Thermoregulatory responses of middle-aged and young men during dry-heat acclimation

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 4: evolution, thermal adaptation and unsupported theories of thermoregulation

This review is the final contribution to a four-part, historical series on human exercise physiology in thermally stressful conditions. The series opened with reminders of the principles governing heat exchange and an overview of our contemporary understanding of thermoregulation (Part 1). We then reviewed the development of physiological measurements (Part 2) used to reveal the autonomic processes at work during heat and cold stresses. Next, we re-examined thermal-stress tolerance and intolerance, and critiqued the indices of thermal stress and strain (Part 3). Herein, we describe the evolutionary steps that endowed humans with a unique potential to tolerate endurance activity in the heat, and we examine how those attributes can be enhanced during thermal adaptation. The first of our ancestors to qualify as an athlete was Homo erectus, who were hairless, sweating specialists with eccrine sweat glands covering almost their entire body surface. Homo sapiens were skilful behavioural thermoregulators, which preserved their resource-wasteful, autonomic thermoeffectors (shivering and sweating) for more stressful encounters. Following emigration, they regularly experienced heat and cold stress, to which they acclimatised and developed less powerful (habituated) effector responses when those stresses were re-encountered. We critique hypotheses that linked thermoregulatory differences to ancestry. By exploring short-term heat and cold acclimation, we reveal sweat hypersecretion and powerful shivering to be protective, transitional stages en route to more complete thermal adaptation (habituation). To conclude this historical series, we examine some of the concepts and hypotheses of thermoregulation during exercise that did not withstand the tests of time.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 3: Heat and cold tolerance during exercise

In this third installment of our four-part historical series, we evaluate contributions that shaped our understanding of heat and cold stress during occupational and athletic pursuits. Our first topic concerns how we tolerate, and sometimes fail to tolerate, exercise-heat stress. By 1900, physical activity with clothing- and climate-induced evaporative impediments led to an extraordinarily high incidence of heat stroke within the military. Fortunately, deep-body temperatures > 40 °C were not always fatal. Thirty years later, water immersion and patient treatments mimicking sweat evaporation were found to be effective, with the adage of cool first, transport later being adopted. We gradually acquired an understanding of thermoeffector function during heat storage, and learned about challenges to other regulatory mechanisms. In our second topic, we explore cold tolerance and intolerance. By the 1930s, hypothermia was known to reduce cutaneous circulation, particularly at the extremities, conserving body heat. Cold-induced vasodilatation hindered heat conservation, but it was protective. Increased metabolic heat production followed, driven by shivering and non-shivering thermogenesis, even during exercise and work. Physical endurance and shivering could both be compromised by hypoglycaemia. Later, treatments for hypothermia and cold injuries were refined, and the thermal after-drop was explained. In our final topic, we critique the numerous indices developed in attempts to numerically rate hot and cold stresses. The criteria for an effective thermal stress index were established by the 1930s. However, few indices satisfied those requirements, either then or now, and the surviving indices, including the unvalidated Wet-Bulb Globe-Thermometer index, do not fully predict thermal strain.

Physiological costs of undocumented human migration across the southern United States border

[Figure: see text].

Individual Anthropometric, Aerobic Capacity and Demographic Characteristics as Predictors of Heat Intolerance in Military Populations

Background and objectives: The Australian Defence Force (ADF) engages in combat-related activities in hot climatic conditions, which exposes ADF members to the threat of exertional heat illness (EHI). After an episode of EHI, the heat tolerance test (HTT) is conducted to determine heat tolerance. Heat intolerance is the inability to maintain thermal balance while exercising in a hot environment. This study investigated the predictive roles of individual characteristics (age, gender, aerobic capacity (VO₂max) and body composition) on physiological responses to the HTT in a group comprising ADF personnel and civilian volunteers. Materials and Methods: A quasi-experimental design was used and 52 (38 males and 14 females) participants were recruited from the ADF and the general population for the HTT. Heat intolerance was defined following the standard criteria for the HTT (temperature and heart rate). Data were analysed using inferential statistics. Results: The mean age of the participants was 31.1 ± 11.6 years, and 44% (23 people: 19 males and 4 females) of the participants were heat intolerant. Independent samples T-test showed that body mass index (p = 0.011) and body fat% (p = 0.034) of heat-intolerant participants were significantly higher than their heat-tolerant counterparts. Body surface area to mass ratio (p = 0.005) and aerobic capacity (p = 0.001) were significantly lower in heat-intolerant participants. Regression analyses showed that age, gender, aerobic capacity and body fat% were significant (p < 0.001) predictors of heat tolerance outcomes, with R² values ranging from 0.505 to 0.636. Conclusions: This study showed that aerobic capacity, body fat%, age and gender are predictors of heat intolerance among military and non-military populations. However, there may be a need for future studies to consider identifying other indicators such as clinical biomarkers of heat intolerance, which could be used to develop a more reliable HTT protocol.

The sweat glands' maximum ion reabsorption rates following heat acclimation in healthy older adults

NEW FINDINGS

What is the central question to this study? Do the sweat glands' maximum ion reabsorption rates increase following heat acclimation in healthy older individuals and is this associated with elevated aldosterone concentrations? What is the main finding and its importance? Sweat gland maximum ion reabsorption rates improved heterogeneously across body sites, which occurred without any changes in aldosterone concentration following a controlled hyperthermic heat acclimation protocol in healthy older individuals.

ABSTRACT

We examined whether the eccrine sweat glands' ion reabsorption rates improved following heat acclimation (HA) in older individuals. Ten healthy older adults (>65 years) completed a controlled hyperthermic (+0.9°C rectal temperature, T_re ) HA protocol for nine non-consecutive days. Participants completed a passive heat stress test (lower leg 42°C water submersion) pre-HA and post-HA to assess physiological regulation of sweat gland ion reabsorption at the chest, forearm and thigh. The maximum ion reabsorption rate was defined as the inflection point in the slope of the relation between galvanic skin conductance and sweat rate (SR). We explored the responses again after a 7-day decay. During passive heating, the T_b thresholds for sweat onset on the chest and forearm were lowered after HA (P < 0.05). However, sweat sensitivity (i.e. the slope), the SR at a given T_re and gross sweat loss did not improve after HA (P > 0.05). Any changes observed were lost during the decay. Pilocarpine-induced sudomotor responses to iontophoresis did not change after HA (P ≥ 0.801). Maximum ion reabsorption rate was only enhanced at the chest (P = 0.001) despite unaltered aldosterone concentration after HA. The data suggest that this adaptation is lost after 7 days' decay. The HA protocol employed in the present study induced partial adaptive sudomotor responses. Eccrine sweat gland ion reabsorption rates improved heterogeneously across the skin sites. It is likely that aldosterone secretion did not alter the chest sweat ion reabsorption rates observed in the older adults.

