Draft - University of Toronto T-Space€¦ · Draft 1 1 Local vs. whole-body sweating adaptations following 14 days of traditional 2 heat acclimation 3 Martin P. Poirier1, Daniel

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Local vs. whole-body sweating adaptations following 14

days of traditional heat acclimation

Journal: Applied Physiology, Nutrition, and Metabolism

Manuscript ID apnm-2015-0698.R1

Manuscript Type: Article

Date Submitted by the Author: 15-Mar-2016

Complete List of Authors: Poirier, Martin; University of Ottawa Gagnon, Daniel; University of Texas Southwestern Medical Center at Dallas, Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital of Dallas Kenny, Glen; University of Ottawa,

Keyword: Heat acclimatization, Local sweating, Mean body temperature, Whole-body

sweat rate

https://mc06.manuscriptcentral.com/apnm-pubs

Applied Physiology, Nutrition, and Metabolism

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Local vs. whole-body sweating adaptations following 14 days of traditional 1

heat acclimation 2

Martin P. Poirier1, Daniel Gagnon

2, and Glen P. Kenny

1 3

4

5 1School of Human Kinetics, Faculty of Health Sciences, 6

University of Ottawa 7

125 University Private, Montpetit Hall 8

Ottawa, ON, Canada K1N 6N5 9

. 10 2Institute for Exercise and Environmental Medicine, 11

Texas Health Presbyterian Hospital of Dallas and 12

University of Texas Southwestern Medical Center, 13

Dallas, TX, USA 14

15

16

17

Authors’ email addresses: 18

Martin P. Poirier, [email protected] 19

Daniel Gagnon,[email protected] 20

Glen P. Kenny, [email protected] 21

22

23

24

Address for correspondence: 25

Dr. Glen P. Kenny 26

School of Human Kinetics, Faculty of Health Sciences, 27

University of Ottawa 28

125 University Private, Montpetit Hall 29

Ottawa, ON, Canada K1N 6N5 30

Tel: (613) 562-5800 ext. 4282 31

Fax: (613) 562-5149 32

Email: [email protected] 33

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ABSTRACT

PURPOSE: To examine if local changes in sweat rate following 14 days of heat acclimation

reflect those that occur at the whole-body level.

METHODS: Both prior to and following a 14-day traditional heat acclimation protocol, 10

males exercised in the heat (35ºC, ~20% relative humidity) at increasing rates of heat production

equal to 300 (Ex1), 350 (Ex2) and 400 (Ex3) W·m-2

. A 10-min recovery period followed Ex1,

while a 20-min recovery period separated Ex2 and Ex3. The exercise protocol was performed in

a direct calorimeter to measure whole-body sweat rate and, on a separate day, in a thermal

chamber to measure local sweat rate (LSR), sweat gland activation (SGA), and sweat gland

output (SGO) on the upper back, chest, and mid-anterior forearm.

RESULTS: Post-acclimation, whole-body sweat rate was greater during each exercise bout

(Ex1: 14.3±0.9; Ex2: 17.3±1.2; Ex3: 19.4±1.3 g/min, all p≤0.05) relative to pre-acclimation

(Ex1: 13.1±0.6; Ex2: 15.4±0.8; Ex3: 16.5±1.3 g/min). In contrast, only LSR on the forearm

increased with acclimation, and this increase was only observed during Ex2 (Post: 1.32±0.33 vs.

Pre: 1.06±0.22 mg·min-1

·cm-2

, p=0.03) and Ex3 (Post: 1.47±0.41 vs. Pre: 1.17±0.23 mg·min-

1·cm

-2, p=0.05). The greater forearm LSR post-acclimation was due to an increase in SGO, as no

changes in SGA were observed.

CONCLUSION: Overall, these data demonstrate marked regional variability in the effect of

heat acclimation on LSR, such that not all local measurements of sweat rate reflect the

improvements observed at the whole-body level.

