, LF, ML phases of their menstrual cycle. All visits were completed using a block‐randomized, crossover design. Each hour participants walked on a treadmill for 30 min at an intensity evoking a rate of metabolic heat production (Hprod) of ~430 W (the average Hprod for outdoor workers in the U.S.), while the work‐rest ratio is consistent with the NIOSH heat stress recommendations. Participants drank a cool (12 ± 1°C) flavor preferred non‐caloric sport drink (Gatorade Zero) ad libitum. Nude body weight was measured pre‐ and post‐ exposure, and percent changes in body weight loss (%BWL) were interpreted as an index of changes in total body water. Total fluid intake and urine output were also measured and sweat loss was estimated from changes in body weight corrected for fluid intake and urine loss. Rectal temperature is reported as a change from pre‐ to post‐ exposure. Data are presented as mean ± SD.ResultsChanges in rectal temperature did not differ between menstrual cycle phases (EF: 0.3 ± 0.2°C, LF: 0.5 ± 0.5°C, ML: 0.3 ± 0.3°C, p = 0.544). Sweat loss (EF: 2628 ± 1504 mL, LF: 2804 ± 1204 mL, ML: 2817 ± 1267 mL, p = 0.762) and urine output (EF: 864 ± 494 mL, LF: 955 ± 540 mL, ML: 925 ± 480 mL, p = 0.725) also did not differ between phases. Likewise, there were no differences in fluid intake between phases (EF: 1764 ± 1039 mL, LF: 1849 ± 694 mL, ML: 1892 ± 883 mL, p = 0.785). Thus, %BWL did not differ between phases (EF: ‐0.3 ± 0.7%, LF: ‐0.1 ± 0.7%, ML: ‐0.3 ± 0.8, p = 0.530) and %BWL did not statistically differ from zero (p ≥ 0.247).ConclusionWhen cool, flavor preferred fluids are freely accessible, fluid balance in women likely does not differ across three distinct phases of the menstrual cycle and thus, is unlikely to result in a net loss of body water during occupationally relevant heat stress scenarios.

Critical environmental limits for young, healthy adults (PSU HEAT Project).

Critical environmental limits are those combinations of ambient temperature and humidity above which heat balance cannot be maintained for a given metabolic heat production, limiting exposure time, and placing individuals at increased risk of heat-related illness. The aim of this study was to establish those limits in young (18-34 yr) healthy adults during low-intensity activity approximating the metabolic demand of activities of daily living. Twenty-five (12 men/13 women) subjects were exposed to progressive heat stress in an environmental chamber at two rates of metabolic heat production chosen to represent minimal activity (MinAct) or light ambulation (LightAmb). Progressive heat stress was performed with either 1) constant dry-bulb temperature (Tdb) and increasing ambient water vapor pressure (Pa) (Pcrit trials; 36°C, 38°C, or 40°C) or 2) constant Pa and increasing Tdb (Tcrit trials; 12, 16, or 20 mmHg). Each subject was tested during MinAct and LightAmb in two to three experimental conditions in random order, for a total of four to six trials per participant. Higher metabolic heat production (P < 0.001) during LightAmb compared with MinAct trials resulted in significantly lower critical environmental limits across all Pcrit and Tcrit conditions (all P < 0.001). These data, presented graphically herein on a psychrometric chart, are the first to define critical environmental limits for young adults during activity resembling those of light household tasks or other activities of daily living and can be used to develop guidelines, policy decisions, and evidence-based alert communications to minimize the deleterious impacts of extreme heat events.NEW & NOTEWORTHY Critical environmental limits are those combinations of temperature and humidity above which heat balance cannot be maintained, placing individuals at increased risk of heat-related illness. Those limits have been investigated in young adults during exercise at 30% V̇o2max, but not during metabolic rates that approximate those of light activities of daily living. Herein, we establish critical environmental limits for young adults at two metabolic rates that reflect activities of daily living and leisurely walking.

The Change in Core Temperature and Sweating Response during Exercise Are Unaffected by Time of Day within the Wake Period.