Effect of regular precooling on adaptation to training in the heat

PURPOSE

This study investigated whether regular precooling would help to maintain day-to-day training intensity and improve 20-km cycling time trial (TT) performed in the heat. Twenty males cycled for 10 day × 60 min at perceived exertion equivalent to 15 in the heat (35 °C, 50% relative humidity), preceded by no cooling (CON, n = 10) or 30-min water immersion at 22 °C (PRECOOL, n = 10).

METHODS

19 participants (n = 9 and 10 for CON and PRECOOL, respectively) completed heat stress tests (25-min at 60% [Formula: see text] and 20-km TT) before and after heat acclimation.

RESULTS

Changes in mean power output (∆MPO, P = 0.024) and heart rate (∆HR, P = 0.029) during heat acclimation were lower for CON (∆MPO - 2.6 ± 8.1%, ∆HR - 7 ± 7 bpm), compared with PRECOOL (∆MPO + 2.9 ± 6.6%, ∆HR - 1 ± 8 bpm). HR during constant-paced cycling was decreased from the pre-acclimation test in both groups (P < 0.001). Only PRECOOL demonstrated lower rectal temperature (T_re) during constant-paced cycling (P = 0.002) and lower T_re threshold for sweating (P = 0.042). However, skin perfusion and total sweat output did not change in either CON or PRECOOL (all P > 0.05). MPO (P = 0.016) and finish time (P = 0.013) for the 20-km TT were improved in PRECOOL but did not change in CON (P = 0.052 for MPO, P = 0.140 for finish time).

CONCLUSION

Precooling maintains day-to-day training intensity and does not appear to attenuate adaptation to training in the heat.

A retrospective analysis to determine if exercise training-induced thermoregulatory adaptations are mediated by increased fitness or heat acclimation

NEW FINDINGS

What is the central question of this study? Are fitness-related improvements in thermoregulatory responses during uncompensable heat stress mediated by aerobic capacity V ̇ O 2 max or is it the partial heat acclimation associated with training? What is the main finding and its importance? During uncompensable heat stress, individuals with high and low V ̇ O 2 max displayed similar sweating and core temperature responses whereas exercise training in previously untrained individuals resulted in a greater sweat rate and a smaller rise in core temperature. These observations suggest that it is training, not V ̇ O 2 max per se, that mediates thermoregulatory improvements during uncompensable heat stress.

ABSTRACT

It remains unclear whether aerobic fitness, as defined by the maximum rate of oxygen consumption V ̇ O 2 max , independently improves heat dissipation in uncompensable environments, or whether the thermoregulatory adaptations associated with heat acclimation are due to repeated bouts of exercise-induced heat stress during regular aerobic training. The present analysis sought to determine if V ̇ O 2 max independently influences thermoregulatory sweating, maximum skin wettedness (ω_max ) and the change in rectal temperature (ΔT_re ) during 60 min of exercise in an uncompensable environment (37.0 ± 0.8°C, 4.0 ± 0.2 kPa, 64 ± 3% relative humidity) at a fixed rate of heat production per unit mass (6 W kg^-1 ). Retrospective analyses were performed on 22 participants (3 groups), aerobically unfit (UF; n = 7; V ̇ O 2 max : 41.7 ± 9.4 ml kg^-1  min^-1 ), aerobically fit (F; n = 7; V ̇ O 2 max : 55.6 ± 4.3 ml kg^-1  min^-1 ; P < 0.01) and aerobically unfit (n = 8) individuals, before (pre; V ̇ O 2 max : 45.8 ± 11.6 ml kg^-1  min^-1 ) and after (post; V ̇ O 2 max : 52.0 ± 11.1 ml kg^-1  min^-1 ; P < 0.001) an 8-week training intervention. ω_max was similar between UF (0.74 ± 0.09) and F (0.78 ± 0.08, P = 0.22). However, ω_max was greater post- (0.84 ± 0.08) compared to pre- (0.72 ± 0.06, P = 0.02) training. During exercise, mean local sweat rate (forearm and upper-back) was greater post- (1.24 ± 0.20 mg cm^-2  min^-1 ) compared to pre- (1.04 ± 0.25 mg cm^-2  min^-1 , P < 0.01) training, but similar between UF (0.94 ± 0.31 mg cm^-2  min^-1 , P = 0.90) and F (1.02 ± 0.30 mg cm^-2  min^-1 ). The ΔT_re at 60 min of exercise was greater pre- (1.13 ± 0.16°C, P < 0.01) compared to post- (0.96 ± 0.14°C) training, but similar between UF (0.85 ± 0.29°C, P = 0.22) and F (0.95 ± 0.22°C). Taken together, aerobic training, not V ̇ O 2 max per se, confers an increased ω_max , greater sweat rate, and smaller rise in core temperature during uncompensable heat stress in fit individuals.

Physiological mechanisms determining eccrine sweat composition

Purpose

The purpose of this paper is to review the physiological mechanisms determining eccrine sweat composition to assess the utility of sweat as a proxy for blood or as a potential biomarker of human health or nutritional/physiological status.

Methods

This narrative review includes the major sweat electrolytes (sodium, chloride, and potassium), other micronutrients (e.g., calcium, magnesium, iron, copper, zinc, vitamins), metabolites (e.g., glucose, lactate, ammonia, urea, bicarbonate, amino acids, ethanol), and other compounds (e.g., cytokines and cortisol).

Results

Ion membrane transport mechanisms for sodium and chloride are well established, but the mechanisms of secretion and/or reabsorption for most other sweat solutes are still equivocal. Correlations between sweat and blood have not been established for most constituents, with perhaps the exception of ethanol. With respect to sweat diagnostics, it is well accepted that elevated sweat sodium and chloride is a useful screening tool for cystic fibrosis. However, sweat electrolyte concentrations are not predictive of hydration status or sweating rate. Sweat metabolite concentrations are not a reliable biomarker for exercise intensity or other physiological stressors. To date, glucose, cytokine, and cortisol research is too limited to suggest that sweat is a useful surrogate for blood.

Conclusion

Final sweat composition is not only influenced by extracellular solute concentrations, but also mechanisms of secretion and/or reabsorption, sweat flow rate, byproducts of sweat gland metabolism, skin surface contamination, and sebum secretions, among other factors related to methodology. Future research that accounts for these confounding factors is needed to address the existing gaps in the literature.