Key Words: heat acclimatization, whole-body sweat rate, local sweating, mean body

temperature.

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INTRODUCTION

Heat acclimation is widely recognized as an effective strategy for improving work

tolerance and exercise performance in the heat (Taylor 2014). These improvements are generally

ascribed to beneficial adaptations to both the cardiovascular and thermoregulatory systems that

accompany short (5-7 days) to longer (7-14 days) duration heat acclimation protocols.

Particularly important for thermoregulatory control are the improvements in sweating that

accompany heat acclimation, which provide a greater potential for evaporative heat loss and,

therefore, attenuated increases in core temperature during exercise.

A number of studies have examined potential improvements in sweating with heat

acclimation, with most studies using changes in body weight and measurements of local sweat

rate (see Table 1), the latter showing a high level of heterogeneity between skin sites.

Nevertheless, it is noticeable that most studies have noted improvements in sweating with heat

acclimation (Table 1), which have been attributed to an earlier onset threshold (Cotter et al.

1997; Fox et al. 1963; Lee et al. 2010; Nadel et al. 1974; Patterson et al. 2004; Roberts et al.

1977; Shvartz et al. 1979) as well as an increase in thermosensitivity (Nadel et al. 1974;

Patterson et al. 2004; Peter and Wyndham 1966; Shvartz et al. 1979) and sweat output per

individual gland (Candas et al. 1980; Inoue et al. 1999; Lee et al. 2010; Peter and Wyndham

1966; Sato et al. 1990). On the other hand, it is surprising to note that a number of studies have

observed a lack of improvement in sweating with heat acclimation (Cotter et al. 1997; Gisolfi

1973; Havenith and Middendorp 1986; Hessemer et al. 1986; Inoue et al. 1999). Such

contradicting observations could result from: i) local sweat rate measurements, sampled from a

small area of skin (0.7 to 13 cm2), not reflecting changes that occur at the whole-body level, ii)

different skin sites being measured between studies, and/or; iii) the combination of exercise

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intensity and environmental conditions being compensable prior to and after heat acclimation

which would not allow for measurable improvements in sweating (Taylor 2014).

Our laboratory has recently demonstrated that direct measurements of whole-body

evaporative heat loss improve by ~18% following a 14-day traditional heat acclimation protocol

(Poirier et al. 2015). Importantly, these improvements were most notable at combinations of

exercise intensity and environmental conditions that were uncompensable both prior to and

following the heat acclimation protocol. Since previous studies have mostly relied on local sweat

rate measurements to determine how the body’s capacity to dissipate heat changes with heat

acclimation, we sought to examine whether improvements in whole-body evaporative heat loss,

and therefore whole-body sweat rate, are paralleled by similar improvements in local sweat rate

measured at commonly measured skin sites (i.e., upper back, chest, and mid-anterior forearm).

Such information is critical in identifying local sites that best represent improvements in whole-

body sweat rate during heat acclimation. Furthermore, we assessed potential mechanisms

governing changes in sweating as determined by the onset threshold and the thermosensitivity of

the sweating response as well as the number of locally activated sweat glands and the sweat

output per gland. We hypothesized that improvements in whole-body evaporative heat loss

following 14 days of traditional heat acclimation would be paralleled by improvements in local

sweat rate measurements. It was further hypothesized that improvements in local sweat rate

following acclimation would be associated with a lower onset threshold, a greater

thermosensitivity and a greater sweat output per gland.

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MATERIALS AND METHODS

The current experimental protocol was approved by the University of Ottawa Health

Sciences and Science Research Ethics Board in accordance with the Declaration of Helsinki.

Written informed consent was obtained from all participants.

