Heather E. Wright

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.

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 (HE) 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). HE 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 HE 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). HE 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.

Do older females store more heat than younger females during exercise in the heat

INTRODUCTION Aging is associated with a reduction in the bodys capacity to dissipate heat. To date, few studies have examined age-related changes in thermoregulatory function during short exercise periods in the heat in older females. PURPOSE This study aimed to investigate the effects of age on whole-body heat loss during intermittent exercise in the heat in young and older females. METHODS Direct and indirect calorimetry was used to measure whole-body evaporative heat loss (EHL), change in body heat content, and metabolic heat production. Eleven young (Y) (mean ± SD age = 24 ± 4 yr) and 13 older (O) (51 ± 8 yr) females matched for body surface area (Y, 1.72 ± 0.15; O, 1.75 ± 0.12 m²) and fitness (V(˙)O(2max)) (Y, 36.7 ± 6.8 mL O₂·kg⁻¹·min⁻¹; O, 33.8 ± 8.0 mL O₂·kg⁻¹·min⁻¹) performed four bouts of 15-min cycling (Ex1, Ex2, Ex3, and Ex4) at a constant rate of heat production (300 W) at 35°C and 20% relative humidity. Each exercise bout was separated by 15 min of rest. RESULTS EHL was reduced in O compared with Y during Ex1 (O, 199 ± 6 W; Y, 240 ± 9 W; P = 0.001), Ex2 (O, 238 ± 4 W; Y, 261 ± 9 W, P = 0.023), and Ex3 (O, 249 ± 4 W; Y, 274 ± 11 W; P = 0.040). EHL was not different between groups during Ex4 or during the recovery periods. Older females had a greater change in body heat content compared with young females (O, 270 ± 20 kJ; Y, 166 ± 20 kJ; P = 0.001). CONCLUSION These findings suggest that older females have a lower capacity for whole-body EHL compared with younger females during short intermittent exercise in the heat performed at a fixed rate of metabolic heat production.

Do older firefighters show long-term adaptations to work in the heat?

Older experienced firefighters may show signs of heat adaptation, and thus reduced physiological strain, due to repeated occupational heat stress exposure. The aim was to examine physiological and perceptual strain, and hydration, responses to intermittent exercise in the heat in 12 older Non-Firefighter (Non-FF) and experienced Firefighter (FF) males, pair matched for age (Group mean ± SE: Non-FF = 51.7 ± 1.5, FF = 49.8 ± 1.1 years), VO2peak (Non-FF = 39.4 ± 2.2, FF = 40.7 ± 1.8 mL·kg−1·min−1), body surface area (Non-FF = 1.94 ± 0.04, FF = 2.03 ± 0.03 m2), and percent body fat (Non-FF = 24.4 ± 2.3, FF = 19.3 ± 1.8%). Rectal (Tre) and mean skin (MTsk) temperatures, heart rate (HR), local sweat rate (LSR), hydration indices, and ratings of thermal sensation and perceived exertion were measured during 4 ×15-min (rest 15-min) moderate-to-heavy cycling bouts (400 W heat production) in Dry and Humid heat (35°C, ∼20 and ∼60% relative humidity, respectively). No differences were observed between the Non-FF and FF for Tre, Tre change, MTsk, HR,% max HR, LSR, physiological strain index (PhSI), or % plasma volume change. Plasma protein concentration was reduced at baseline for the Non-FF (7.6 ± 0.1 g·100 mL−1) than FF (8.0 ± 0.1 g·100 mL−1). The Perceptual Strain Index overestimated PhSI for Non-FF and FF in both thermal conditions. At the end of exercise, the Non-FF showed a greater Tre difference between thermal conditions (0.27 ± 0.05°C) compared to the FF (0.10 ± 0.09°C). Although the Non-Firefighters and Firefighters demonstrate similar cardiovascular and hydration responses during moderate-to-heavy intensity exercise within each of the thermal conditions, the attenuated thermal effects between the two heat stress conditions in the Firefighters suggests a protective adaptation.