Electronic supplementary material

The online version of this article (10.1007/s00421-020-04323-7) contains supplementary material, which is available to authorized users.

Performance Changes Following Heat Acclimation and the Factors That Influence These Changes: Meta-Analysis and Meta-Regression

Heat acclimation (HA) is the process of intentional and consistent exercise in the heat that results in positive physiological adaptations, which can improve exercise performance both in the heat and thermoneutral conditions. Previous research has indicated the many performance benefits of HA, however, a meta-analysis examining the magnitude of different types of performance improvement is absent. Additionally, there are several methodological discrepancies in the literature that could lead to increased variability in performance improvement following HA and no previous study has examined the impact of moderators on performance improvement following HA. Therefore, the aim of this study was two-fold; (1) to perform a meta-analysis to examine the magnitude of changes in performance following HA in maximal oxygen consumption (VO2max), time to exhaustion, time trial, mean power, and peak power tests; (2) to determine the impact of moderators on results of these performance tests. Thirty-five studies met the inclusion/exclusion criteria with 23 studies that assessed VO2max (n = 204), 24 studies that assessed time to exhaustion (n = 232), 10 studies that performed time trials (n = 101), 7 studies that assessed mean power (n = 67), and 10 papers that assessed peak power (n = 88). Data are reported as Hedge's g effect size (ES), and 95% confidence intervals (95% CI). Statistical significance was set to p < 0.05, a priori. The magnitude of change following HA was analyzed, with time to exhaustion demonstrating the largest performance enhancement (ES [95% CI], 0.86 [0.71, 1.01]), followed by time trial (0.49 [0.26, 0.71]), mean power (0.37 [0.05, 0.68]), VO2max (0.30 [0.07, 0.53]), and peak power (0.29 [0.09, 0.48]) (p < 0.05). When all of the covariates were analyzed as individual models, induction method, fitness level, heat index in time to exhaustion (coefficient [95% CI]; induction method, -0.69 [-1.01, -0.37], p < 0.001; fitness level, 0.04 [0.02, 0.06], p < 0.001; heat index, 0.04 [0.02, 0.07], p < 0.0001) and induction length in mean power (coefficient [95% CI]; induction length 0.15 [0.05, 0.25], p = 0.002) significantly impacted the magnitude of change. Sport scientists and researchers can use the findings from this meta-analysis to customize HA induction. For time to exhaustion improvements, HA implementation should focus on induction method and baseline fitness, while the training and recovery balance could lead to optimal time trial performance.

Heat alleviation strategies for athletic performance: A review and practitioner guidelines

International competition inevitably presents logistical challenges for athletes. Events such as the Tokyo 2020 Olympic Games require further consideration given historical climate data suggest athletes will experience significant heat stress. Given the expected climate, athletes face major challenges to health and performance. With this in mind, heat alleviation strategies should be a fundamental consideration. This review provides a focused perspective of the relevant literature describing how practitioners can structure male and female athlete preparations for performance in hot, humid conditions. Whilst scientific literature commonly describes experimental work, with a primary focus on maximizing magnitudes of adaptive responses, this may sacrifice ecological validity, particularly for athletes whom must balance logistical considerations aligned with integrating environmental preparation around training, tapering and travel plans. Additionally, opportunities for sophisticated interventions may not be possible in the constrained environment of the athlete village or event arenas. This review therefore takes knowledge gained from robust experimental work, interprets it and provides direction on how practitioners/coaches can optimize their athletes' heat alleviation strategies. This review identifies two distinct heat alleviation themes that should be considered to form an individualized strategy for the athlete to enhance thermoregulatory/performance physiology. First, chronic heat alleviation techniques are outlined, these describe interventions such as heat acclimation, which are implemented pre, during and post-training to prepare for the increased heat stress. Second, acute heat alleviation techniques that are implemented immediately prior to, and sometimes during the event are discussed. Abbreviations: CWI: Cold water immersion; HA: Heat acclimation; HR: Heart rate; HSP: Heat shock protein; HWI: Hot water immersion; LTHA: Long-term heat acclimation; MTHA: Medium-term heat acclimation; ODHA: Once-daily heat acclimation; RH: Relative humidity; RPE: Rating of perceived exertion; STHA: Short-term heat acclimation; TCORE: Core temperature; TDHA: Twice-daily heat acclimation; TS: Thermal sensation; TSKIN: Skin temperature; V̇O2max: Maximal oxygen uptake; WGBT: Wet bulb globe temperature.

Thermoregulatory adaptations with progressive heat acclimation are predominantly evident in uncompensable, but not compensable, conditions

This study assessed whether, notwithstanding lower resting absolute core temperatures, alterations in time-dependent changes in thermoregulatory responses following partial and complete heat acclimation (HA) are only evident during uncompensable heat stress. Eight untrained individuals underwent 8 wk of aerobic training (i.e., partial HA) followed by 6 days of HA in 38°C/65% relative humidity (RH) (i.e., complete HA). On separate days, esophageal temperature (T_es), arm (LSR_arm), and back (LSR_back) sweat rate, and whole body sweat rate (WBSR) were measured during a 45-min compensable (37°C/30% RH) and 60-min uncompensable (37°C/60% RH) heat stress trial pre-training (PRE-TRN), post-training (POST-TRN), and post-heat acclimation (POST-HA). For compensable heat stress trials, resting T_es was lower POST-TRN (36.74 ± 0.27°C, P = 0.05) and POST-HA (36.60 ± 0.27°C, P = 0.001) compared with PRE-TRN (36.99 ± 0.19°C); however, ΔT_es was similar in all trials (PRE-TRN:0.40 ± 0.23°C; POST-TRN:0.42 ± 0.20°C; POST-HA:0.43 ± 0.12°C, P = 0.97). While LSR_back was unaltered by HA (P = 0.94), end-exercise LSR_arm was higher POST-TRN (0.70 ± 0.14 mg·cm^-2·min^-1, P < 0.001) and POST-HA (0.75 ± 0.16 mg·cm^-2·min^-1, P < 0.001) compared with PRE-TRN (0.61 ± 0.15 mg·cm^-2·min^-1). Despite matched evaporative heat balance requirements, steady-state WBSR (31st-45th min) was greater POST-TRN (12.7 ± 1.0 g/min, P = 0.02) and POST-HA (12.9 ± 0.8 g/min, P = 0.004), compared with PRE-TRN (11.7 ± 0.9 g/min). For uncompensable heat stress trials, resting T_es was lower POST-TRN (36.77 ± 0.22°C, P = 0.05) and POST-HA (36.62 ± 0.15°C, P = 0.03) compared with PRE-TRN (36.86 ± 0.24°C). But ΔT_es was smaller POST-TRN (0.77 ± 0.19°C, P = 0.05) and POST-HA (0.75 ± 0.15°C, P = 0.04) compared with PRE-TRN (1.10 ± 0.32°C). LSR_back and LSR_arm increased with HA (P < 0.007), supporting the greater WBSR with HA (POST-TRN:14.4 ± 2.4 g/min, P < 0.001; POST-HA:16.8 ± 2.8 g/min, P < 0.001) compared with PRE-TRN (12.7 ± 3.2 g/min). In conclusion, the thermal benefits of HA are primarily evident when conditions challenge the physiological capacity to dissipate heat.NEW & NOTEWORTHY We demonstrate that neither partial nor complete heat acclimation alters the change in core temperature during compensable heat stress compared with an unacclimated state, despite a marginally greater whole body sweat rate. However, the greater local and whole body sweat rate with partial and complete heat acclimation reduced the rise in core temperature during 60 min of uncompensable heat stress compared with an unacclimated state, suggesting the improvements in heat dissipation associated with heat acclimation are best observed when the upper physiological limits for evaporative heat loss are challenged.