Participants

Ten healthy male participants were recruited within the University of Ottawa community

and volunteered to take part in the study. The participants were recruited to be part of a larger

study examining changes in whole-body heat exchange during and following heat acclimation,

the results of which have been published (Poirier et al. 2015). The current study focuses on

separate experimental sessions performed as part of this larger study, which examined potential

improvements in local sweat rate with heat acclimation. However, the whole-body evaporative

heat loss data from the previously published study was used to calculate changes in whole-body

sweat rate to address the specific research question of the current study. All experimental trials

were carried out between late September and mid-April; therefore, it was assumed that

participants were not already heat acclimatized from environmental heat exposure. In addition,

all participants were not endurance-trained in order to avoid partial acclimation from endurance

training (Gisolfi 1973), albeit they were habitually active (≥ 3 days per week of structured

physical activity for at least 30 min per session). Mean ± standard deviation characteristics of the

participants were: age, 23 ± 3 yrs.; height, 180 ± 5 cm; body mass, 79.5 ± 3.5 kg; body fat

percentage, 15.2 ± 4.5 %; body surface area, 1.99 ± 0.05 m2; and, V� O��: 51.1 ± 4.6 mL·kg

-

1·min

-1. Participants did not report smoking or having any cardiovascular, metabolic or

respiratory diseases.

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Experimental Design

Participants volunteered for a 14-day traditional heat acclimation protocol, during which

they cycled daily in dry heat (35-40ºC, 16-20% relative humidity [RH]) for 90 min at 50% of

their pre-determined V� O��. Whole-body sweat rate (days 0 and 14) and local sweat rate (days

1 and 13) were examined prior to and following the heat acclimation protocol in a direct

calorimeter and a thermal chamber, respectively. A preliminary session was performed within 2

weeks of the experimental protocol.

Preliminary session. During the preliminary session, body height was measured using a

stadiometer (Detecto, model2391, Webb City, MO, USA) and body mass was determined using

a digital high-performance weighing platform (model CBU150X, Mettler Toledo Inc.,

Mississauga, ON, CAN). Subsequently, both measurements were used to calculate body surface

area (DuBois and DuBois 1916). The hydrostatic weighing technique was employed to estimate

body density, which was used to calculate body fat percentage using the Siri Equation (Siri

1956). Maximum oxygen consumption was also measured as previously described (Poirier et al.

2015).

Direct Calorimetry Sessions. Whole-body sweat rate was evaluated using direct

calorimetry on days 0 and 14 of the heat acclimation protocol. Upon arrival to the laboratory,

participants were asked to provide a urine sample as well as a nude body weight. Participants

subsequently changed into running shorts and sandals before being instrumented at a temperature

of ~24°C. Once instrumented, participants entered the modified Snellen whole-body calorimeter

which was regulated at an ambient temperature of 35.2 ± 0.1°C and an absolute humidity of 5.56

± 2.39 grams of water vapour per kilogram of dry air (16 % RH). After 30 min of baseline rest,

intermittent exercise was performed at fixed rates of metabolic heat production equal to 300

(Ex1), 350 (Ex2) and 400 W·m-2

(Ex3), each bout being 30 min in duration. A 10 min recovery

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period separated Ex1 and Ex2, while a 20 min recovery period was allocated between Ex2 and

Ex3 to minimize fatigue. The baseline session (day 0) was always performed within one week of

the start of the 14-day heat acclimation period (day 1 to day 14).

Thermal Chamber Sessions. On days 1 and 13 of the heat acclimation protocol, local

sweat rate was assessed within a thermal chamber. Following a similar instrumentation period as

the direct calorimetry sessions, participants entered a thermal chamber maintained at 35°C and

~20% RH. After 30 min of baseline rest, intermittent exercise was performed at fixed rates of

metabolic rate heat production similar to that performed in the calorimeter (see above section).

Local sweat rate was measured continuously, while sweat gland activation was determined at

baseline, as well as within the last 5 min of each exercise bout.