Exercise thermoregulation studies typically control for time of day. The present study assessed whether circadian rhythm independently alters time-dependent changes in core temperature and sweating during exercise at a fixed rate of metabolic heat production (Hprod) during the wake period. Ten men (26 ± 2 yr, 76.6 ± 6.3 kg, 1.95 ± 0.10 m2) cycled for 60 min in three combinations of ambient temperature and Hprod (23°C-7.5 W·kg-1, 33°C-5.5 W·kg-1, and 33°C-7.5 W·kg-1) at two times of day (a.m.: 0800 h, p.m.: 1600 h). Rectal temperature (Tre), local sweat rate, and whole-body sweat losses were measured. Absolute Tre was lower at baseline in a.m. versus p.m. for all three conditions (a.m.: 36.8°C ± 0.2°C, p.m.: 37.0°C ± 0.2°C, P < 0.01). The ΔTre was not altered by time of day (P > 0.22) and not different at 60 min between a.m. and p.m. for 23°C-7.5 W·kg-1 (a.m.: 0.83°C ± 0.14°C, p.m.: 0.75°C ± 0.20°C; P = 0.20), 33°C-5.5 W·kg-1 (a.m.: 0.51°C ± 0.14°C, p.m.: 0.47°C ± 0.14°C; P = 0.22), and 33°C-7.5 W·kg-1 (a.m.: 0.77°C ± 0.20°C, p.m.: 0.73°C ± 0.21°C; P = 0.80). The change in local sweat rate was unaffected by time of day (P > 0.16) and not different at 60 min in 23°C-7.5 W·kg-1 (a.m.: 0.67 ± 0.20 mg·cm-2·min-1, p.m.: 0.62 ± 0.21 mg·cm-2·min-1; P = 0.55), 33°C-5.5 W·kg-1 (a.m.: 0.59 ± 0.13 mg·cm-2·min-1, p.m.: 0.57 ± 0.12 mg·cm-2·min-1; P = 0.65), and 33°C-7.5 W·kg-1 (a.m.: 0.91 ± 0.19 mg·cm-2·min-1, p.m.: 0.84 ± 0.15 mg·cm-2·min-1; P = 0.33). Whole-body sweat loss was not different between a.m. and p.m. for 23°C-7.5 W·kg-1 (a.m.: 579 ± 72 g, p.m.: 579 ± 96 g; P = 0.99), 33°C-5.5 W·kg-1 (a.m.: 558 ± 48 g, p.m.: 555 ± 83 g; P = 0.89), and 33°C-7.5 W·kg-1 (a.m.: 796 ± 72 g, p.m.: 783 ± 75 g; P = 0.31). The change in core temperature and sweating throughout a 60-min exercise bout in 23°C and 33°C were unaffected by circadian rhythm during the wake period when exercise intensity was prescribed to elicit comparable rates of Hprod, suggesting that scheduling thermoregulatory exercise trials for the same time of day is unnecessary.

Independent Influence of Skin Temperature on Whole-Body Sweat Rate.

It is often assumed that a person with a higher mean skin temperature (Tsk) will sweat more during exercise. However, it has not yet been demonstrated whether Tsk describes any individual variability in whole-body sweat rate (WBSR) independently of the evaporative requirement for heat balance (Ereq). One hundred forty bouts of 2-h treadmill walking completed by a pool of 21 participants (23 ± 4 yr, 174 ± 8 cm, 76 ± 11 kg, 1.9 ± 0.2 m) under up to nine conditions were analyzed. Trials employed varying rates of metabolic heat production (Hprod; 197-813 W), and environmental conditions (15°C, 20°C, 25°C, 30°C; all 50% relative humidity), yielding a wide range of Ereq (86-684 W) and Tsk values (26.9°C-34.4°C). The individual variation observed in WBSR was best described using Ereq (in watts; R = 0.784) as a sole descriptor, relative to Ereq (in watts per meter squared; R = 0.735), Hprod (in watts; R = 0.639), Hprod (in watts per meter squared; R = 0.584), ambient air temperature (Ta) (R = 0.263), and Tsk (R = 0.077; all, P < 0.001). A multiple stepwise linear regression included only Ereq (in watts; adjusted R = 0.784), with Tsk not significantly correlating with the residual variance (P = 0.285), independently of Ereq (in watts). Hprod (in watts) had similar predictive strength to Ereq (in watts) at a fixed air temperature, explaining only 5.2% at 30°C, 4.9% at 25°C, 2.7% at 20°C, and 0.5% at 15°C (all, P < 0.001) less variance in WBSR compared with Ereq. However, when data from all ambient temperatures were pooled, Hprod alone was a markedly worse predictor of WBSR than Ereq (R = 0.639 vs 0.784; P < 0.001). Ereq (in watts) explained approximately four-fifths of the individual variation in WBSR over a range of ambient temperatures and exercise intensities, whereas Tsk did not explain any residual variance independently of Ereq.