Physiology of sweat gland function: The roles of sweating and sweat composition in human health

The purpose of this comprehensive review is to: 1) review the physiology of sweat gland function and mechanisms determining the amount and composition of sweat excreted onto the skin surface; 2) provide an overview of the well-established thermoregulatory functions and adaptive responses of the sweat gland; and 3) discuss the state of evidence for potential non-thermoregulatory roles of sweat in the maintenance and/or perturbation of human health. The role of sweating to eliminate waste products and toxicants seems to be minor compared with other avenues of excretion via the kidneys and gastrointestinal tract; as eccrine glands do not adapt to increase excretion rates either via concentrating sweat or increasing overall sweating rate. Studies suggesting a larger role of sweat glands in clearing waste products or toxicants from the body may be an artifact of methodological issues rather than evidence for selective transport. Furthermore, unlike the renal system, it seems that sweat glands do not conserve water loss or concentrate sweat fluid through vasopressin-mediated water reabsorption. Individuals with high NaCl concentrations in sweat (e.g. cystic fibrosis) have an increased risk of NaCl imbalances during prolonged periods of heavy sweating; however, sweat-induced deficiencies appear to be of minimal risk for trace minerals and vitamins. Additional research is needed to elucidate the potential role of eccrine sweating in skin hydration and microbial defense. Finally, the utility of sweat composition as a biomarker for human physiology is currently limited; as more research is needed to determine potential relations between sweat and blood solute concentrations.

Impact of environmental stressors on tolerance to hemorrhage in humans

Hemorrhage is a leading cause of death in military and civilian settings, and ~85% of potentially survivable battlefield deaths are hemorrhage-related. Soldiers and civilians are exposed to a number of environmental and physiological conditions that have the potential to alter tolerance to a hemorrhagic insult. The objective of this review is to summarize the known impact of commonly encountered environmental and physiological conditions on tolerance to hemorrhagic insult, primarily in humans. The majority of the studies used lower body negative pressure (LBNP) to simulate a hemorrhagic insult, although some studies employed incremental blood withdrawal. This review addresses, first, the use of LBNP as a model of hemorrhage-induced central hypovolemia and, then, the effects of the following conditions on tolerance to LBNP: passive and exercise-induced heat stress with and without hypohydration/dehydration, exposure to hypothermia, and exposure to altitude/hypoxia. An understanding of the effects of these environmental and physiological conditions on responses to a hemorrhagic challenge, including tolerance, can enable development and implementation of targeted strategies and interventions to reduce the impact of such conditions on tolerance to a hemorrhagic insult and, ultimately, improve survival from blood loss injuries.

Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability

Athletes lose water and electrolytes as a consequence of thermoregulatory sweating during exercise and it is well known that the rate and composition of sweat loss can vary considerably within and among individuals. Many scientists and practitioners conduct sweat tests to determine sweat water and electrolyte losses of athletes during practice and competition. The information gleaned from sweat testing is often used to guide personalized fluid and electrolyte replacement recommendations for athletes; however, unstandardized methodological practices and challenging field conditions can produce inconsistent/inaccurate results. The primary objective of this paper is to provide a review of the literature regarding the effect of laboratory and field sweat-testing methodological variations on sweating rate (SR) and sweat composition (primarily sodium concentration [Na⁺]). The simplest and most accurate method to assess whole-body SR is via changes in body mass during exercise; however, potential confounding factors to consider are non-sweat sources of mass change and trapped sweat in clothing. In addition, variability in sweat [Na⁺] can result from differences in the type of collection system used (whole body or localized), the timing/duration of sweat collection, skin cleaning procedure, sample storage/handling, and analytical technique. Another aim of this paper is to briefly review factors that may impact intra/interindividual variability in SR and sweat [Na⁺] during exercise, including exercise intensity, environmental conditions, heat acclimation, aerobic capacity, body size/composition, wearing of protective equipment, sex, maturation, aging, diet, and/or hydration status. In summary, sweat testing can be a useful tool to estimate athletes’ SR and sweat Na⁺ loss to help guide fluid/electrolyte replacement strategies, provided that data are collected, analyzed, and interpreted appropriately.

Short-term heat acclimation training improves physical performance: a systematic review, and exploration of physiological adaptations and application for team sports

BACKGROUND

Studies have demonstrated that longer-term heat acclimation training (≥8 heat exposures) improves physical performance. The physiological adaptations gained through short-term heat acclimation (STHA) training suggest that physical performance can be enhanced within a brief timeframe.

OBJECTIVE

The aim of this systematic review was to determine if STHA training (≤7 heat exposures) can improve physical performance in healthy adults.

DATA SOURCES

MEDLINE, PubMed, and SPORTDiscus™ databases were searched for available literature.

STUDY SELECTION

Studies were included if they met the following criteria: STHA intervention, performance measure outcome, apparently healthy participants, adult participants (≥18 years of age), primary data, and human participants.

STUDY APPRAISAL

A modified McMaster critical appraisal tool determined the level of bias in each included study.

RESULTS

Eight papers met the inclusion criteria. Studies varied from having a low to a high risk of bias. The review identified aerobic-based tests of performance benefit from STHA training. Peak anaerobic power efforts have not been demonstrated to improve.