Traditional heat acclimation sessions. On days 2 to 6 and 8 to 12 inclusively, each

participant performed 90 min of upright cycling at an intensity of 50% of their predetermined

�� in a thermal chamber regulated at 40ºC and ~20% RH. The air flow in the chamber was

minimal during the heat acclimation sessions (ranging from 0 to 0.5 m/s). Participants were

asked to refrain from consuming coffee or alcohol 12 hours prior to each acclimation session and

were asked to replace their total weight loss with water after each session. Participants wore

running shorts and sandals during the acclimation sessions. On day 7 of the acclimation protocol,

whole-body heat exchange was evaluated in the Snellen whole-body calorimeter using the same

experimental protocol as described above, the results of which have been published elsewhere

(Poirier et al. 2015).

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Measurements

The modified Snellen whole-body calorimeter was employed for the measurement of whole-

body evaporative heat loss. Since a fundamental characteristic of the direct calorimeter is to

ensure 100% evaporation of the sweat produced, we subsequently calculated whole-body sweat

rate (in g·min-1

) as: evaporative heat loss (in W) multiplied by 60 s and divided by the latent heat

of vaporization of sweat at 30°C (2426 J·g of sweat-1

). For a detailed description of the

calorimeter, the reader is referred to a previously published technical description (Reardon et al.

2006). Whole-body sweat rate was also estimated from the change in body weight (digital high-

performance weighing platform, model CBU150X, Mettler Toledo Inc., Mississauga, ON,

CAN). Due to inherent limitations associated with performing trials in the calorimeter, it was not

always possible to standardize the measurement periods within participants in order to obtain

reliable values for the change in body mass.

During the thermal chamber trials, local sweat rate was measured on the upper back,

chest and mid-anterior forearm using 3.8-cm² ventilated capsules. The sweat capsules were

attached to the skin with adhesive rings and topical skin glue (Collodion HV, Mavidon Medical

products, Lake Worth, FL, USA). Dry compressed air was supplied to each capsule at a rate of

1.0 L·min−1

, while water content of the effluent air was measured with capacitance hygrometers

(Model HMT333, Vaisala, Helsinki, Finland). Local sweat rate was calculated using the

difference in water content between effluent and influent air multiplied by the flow rate and

normalised for the skin surface area under the capsule.

The modified iodine-paper technique (Gagnon et al. 2012) was used to measure the

number of heat activated sweat glands on the upper back, chest and mid-anterior forearm in the

thermal chamber. Cotton paper (32 lb, Southworth Cie, Agawam, MA, USA) was cut into 3 x 3

cm pieces and placed in a sealed container with solid iodine (Sigma-Aldrich Corporation, St

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Louis MO, USA) for 48 hours to ensure the paper was completely saturated with iodine. Before

applying the iodine paper to the skin surface, the area adjacent to the sweat capsule was dried

thoroughly. A minimum of two measurements were taken at each site for each time point and

subsequently scanned and analyzed using ImageJ software (Abramoff et al. 2004). The number

of heat activated sweat glands was divided by the surface area of the paper to provide the number

of active sweat glands per square centimeter. The sweat output per gland was determined by

dividing the sweat rate at the corresponding measurement time by the number of active sweat

glands. In order to obtain the sweat gland activation and output values used for analysis, we

averaged the results obtained from a minimum of two measurements. However, in the case of a

major variance between two measurements, we chose the one that best represented the activation

of the area measured.

Indirect calorimetry was used for the measurement of metabolic energy expenditure

during sessions in the calorimeter and thermal chamber. Expired gas samples were analyzed for

oxygen and carbon dioxide concentrations using electrochemical gas analyzers (AMETEK

model S-3A/1 and CD 3A, Applied Electrochemistry, Pittsburgh, PA, USA). For the direct

calorimetry sessions, expired air was recycled back into the calorimeter chamber to account for

respiratory dry and evaporative heat loss. Prior to each session, gas mixtures of 17% O2, 4%

CO2, and balanced nitrogen were used to calibrate the gas analyzers and a 3 L syringe was used

to calibrate the turbine ventilometer.