Cortisol and Interleukin-6 Responses During Intermittent Exercise in Two Different Hot Environments with Equivalent WBGT

Blood marker concentrations such as cortisol (COR) and interleukin (IL)-6 are commonly used to evaluate the physiological strain associated with work in the heat. It is unclear, however, if hot environments of an equivalent thermal stress, as defined by a similar wet bulb globe temperature (WBGT), result in similar response patterns. This study examined markers of neuroendocrine (COR) and immune (IL-6) responses, as well as the cardiovascular and thermal responses, relative to changes in body heat content measured by whole-body direct calorimetry during work in two different hot environments with equivalent WBGT. Eight males performed a 2-hr heavy intermittent exercise protocol (six 15-min bouts of cycling at a constant rate of metabolic heat production (360W) interspersed by 5-min rest periods) in Hot/Dry (46°C, 10% relative humidity [RH]) and Warm/Humid (33°C, 60% RH) conditions (WBGT ∼ 29°C). Whole-body evaporative and dry heat exchange, change in body heat content (ΔHb), rectal temperature (Tre), and heart rate were measured continuously. Venous blood was obtained at rest (PRE) and the end of each exercise bout for the measurement of changes in plasma volume (PV), plasma protein (an estimate of plasma water changes), COR, and IL-6. Ratings of perceived exertion and thermal sensation were measured during the last minute of each exercise bout. No differences existed for ΔHb, heart rate, Tre,%ΔPV, plasma protein concentration, perceptual strain (thermal sensation, perceived exertion), and COR between the Hot/Dry and Warm/Humid conditions. IL-6 exhibited an interaction effect (p = 0.041), such that greater increases were observed in the Hot/Dry (Δ = 1.61 pg·mL−1) compared with the Warm/Humid (Δ = 0.64 pg·mL−1) environment. These findings indicate that work performed in two different hot environments with equivalent WBGT resulted in similar levels of thermal, cardiovascular, and perceptual strain, which support the use of the WBGT stress index. However, the greater IL-6 response in the Hot/Dry requires further research to elucidate the effects of different hot environments and work intensities.

Influence of circulating cytokines on prolactin during slow vs. fast exertional heat stress followed by active or passive recovery

Prolactin (PRL) has been suggested as an indicator of fatigue during exertional heat stress (EHS), given its strong relationship with body core temperature (T(c)); however, the strength of this relationship during different rates of T(c) increase and subsequent recovery is unknown. In addition, given the influence that systemic cytokines, such as interleukin (IL)-6 and tumor necrosis factor (TNF)-α, have on the pituitary gland, it would be of interest to determine the relationship between PRL, IL-6, and TNF-α during EHS. The purpose was to examine the PRL, IL-6, and TNF-α heat stress responses during slow and fast heating and subsequent resting or cold water immersion recovery. On 4 days, nine individuals walked at ≈ 45% (slow heating) or ran at ≈ 65% (fast heating) maximal oxygen consumption on a treadmill in the heat (40°C, 30% relative humidity) until rectal temperature (T(re)) reached 39.5°C (esophageal temperature; fast = 39.41 ± 0.04°C, slow = 39.82 ± 0.09°C). Post-EHS, subjects were either immersed in 2°C water or rested seated until T(re) returned to 38.0°C. Venous blood, analyzed for PRL, IL-6, and TNF-α, was obtained at rest, during exercise (T(re) 38.0, 39.0, 39.5°C), the start of recovery (≈ 5 min after 39.5°C), and subsequent recovery (T(re) 39.0, 38.0°C). IL-6 exhibited myokine properties, given the greater increases with slow heating and lack of increase in TNF-α. A strong temperature-dependent PRL response during slow and fast heating provides additional support for the use of PRL as a peripheral marker of impending fatigue, which is independent of IL-6 and TNF-α cytokine responses.

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.

Are circulating cytokine responses to exercise in the heat augmented in older men