LIMITATIONS

At the review level, this systematic review did not include tolerance time exercise tests; however, certain professions may be interested in this type of exercise (e.g. fire-fighters). At the outcome level, the review was limited by the moderate level of bias that exists in the field. Only two randomized controlled trials were included. Furthermore, a limited number of studies could be identified (eight), and only one of these articles focused on women participants.

CONCLUSIONS

The review identified that aerobic-based tests of performance benefit from STHA training. This is possibly through a number of cardiovascular, thermoregulatory, and metabolic adaptations improving the perception of effort and fatigue through a reduction in anaerobic energy release and elevation of the anaerobic threshold. These results should be viewed with caution due to the level of available evidence, and the limited number of papers that met the inclusion criteria of the review. STHA training can be applied in the team-sport environment during a range of instances within the competitive season. A mixed high-intensity protocol may only require five sessions with a duration of 60 min to potentially improve aerobic-based performance in trained athletes.

Thermometry, calorimetry, and mean body temperature during heat stress

Heat balance in humans is maintained at near constant levels through the adjustment of physiological mechanisms that attain a balance between the heat produced within the body and the heat lost to the environment. Heat balance is easily disturbed during changes in metabolic heat production due to physical activity and/or exposure to a warmer environment. Under such conditions, elevations of skin blood flow and sweating occur via a hypothalamic negative feedback loop to maintain an enhanced rate of dry and evaporative heat loss. Body heat storage and changes in core temperature are a direct result of a thermal imbalance between the rate of heat production and the rate of total heat dissipation to the surrounding environment. The derivation of the change in body heat content is of fundamental importance to the physiologist assessing the exposure of the human body to environmental conditions that result in thermal imbalance. It is generally accepted that the concurrent measurement of the total heat generated by the body and the total heat dissipated to the ambient environment is the most accurate means whereby the change in body heat content can be attained. However, in the absence of calorimetric methods, thermometry is often used to estimate the change in body heat content. This review examines heat exchange during challenges to heat balance associated with progressive elevations in environmental heat load and metabolic rate during exercise. Further, we evaluate the physiological responses associated with heat stress and discuss the thermal and nonthermal influences on the body's ability to dissipate heat from a heat balance perspective.

Heat stress and thermal strain challenges in running

Running well and safely in the heat is challenging for all runners, from recreational to elite. As environmental heat stress (heat stress modulated or augmented by air temperature, humidity, wind speed, and solar radiation) and the intensity and duration of a training run or race increase, so are metabolic heat production, the parallel need for heat transfer from the body to maintain thermal equilibrium, the consequent increase in blood flow to the skin, and the concomitant sweating response progressively and proportionally amplified. An accumulating total body-water deficit from extensive sweating and escalating level of cardiovascular and thermal strain will, in due course, considerably challenge a runner's physiology, perception of effort, and on-course well-being and performance. However, with the appropriate preparation and modifications to planned running intensity and distance, runners can safely tolerate and effectively train and compete in a wide range of challenging environmental conditions. Clinicians play a key role in this regard as an effective resource for providing the most effective guidelines and making the best overall individual recommendations regarding training and competing in the heat.

Moderate-intensity intermittent work in the heat results in similar low-level dehydration in young and older males

Older individuals may be more susceptible to the negative thermal and cardiovascular consequences of dehydration during intermittent work in the heat. This study examined the hydration, thermal, and cardiovascular responses to intermittent exercise in the heat in 14 Young (Y, Mean ± SE; 25.8 ± 0.8 years), Middle-age (MA, 43.6 ± 0.9 years), and Older (O, 57.2 ± 1.5 years) healthy, non-heat acclimated males matched for height, mass, body surface area, and percent body fat. Rectal temperature (Tre), heart rate (HR), local sweat rate (LSR), and hydration indices were measured during 4 × 15-min moderate to heavy cycling bouts at 400 W heat production, each followed by a 15-min rest period, in Warm/Dry (35°C, 20% relative humidity [RH]) and Warm/Humid (35°C, 60% RH) heat. No differences were observed between the age groups for Tre, Tre change, HR, LSR, mass change, urine specific gravity, and plasma protein concentration in either condition, irrespective of the greater level of thermal and cardiovascular strain experienced in the Warm/Humid environment. Plasma volume changes (Dry Y: -5.4 ± 0.7, MA: -6.2 ± 0.9, O: -5.7 ± 0.9%, Humid Y: -7.3 ± 1.0, MA: -7.9 ± 0.8, O: -8.4 ± 1.0%) were similar between groups, as were urine specific gravity and plasma protein concentrations. Thus, physically active Young, Middle-age, and Older males demonstrate similar hydration, thermal, and cardiovascular responses during moderate- to high-intensity intermittent exercise in the heat.

Age-related decrements in heat dissipation during physical activity occur as early as the age of 40

Older adults typically experience greater levels of thermal strain during physical efforts in the heat compared to young individuals. While this may be related to an age-dependent reduction in whole-body sweating, no study has clearly delineated at what age this occurs. In the present study, we report direct measurements of human heat dissipation during physical activity in the heat in males ranging in age from 20–70 years. Eighty-five males performed four 15-min bouts of cycling separated by 15-min rest periods, in a calorimeter regulated to 35°C and 20% relative humidity. Direct calorimetry was used to measure total heat loss (whole-body evaporative heat loss and dry heat exchange). We also used indirect calorimetry as a continuous measure of metabolic heat production. Body heat storage was calculated as the temporal summation of heat production and total heat loss over the experimental session. Whole-body sweat rate (WBSR) was calculated from measurements of evaporative heat loss. Males were divided into five age categories for the analysis of WBSR and body heat storage: 20–31 years (n = 18), 40–44 years (n = 15), 45–49 years (n = 15), 50–55 years (n = 21) and 56–70 years (n = 16). Relative to young males, WBSR was reduced in males aged 56–70 during each exercise (all P<0.05), in males aged 50–55 during the second (P = 0.031) and third exercises (P = 0.028) and in males aged 45–49 during the final exercise bout (P = 0.046). Although not significantly different, 40–44 years old males also had a lower rate of heat loss compared to younger males. Over the sum of two hours, the change in body heat content was greater in males 40–70 years compared to young males (all P<0.05). Our findings suggest that middle-aged and older adults have impairments in heat dissipation when doing physical activity in the heat, thus possibly increasing their risk of heat-related illness under such conditions.