For both the calorimeter and thermal chamber sessions, esophageal temperature was

measured using a paediatric thermocouple probe (Mon-a-therm General Purpose Temperature

Probe, Mallinckrodt Medical, St-Louis, MO, USA) inserted 40 cm past the nostril while

participants sipped water (100-300 mL) through a straw. Likewise, skin temperature was

measured using type T thermocouples (Concept Engineering, Old Saybrook, CT, USA) and

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mean skin temperature was calculated using 4 skin temperature measurements weighted to the

following regional proportions (Ramanathan 1964): chest 30%, biceps 30%, thigh 20%, and calf

20%. All temperature data were collected using a data acquisition module (HP Agilent model

3497A; Agilent Technologies Canada Inc., Mississauga, ON, Canada) at a sampling rate of 15

seconds and simultaneously displayed and recorded in spreadsheet format on a personal

computer with LabVIEW software (Version 7.0, National Instruments, Austin, TX, USA). The

measurements of esophageal and skin temperatures were subsequently used to calculate mean

body temperature as: 0.9 x esophageal temperature + 0.1 x mean skin temperature (Shibasaki et

al. 2006).

Heart rate was monitored during all sessions using a Polar coded transmitter, recorded

every 15 seconds and stored with a Polar Advantage interface and Polar Precision Performance

software (Polar Electro Oy, Finland). Urine specific gravity was determined in duplicate prior to

each session using a handheld total solids refractometer (model TS400, Reichter Inc., Depew,

NY, USA).

Data and Statistical Analyses

Minute averages were calculated for all dependent variables to perform the statistical

analyses. The onset threshold (i.e., core temperature at which sweating is initiated) and

thermosensitivity (i.e., rate of increase in sweat rate as a function of changes in mean body

temperature) of local and whole-body sweat rates were determined for each exercise bout by

plotting the sweating response against mean body temperature. Using segmental regression

analysis (Cheuvront et al. 2009), regression lines were fitted to the sweating response prior to

exercise, as well as to the linear portion of the increase in sweat rate during exercise. The “best-

fit” intercept of the regression lines was taken as the onset threshold of the response, while the

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slope of the regression line for the linear increase in sweat rate during exercise was taken as the

thermosensitivity. Repeated measures were analyzed using a two-way analysis of variance with

the repeated factors of exercise bout (3 levels: Ex1, Ex2, Ex3) and acclimation state (2 levels: pre

and post acclimation). When a significant main effect was observed, post hoc comparisons were

carried out using paired samples t tests. The level of significance for all analyses was set at an

alpha level of p<0.05. Statistical analyses were performed using commercially available

statistical software (SPSS 23.0 for Windows, SPSS Inc., Chicago, IL, USA). Regression analyses

were performed using GraphPad Prism 6.0 (Graph Pad Software, La Jolla, CA, USA). All values

are reported as mean ± standard deviation.

RESULTS

Participant’s hydration status, based on measurements of urine specific gravity, was

similar between sessions prior to (calorimeter: 1.012 ± 0.009, thermal chamber: 1.016 ± 0.05)

and following (calorimeter: 1.016 ± 0.009, thermal chamber: 1.016 ± 0.09) heat acclimation

(p=0.211). Pre-trial body weight measurements were also similar before (calorimeter: 80.3 ± 4.2,

chamber: 80.3 ± 3.7 kg) and after (calorimeter: 79.5 ± 3.7, thermal chamber: 79.6 ± 3.5 kg) the

14-day heat acclimation period (p=0.101).

Calorimetry sessions: Whole-body sweat rate

Mean body temperature. A main effect of heat acclimation was observed for mean body

temperature (p=0.024) such that it was significantly lower at baseline (Post: 36.43 ± 0.13˚C vs.