Thermoregulatory and perceptual responses of lean and obese fit and unfit girls exercising in the heat

ObjectiveTo verify the thermoregulatory and perceptual responses of obese and lean girls, either fit or unfit, exercising in the heat at a similar rate of metabolic heat production per unit body mass. MethodsA total of 34 pubescent girls were allocated in four groups: 12 obese fit, 9 obese unfit, 5 lean fit, and 8 lean unfit. The obese groups (13.2±1.4 years, 40.5%±5.8% fat by DXA) differed in their aerobic fitness (V˙O2peak 76.0±8.1 vs. 56.6±5.8mL.kgmusclemass‐1.min−1), as well as the lean groups (13.1±1.6 years, 24.0%±4.8% fat) (V˙O2peak 74.5±2.9 vs. 56.2±5.0mL.kgmuscle mass−1min−1). Girls cycled two bouts of 25min with a 10min rest in between, at ∼5.4W.kg−1 in the heat (36°C and 40% relative humidity) and they were kept euhydrated. Rectal and skin temperatures and heart rate were measured every 5min. Perceptual responses were evaluated throughout the exercise. ResultsInitial rectal temperature was higher in the obese subjects compared to the lean subjects (37.5±0.3 and 37.2±0.3°C). No difference was observed among the girls whom were obese (eight fit or unfit) and lean (also fit or unfit) throughout the exercise in rectal temperature (37.6±0.2, 37.5±0.3, 37.5±0.3, 37.4±0.3°C, respectively), skin temperature (34.8±0.8, 35.1±1.0, 34.4±0.9, 35.2±0.9°C), and heart rate (128±18; 118±12, 130±16, 119±16beatsmin−1). No differences were observed in perceptual responses among groups. ConclusionRegardless of the adiposity or aerobic fitness, pubescent girls had similar thermoregulatory and perceptual responses while cycling in the heat at similar metabolic heat production.

Voluntary Cooling During Exercise Is Augmented In Heat Sensitive People With Multiple Sclerosis

PURPOSE: Body cooling improves exercise tolerance in heat sensitive people with Multiple Sclerosis (MS). The effectiveness of cooling modalities in real-world settings requires appropriately engaging in cooling behavior. We tested the hypothesis that people with MS voluntarily engage in cooling behavior during exercise to a greater extent than healthy controls. METHODS: In a 27.0±0.2°C, 41±2% RH environment, 7 subjects with relapsing-remitting MS who exhibited heat sensitivity (34±7y, 167±5cm, 72±15kg, EDSS: 1.9±0.8, 1 male) and 7 healthy subjects (CON, 37±7y, 168±7cm, 71±11kg, 1 male) completed two randomized trials cycling for 40 min (EX) at a fixed rate of metabolic heat production (3.5 W/kg) followed by 30 min recovery (REC). In one trial, subjects were restricted from engaging in cooling (NONE). In the other, subjects pressed a button to receive 2 min of ~2°C water perfusing a suit top as often as desired (COOL). Mean skin (Tsk, 8 site) and core (Tcore, telemetry pill) temperatures and mean skin wettedness (Wsk, 8 site) were recorded continuously. Total voluntary time in cooling provided an index of cooling behavior. Ratings of perceived exertion (RPE), composite heat sensitivity symptom scores (HSSS, MS only) and subjective fatigue (MS only) were recorded every 10 min. RESULTS: Tcore (+0.5±0.1°C, P<0.01) and Wsk (+0.52±0.02 a.u., P<0.01) increased in EX and remained elevated in REC (P<0.01) but were not different between trials (P≥0.35) or groups (P≥0.60). Tsk decreased in MS COOL compared to MS NONE from min 20 EX to 5 REC (P≤0.02). There were no other differences in Tsk (P≥0.25). MS spent more total time in cooling in EX (MS: 13.4±3.0 min; CON: 7.4±3.6 min, P<0.01) but not REC (MS: 1.7±1.4 min; CON: 0.3±0.7 min, P=0.40). In both trials, RPE was higher in MS vs. CON (P<0.01). HSSS increased in EX (P<0.01) but was not different between trials (P=0.25). Subjective fatigue was not different between trials in EX (P≥0.97) but was lower at 10 and 20 min of REC in COOL (P<0.02). CONCLUSIONS: MS engaged in body cooling during EX to a greater extent than CON. While cooling did not affect HSSS or fatigue during EX, MS reported greater reductions in fatigue following exercise when cooling was permitted. Cooling during exercise could improve exercise participation and adherence. Supported by: ACSM Foundation Research Endowment Grant