Encapsulated environment

In many occupational settings, clothing must be worn to protect individuals from hazards in their work environment. However, personal protective clothing (PPC) restricts heat exchange with the environment due to high thermal resistance and low water vapor permeability. As a consequence, individuals who wear PPC often work in uncompensable heat stress conditions where body heat storage continues to rise and the risk of heat injury is greatly enhanced. Tolerance time while wearing PPC is influenced by three factors: (i) initial core temperature (Tc), affected by heat acclimation, precooling, hydration, aerobic fitness, circadian rhythm, and menstrual cycle (ii) Tc tolerated at exhaustion, influenced by state of encapsulation, hydration, and aerobic fitness; and (iii) the rate of increase in Tc from beginning to end of the heat-stress exposure, which is dependent on the clothing characteristics, thermal environment, work rate, and individual factors like body composition and economy of movement. Methods to reduce heat strain in PPC include increasing clothing permeability for air, adjusting pacing strategy, including work/rest schedules, physical training, and cooling interventions, although the additional weight and bulk of some personal cooling systems offset their intended advantage. Individuals with low body fatness who perform regular aerobic exercise have tolerance times in PPC that exceed those of their sedentary counterparts by as much as 100% due to lower resting Tc, the higher Tc tolerated at exhaustion and a slower increase in Tc during exercise. However, questions remain about the importance of activity levels, exercise intensity, cold water ingestion, and plasma volume expansion for thermotolerance.

Whole body heat loss is reduced in older males during short bouts of intermittent exercise

Studies in young adults show that a greater proportion of heat is gained shortly following the start of exercise and that temporal changes in whole body heat loss during intermittent exercise have a pronounced effect on body heat storage. The consequences of short-duration intermittent exercise on heat storage with aging are unclear. We compared evaporative heat loss (H E) and changes in body heat content (ΔHb) between young (20–30 yr), middle-aged (40–45 yr), and older males (60–70 yr) of similar body mass and surface area, during successive exercise (4 × 15 min) and recovery periods (4 × 15 min) at a fixed rate of heat production (400 W) and under fixed environmental conditions (35°C/20% relative humidity). H E was lower in older males vs. young males during each exercise (Ex1: 283 ± 10 vs. 332 ± 11 kJ, Ex2: 334 ± 10 vs. 379 ± 5 kJ, Ex3: 347 ± 11 vs. 392 ± 5 kJ, and Ex4: 347 ± 10 vs. 387 ± 5 kJ, all P < 0.02), whereas H E in middle-aged males was intermediate to that measured in young and older adults (Ex1: 314 ± 13, Ex2: 355 ± 13, Ex3: 371 ± 13, and Ex4: 365 ± 8 kJ). H E was not significantly different between groups during the recovery periods. The net effect over 2 h was a greater ΔHb in older (267 ± 33 kJ; P = 0.016) and middle-aged adults (245 ± 16 kJ; P = 0.073) relative to younger counterparts (164 ± 20 kJ). As a result of a reduced capacity to dissipate heat during exercise, which was not compensated by a sufficiently greater rate of heat loss during recovery, both older and middle-aged males had a progressively greater rate of heat storage compared with young males over 2 h of intermittent exercise.

Is active sweating during heat acclimation required for improvements in peripheral sweat gland function?

We investigated whether the eccrine sweat glands must actively produce sweat during heat acclimation if they are to adapt and increase their capacity to sweat. Eight volunteers received intradermal injections of BOTOX, to prevent neural stimulation and sweat production of the sweat glands during heat acclimation, and saline injections as a control in the contralateral forearm. Subjects performed 90 min of moderate-intensity exercise in the heat (35 degrees C, 40% relative humidity) on 10 consecutive days. Heat acclimation decreased end-exercise heart rate (156 +/- 22 vs. 138 +/- 17 beats/min; P = 0.0001) and rectal temperature (38.2 +/- 0.3 vs. 37.9 +/- 0.3 degrees C; P = 0.0003) and increased whole body sweat rate (0.70 +/- 0.29 vs. 1.06 +/- 0.50 l/h; P = 0.030). During heat acclimation, there was no measurable sweating in the BOTOX-treated forearm, but the control forearm sweat rate during exercise increased 40% over the 10 days (P = 0.040). Peripheral sweat gland function was assessed using pilocarpine iontophoresis before and after heat acclimation. Before heat acclimation, the pilocarpine-induced sweat rate of the control and BOTOX-injected forearms did not differ (0.65 +/- 0.20 vs. 0.66 +/- 0.22 mg x cm(-2) x min(-1)). However, following heat acclimation, the pilocarpine-induced sweat rate in the control arm increased 18% to 0.77 +/- 0.21 mg x cm(-2) x min(-1) (P = 0.021) but decreased 52% to 0.32 +/- 0.18 mg x cm(-2) x min(-1) (P < 0.001) in the BOTOX-treated arm. Using complete chemodenervation of the sweat glands, coupled with direct cholinergic stimulation via pilocarpine iontophoresis, we demonstrated that sweat glands must be active during heat acclimation if they are to adapt and increase their capacity to sweat.

Dehydration and Thermal Strain in Junior Tennis

Playing tennis effectively and safely in the heat can be particularly challenging in junior tennis, especially during organized tournament competition when young players have to compete in demanding environmental conditions several times on the same day. With sweat rates ranging from 0.5 L to more than 2.0 L per hour, a young player can readily incur significant total body water and sodium deficits during a very long match or when participating in multiple matches on the same day over several days in a row. On-court thermal strain can be quite high, as tournament-level tennis can elicit appreciable metabolic heat production and storage, even during doubles; this can be exacerbated by poor hydration as well as carryover effects from previous same-day competition and heat exposure. Appropriate and effective safety and performance guidelines for young tennis players training and competing in the heat should focus on readily modifiable risk factors such as hydration management and scheduling of play versus any purported inherent thermoregulatory disadvantages in this specific age group.

Sodium ion concentration vs. sweat rate relationship in humans

The purpose of this study was to determine the effect of active heat acclimation on the sweat osmolality and sweat sodium ion concentration vs. sweat rate relationship in humans. Eight healthy male volunteers completed 10 days of exercise in the heat. The mean exercising heart rate and core temperature were significantly decreased ( P < 0.05) by 18 beats/min and 0.6°C, respectively, following heat acclimation. Furthermore, sweat osmolality and the sweat sodium ion concentration vs. sweat rate relationships were shifted to the right. Specifically, the slopes of the relationships were not affected by heat acclimation. Rather, heat acclimation significantly reduced the y-intercepts of the sweat osmolality and sweat sodium relationships with sweat rate by 28 mosmol/kgH2O and 15 mmol/l, respectively. Thus there was a significantly lower sweat sodium ion concentration for a given sweat rate following heat acclimation. These results suggest that heat acclimation increases the sodium ion reabsorption capacity of the human eccrine sweat gland.