Pre: 36.63 ± 0.18˚C, p=0.003) as well as at the end of all exercise bouts following the 14-day

heat acclimation protocol (all p<0.05, Figure 1). In contrast, the change in mean body

temperature during the protocol did not differ as a function of heat acclimation (p=0.141).

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Whole-body sweat rate. The time-dependent changes in whole-body sweat rate are

depicted in Figure 2, while each participant’s individual responses are shown in Figure 3. As part

of the experimental design, rate of metabolic heat production was similar for each exercise bout

prior to (Ex1: 304 ± 7, Ex2: 348 ± 6, Ex3: 404 ± 7 W·m-2

) and following (Ex1: 299 ± 4, Ex2:

350 ± 5, Ex3: 405 ± 11 W·m-2

, p=0.528) heat acclimation. After the 14-day heat acclimation

period, a main effect of acclimation status was observed for whole-body sweat rate (p≤0.001)

such that it was increased at the end of all exercise bouts post-acclimation (Figure 2 and 3, all

p≤0.001). Furthermore, heat acclimation reduced the absolute onset threshold for whole-body

sweat rate during Ex1 (p=0.02) and Ex3 (p=0.01), while the difference did not reach statistical

significance during Ex2 (p=0.062, Table 2). The thermosensitivity of whole-body sweat rate was

increased for all exercise bouts following heat acclimation (all p≤0.05, Table 2). The relative

change in whole-body sweat rate as assessed by calorimetry and change in body weight was

significantly increased post-acclimation (0.22 ± 0.15 and 0.23 ± 0.15 kg respectively, both

p=0.002, n=9), with no significant difference measured between both methods (p=0.84).

Thermal chamber sessions: Local sweat rate, sweat gland activation and output

Rates of metabolic heat production during the exercise bouts of the thermal chamber

sessions did not differ before (Ex1: 305 ± 5, Ex2: 352 ± 6, Ex3: 405 ± 4 W·m-2

) and after (Ex1:

305 ± 6, Ex2: 353 ± 5, Ex3: 402 ± 4 W·m-2

, p=0.272) the 14-day heat acclimation period.

Mean body temperature. A main effect of acclimation was observed for absolute mean

body temperature (p<0.001) such that it was reduced at baseline (Post: 36.67 ± 0.38˚C vs. Pre:

36.47 ± 0.34˚C, p=0.016) and at the end of all exercise bouts following the 14-day heat

acclimation protocol (all p<0.001, Figure 1). However, the change in mean body temperature

from baseline did not differ throughout the protocol as a function of acclimation (p=0.091).

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Local sweat rate. Time dependent changes in local sweat rate are depicted in Figure 2,

while individual responses are presented in Figure 3. In contrast to the improvements in whole-

body sweat rate, significant increases in local sweat rate were only observed on the mid-anterior

forearm (p=0.049) at the end of Ex2 (p=0.03) and Ex3 (p=0.05). In contrast, local sweat rate did

not change on the chest (p=0.791) and upper back (p=0.229) following heat acclimation. The

absolute mean body temperature onset threshold for local sweat rate was lower at all

measurement sites following heat acclimation (all p≤0.05, Table 2). Although a main effect of

heat acclimation was observed for the thermosensitivity of local sweat rate at all skin sites (all

p≤0.05), post-hoc analyses revealed it increased at the mid-anterior forearm during each exercise

bout (all p≤0.05), and at the chest during Ex3 (p=0.003, Table 2).

Sweat gland activation and sweat gland output. The number of heat activated sweat

glands did not change as a function of heat acclimation on the mid-anterior forearm (p=0.433)

and upper back (p=0.760), however a main effect was observed on the chest (p=0.041) (Table 2).