Age-related chronic low-grade inflammation may render older individuals more susceptible to heat illnesses. The purpose of this study was to examine the influence of intermittent work in the heat on the circulating cytokine responses of older workers. Fourteen young (aged 25.6 ± 0.7 years) and older (aged 57.7 ± 1.5 years) males, matched for body surface area, cycled for 4 × 15 min (separated by 15-min rest) at moderate to heavy intensity (400 W heat production) in warm/dry (35 °C, 20% relative humidity (RH)) and warm/humid (35 °C, 60% RH) conditions. Rectal (Tre) and mean skin (MTsk) temperatures and heart rate were measured continuously, ratings of perceived exertion and thermal sensation recorded at the end of each exercise bout, and blood samples at baseline (PRE) and following the final 60-min recovery (POST) were analyzed for interleukin (IL)-6, tumor necrosis factor (TNF)-α, and percent changes in blood (BV) and plasma (PV) volumes. No differences were observed between the age groups for Tre, MTsk, heart rate, perceptual strain, or percentage of changes in BV, PV, or ΔTNF-α. Under both conditions, the older males had elevated IL-6 and TNF-α (PRE, POST) compared with the young males. ΔIL-6 tended to be greater in the warm/humid condition (+2.53 ± 0.49 and +1.52 ± 0.41 pg·mL(-1)) compared with the warm/dry condition (+1.02 ± 0.13 and +0.68 ± 0.18 pg·mL(-1)) for older but not young males, respectively. Young and older males experienced similar thermal, cardiovascular, and perceptual strain within the warm/dry and warm/humid conditions.

Do physiological and pathological stresses produce different changes in heart rate variability

Although physiological (e.g., exercise) and pathological (e.g., infection) stress affecting the cardiovascular system have both been documented to be associated with a reduction in overall heart rate variability (HRV), it remains unclear if loss of HRV is ubiquitously similar across different domains of variability analysis or if distinct patterns of altered HRV exist depending on the stressor. Using Continuous Individualized Multiorgan Variability Analysis (CIMVA™) software, heart rate (HR) and four selected measures of variability were measured over time (windowed analysis) from two datasets, a set (n = 13) of patients who developed systemic infection (i.e., sepsis) after bone marrow transplant (BMT), and a matched set of healthy subjects undergoing physical exercise under controlled conditions. HR and the four HRV measures showed similar trends in both sepsis and exercise. The comparison through Wilcoxon sign-rank test of the levels of variability at baseline and during the stress (i.e., exercise or after days of sepsis development) showed similar changes, except for LF/HF, ratio of power at low (LF) and high (HF) frequencies (associated with sympathovagal modulation), which was affected by exercise but did not show any change during sepsis. Furthermore, HRV measures during sepsis showed a lower level of correlation with each other, as compared to HRV during exercise. In conclusion, this exploratory study highlights similar responses during both exercise and infection, with differences in terms of correlation and inter-subject fluctuations, whose physiologic significance merits further investigation.

Body heat storage during intermittent work in hot- dry and warm-wet environments

We examined heat balance using an American Conference of Governmental Industrial Hygienists threshold limit value allocated exercise protocol in hot-dry (HD; 46 °C, 10% relative humidity (RH)) and warm-wet (WW; 33 °C, 60% RH) environments of equivalent WBGT (29 °C) for different clothing ensembles. Whole-body heat exchange and changes in body heat content (ΔH(b)) were measured using simultaneous direct whole-body and indirect calorimetry. Eight males performed six 15-min cycling periods at a constant rate of metabolic heat production (360 W) interspersed by 5-min rest periods for six experimental trials: HD and WW environments for a seminude control (CON), modified work uniform (MWU, moisture permeable top and work pants), and standard work uniform (SWU, work coveralls and cotton undergarments). Whole-body evaporative and dry heat exchange, rectal temperature (T(re)), and heart rate were measured continuously. The cumulative ΔH(b) during the 2 h intermittent exercise protocol was similar between HD and WW environments for each of the clothing ensembles (CON, 387 ± 55 vs. 435 ± 49 kJ; MWU, 485 ± 58 vs. 531 ± 61 kJ; SWU, 585 ± 74 vs. 660 ± 54 kJ, respectively). Similarly, no differences in T(re) (CON, 37.67 ± 0.07 vs. 37.48 ± 0.08 °C; MWU, 37.73 ± 0.08 vs. 37.53 ± 0.09 °C; SWU, 38.01 ± 0.09 vs. 37.94 ± 0.05 °C) or heat rate (CON, 93 ± 3 vs. 84 ± 3 beats·min⁻¹; MWU, 102 ± 5 vs. 95 ± 9 beats·min⁻¹; SWU, 119 ± 8 vs. 110 ± 9 beats·min⁻¹) were observed at the end of the 2 h intermittent exercise protocol in HD vs. WW environments, respectively. We showed similar levels of thermal and cardiovascular strain for intermittent work performed in high heat stress conditions of varying environmental conditions but similar WBGT.