Measurement of cytokines in sweat patches and plasma in healthy women: validation in a controlled study

Cytokines have been detected by ELISA in a variety of body fluids. Recycling immunoaffinity chromatography (RIC) coupled with laser-induced fluorescence detection is a highly sensitive and specific method, which allows simultaneous measurements of many analytes in small volumes of biological fluids. This method has been applied to plasma, cervical secretions and other body fluids, but has not previously been applied to sweat. The aim of this study was to validate the RIC methodology in sweat for measurements of IL-1alpha, IL-1beta, IL-6, TNF-alpha, IL-8 and TGF-beta. Two sweat patches were applied for 24 h on the torso, and blood was collected at one time point during this period in nine healthy women. Cytokines were measured in paired samples of plasma and sweat. Cytokines were detected in sweat in similar concentrations to plasma. Linear regression analysis confirmed that sweat levels of these cytokines accounted for a large percentages of variance in plasma levels: IL-1alpha (R2 = 0.70, p = 0.005), IL-1beta (R2 = 0.79, p = 0.003), IL-6 (R2 = 0.52, p = 0.03), TNF-alpha (R2 = 0.95, p < 0.0001), IL-8 (R2 = 0.81, p = 0.001) and TGF-beta (R2 = 0.94, p = 0.0003). These findings indicate that cytokine levels measured in sweat are informative of circulating levels and that sweat patches combined with RIC represents a viable non-invasive method to measure cytokines in ambulatory settings over time. This method is unobtrusive and requires minimal active compliance on the part of the subjects being studied, without pain or stress. This approach can open a new generation of studies to address the effects of environmental factors on immune responses in a wide range of different settings.

Health benefits for veteran (senior) tennis players

To explore the health benefits of tennis participation in veteran players and to identify future research needs, an electronic literature search using the Ovid (Cinhal, Medline, Sport Discus) library databases from 1966-2005 was undertaken. Specific search words were employed related to tennis, aging, exercise, health, and the psychophysiological systems. Public access internet search engines were also used (Google, PubMed), along with non-electronic searches of library holdings. There is ample research documenting the health benefits of regular participation in moderately intense aerobic activity. There have been research studies targeting veteran tennis players but most were cross sectional. No tennis related study successfully eliminated all confounding cross training effects. The health of veteran tennis players is improved by enhanced aerobic capacity, greater bone densities in specific regions, lower body fat, greater strength, and maintained reaction time performance in comparison with age matched but less active controls. However, it is not certain whether tennis alone can be a sole contributor to these physiological variables. Well controlled longitudinal research among elite veteran and novice older adult players is needed.

Effects of Short-term Exercise in the Heat on Thermoregulation, Blood Parameters, Sweat Secretion and Sweat Composition of Tropic-dwelling Subjects

This study investigates the effects of a short-term aerobic training program in a hot environment on thermoregulation, blood parameters, sweat secretion and composition in tropic-dwellers who have been exposed to passive heat. Sixteen healthy Malaysian-Malay male volunteers underwent heat acclimation (HA) by exercising on a bicycle ergometer at 60% of VO2max for 60 min each day in a hot environment (Ta: 31.1+/-0.1 degrees C, rh: 70.0+/-4.4%) for 14 days. All parameters mentioned above were recorded on Day 1 and at the end of HA (Day 16). On these two days, subjects rested for 10 min, then cycled at 60% of VO2max for 60 min and rested again for 20 min (recovery) in an improvised heat chamber. Rectal temperature (Tre), mean skin temperature (Tsk) heart rate (HR), ratings of perceived exertion (RPE), thermal sensation (TS), local sweat rate and percent dehydration were recorded during the test. Sweat concentration was analysed for sodium [Na+]sweat and potassium. Blood samples were analysed for biochemical changes, electrolytes and hematologic indices. Urine samples were collected before and after each test and analysed for electrolytes.After the period of acclimation the percent dehydration during exercise significantly increased from 1.77+/-0.09% (Day 1) to 2.14+/-0.07% (Day 16). Resting levels of hemoglobin, hematocrit and red blood cells decreased significantly while [Na+]sweat increased significantly. For Tre and Tsk there were no differences at rest. Tre, HR, RPE, TS, plasma lactate concentration, hemoglobin and hematocrit at the 40th min of exercise were significantly lower after the period of acclimation but mean corpuscular hemoglobin and serum osmolality were significantly higher while no difference was seen in [Na+]sweat and Tsk. It can be concluded that tropic-dwelling subjects, although exposed to prolonged passive heat exposure, were not fully heat acclimatized. To achieve further HA, they should gradually expose themselves to exercise-heat stress in a hot environment.

Aging and assessment of physiological strain during exercise-heat stress

The purpose of this study was to evaluate the physiological strain index (PSI) for different age groups during exercise-heat stress (EHS). PSI was applied to three different databases. First, from young and middle-age men (21 +/- 2 and 46 +/- 5 yr, respectively) matched (n = 9 each, P > 0.05) for maximal aerobic power. Subjects were heat acclimated by daily treadmill walking for two 50-min bouts separated by 10-min rest for 10 days in a hot-dry environment [49 degrees C, 20% relative humidity (RH)]. The second database involved a group (n = 8) of young (YA) and a group (n = 7) of older (OA) men (26 +/- 1 and 69 +/- 1 yr, respectively) who underwent 16 wk of aerobic training and two control groups (n = 7 each) who were matched for age to YA and OA. These four groups performed EHS at 36 degrees C, 40% RH on a cycle ergometer for 60 min at 60% maximal aerobic power before and after training. The third database was obtained from three groups of postmenopausal women and a group of 10 men. Two groups of women (n = 8 each) were undergoing hormone replacement therapy, estrogen or estrogen plus progesterone, and the third group (n = 9) received no hormone replacement. Subjects were over 50 yr and performed the same EHS: exercising at 36 degrees C, 40% RH on a cycle ergometer for 60 min. PSI assessed the strain for all three databases and reported differences were significant at P < 0.05. This index rated the strain in rank order, whereas the postacclimation and posttraining groups were assessed as having less strain than the preacclimation and pretraining groups. Furthermore, middle-aged women on estrogen replacement therapy had less strain than estrogen + progesterone and no hormone therapy. PSI evaluation was extended for men and women of different ages (50-70 yr) during acute EHS, heat acclimation, after aerobic training, and inclusive of women undergoing hormone replacement therapy.

Potential applications of heat and cold stress indices to sporting events

Many recreational and elite athletes participate in sporting events every year. However, when these events are conducted under hostile environmental conditions, whether in cold or hot climates, the risk for environmental illnesses increases. The higher the stress, the greater is the potential for performance decrements, injuries and illnesses. The most common expected heat illnesses are heat exhaustion and heatstroke, whereas hypothermia and frostbite are the most common cold injuries. However, heat and cold stress indices can minimise the risk for environmental illnesses and dehydration by following the recommendations and guidelines which accompany these indices. Stress indices should be used by athletes, coaches and officials to prevent injury and improve safety conditions for competitors and participants in recreational activities. All participants should be made aware of warning signs, susceptibility and predisposing conditions. Coaches should be aware of their responsibility with regard to the safety of their trainees, and officials should organise and plan events at times that are likely to be of low environmental stress. However, they must also be prepared and equipped with the means necessary to reduce injuries and treat cases of collapse and environmental illnesses. The lack of a friendly, small and simple device for environmental stress assessment is probably the main reason why stress indices are not commonly used. We believe that developing a new portable heat and cold stress monitor in wristwatch format for use by those exposed to environmental stress could help in the decision making process of expected hazards caused by exercising and working in hostile environments, and might help prevent heat and cold illnesses.

Recovery from transport and acclimatisation of competition horses in a hot humid environment

The aims of the present field-based study were to investigate changes in fit horses undergoing acclimatisation to a hot humid environment and to provide data on which to base recommendations for safe transport and acclimatisation. Six horses (age 7-12 years) were flown from Europe to Atlanta and underwent a 16 day period of acclimatisation. Exercise conditions during acclimatisation (wet bulb globe temperature index 27.6+/-0.0 [mean +/- s.e.]) were more thermally stressful compared with the European climate from which the horses had come (22.0+/-1.8, P<0.001). Following the flight, weight loss was 4.1+/-0.8% bodyweight and took around 7 days to recover. Water intake during the day was significantly increased (P<0.05) compared with night during acclimatisation. Daily mean exercise duration was 72+/-12 min and the majority of work was performed with a heart rate below 120 beats/min. Respiratory rate (fR) was increased (P<0.05) throughout acclimatisation compared with in Europe, but resting morning (AM) and evening (PM) rectal temperature (TREC), heart rate (fC) and plasma volume were unchanged. White blood cell (WBC) count was significantly increased at AM compared with in Europe on Days 4 and 10 of acclimatisation (P<0.01), but was not different by Day 16. In conclusion, horses exposed to hot humid environmental conditions without prior acclimatisation are able to accommodate these stresses and, with appropriate management, remain fit and clinically healthy, without significant risk of heat illness or heat-related disorders, provided they are allowed sufficient time to recover from transport, acclimatisation is undertaken gradually and they are monitored appropriately.

The thermophysiology of uncompensable heat stress. Physiological manipulations and individual characteristics

In many athletic and occupational settings, the wearing of protective clothing in warm or hot environments creates conditions of uncompensable heat stress where the body is unable to maintain a thermal steady state. Therefore, special precautions must be taken to minimise the threat of thermal injury. Assuming that manipulations known to reduce thermoregulatory strain during compensable heat stress would be equally effective in an uncompensable heat stress environment is not valid. In this review, we discuss the impact of hydration status, aerobic fitness, endurance training, heat acclimation, gender, menstrual cycle, oral contraceptive use, body composition and circadian rhythm on heat tolerance while wearing protective clothing in hot environments. The most effective countermeasure is ensuring that the individual is adequately hydrated both before and throughout the exercise or work session. In contrast, neither short term aerobic training or heat acclimation significantly improve exercise-heat tolerance during uncompensable heat stress. While short term aerobic training is relatively ineffective, long term improvements in physical fitness appear to provide some degree of protection. Individuals with higher proportions of body fat have a lower heat tolerance because of a reduced capacity to store heat. Women not using oral contraceptives are at a thermoregulatory disadvantage during the luteal phase of the menstrual cycle. The use of oral contraceptives eliminates any differences in heat tolerance throughout the menstrual cycle but tolerance is reduced during the quasi-follicular phase compared with non-users. Diurnal variations in resting core temperature do not appear to influence tolerance to uncompensable heat stress.

Effect of an exercise-heat acclimation program on body fluid regulatory responses to dehydration in older men

We examined if an exercise-heat acclimation program improves body fluid regulatory function in older subjects, as has been reported in younger subjects. Nine older (Old; 70 +/- 3 yr) and six younger (Young; 25 +/- 3 yr) male subjects participated in the study. Body fluid regulatory responses to an acute thermal dehydration challenge were examined before and after the 6-day acclimation session. Acute dehydration was produced by intermittent light exercise [4 bouts of 20-min exercise at 40% peak rate of oxygen consumption (VO(2 peak)) separated by 10 min rest] in the heat (36 degrees C; 40% relative humidity) followed by 30 min of recovery without fluid intake at 25 degrees C. During the 2-h rehydration period the subjects drank a carbohydrate-electrolyte solution ad libitum. In the preacclimation test, the Old lost approximately 0.8 kg during dehydration and recovered 31 +/- 4% of that loss during rehydration, whereas the Young lost approximately 1.2 kg and recovered 56 +/- 8% (P < 0.05, Young vs. Old). During the 6-day heat acclimation period all subjects performed the same exercise-heat exposure as in the dehydration period. Exercise-heat acclimation increased plasma volume by approximately 5% (P < 0.05) in Young subjects but not in Old. The body fluid loss during dehydration in the postacclimation test was similar to that in the preacclimation in Young and Old. The fractional recovery of lost fluid volume during rehydration increased in Young (by 80 +/- 9%; P < 0.05) but not in Old (by only 34 +/- 5%; NS). The improved recovery from dehydration in Young was mainly due to increased fluid intake with a small increase in the fluid retention fraction. The greater involuntary dehydration (greater fluid deficit) in Old was accompanied by reduced plasma vasopressin and aldosterone concentrations, renin activity, and subjective thirst rating (P < 0.05, Young vs. Old). Thus older people have reduced ability to facilitate body fluid regulatory function by exercise-heat acclimation, which might be involved in attenuation of the acclimation-induced increase in body fluid volume.