A greater number of sweat glands were activated on the chest at the end of Ex2 (p=0.004) after

the 14-day heat acclimation period, yet no differences were measured at the end of Ex1

(p=0.483) and Ex3 (p=0.190) (Table 2). Sweat gland output did not change as a function of heat

acclimation on the chest (p=0.316) and upper back (p=0.305), however a main effect of heat

acclimation was observed on the mid-anterior forearm (p≤0.05). The sweat output per gland was

greater on the mid-anterior forearm at the end of Ex2 (p=0.041) after acclimation, whereas no

differences were measured at the end Ex1 (p=0.137) and Ex3 (p=0.062) (Table 2).

DISCUSSION

The main finding of the current study is that improvements in whole-body sweat rate

following a 14-day heat acclimation are not consistently reflected in local measurements of

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sweat rate when assessed on the mid-anterior forearm, chest and upper back. These findings

suggest that local sweat rate measurements may not always reflect changes in whole-body sweat

rate following 14 days of traditional heat acclimation.

Local vs. Whole-body Sweating

It is well recognized that short-term heat acclimation induces physiological adjustments

that improve the body’s ability to dissipate heat, the most important being enhanced sweating

(see Table 1). However, some studies report a lack of improvement in sweating following heat

acclimation (Cotter et al. 1997; Gisolfi 1973; Havenith and Middendorp 1986; Hessemer et al.

1986; Inoue et al. 1999). We recently reported that whole-body sweat evaporation is improved

by as much as 18% following 14 days of heat acclimation (Poirier et al. 2015), which translated

into greater whole-body sweat rate at the end of each exercise bout during the calorimetry

sessions in the current study. In contrast, improvements in local sweat rate were only observed

on the mid-anterior forearm and only during Ex2 and Ex3 (see Figure 2). Contrary to our

hypothesis, we did not observe significant improvements in local sweat rate on the chest and

upper back. These findings highlight a disparity in the pattern of adaptation between local and

whole-body sweating following a period of traditional heat acclimation. The greater whole-body

sweating response observed is likely a combination of improved sweating in local areas that were

not examined during the thermal chamber sessions (e.g. forehead, thigh, abdomen, lower back,

etc.). It should also be considered that local sweat rate measurements were taken from a small

surface area (~3.8 cm2) which is likely not representative of the entire body segment examined

(Smith and Havenith 2011). Regardless of the discrepancy between whole-body and local

sweating adaptations, the current findings may provide a potential explanation for the lack of

improvement in local sweat rates measured on the chest, back and forearm following heat

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acclimation reported in previous studies (Cotter et al. 1997; Hessemer et al. 1986; Inoue et al.

1999). It is possible that previous studies relied upon local sweat rate measurements that simply

did not reflect changes which occur at the whole-body level; thereby resulting in misleading

conclusions about the benefits of heat acclimation on the body’s physiological capacity to

dissipate heat.

An important observation in this study is the high degree of individual variability

observed in the improvements of local and whole-body sweat rate (see Figure 3). The variability

in local sweating improvements was much greater than those observed for the whole-body, with

important regional differences being evident. Specifically, increases in mid-anterior forearm

sweat rate were observed in 60% of participants during Ex1 compared to 80% during Ex2 and

Ex3. On the upper back, this number was reduced to 50%, 70% and 60% of participants from

Ex1 to Ex3, while only 50%, 30%, and 50% of participants showed improvements on the chest

during Ex1, Ex2, and Ex3, respectively. In contrast, whole-body sweat rate increased in 80% of

participants during Ex1, and all (100%) participants displayed an improvement during Ex2 and

Ex3, albeit the magnitude of improvement varied between individuals (see Figure 3). These

findings highlight the regional variability that exists in local sweating adaptations following heat

acclimation. Studies suggest that increases in local sweat rate are dependent upon each region’s

maximal capacity, which is possibly pre-determined genetically (Taylor 2014). Therefore, it is

conceivable that peak sweat rates at different regions could be at varying levels along an

adaptation continuum, which also depend on the degree of physiological adaptation previously

achieved through direct (heat exposure) or indirect (independent adaptation of sweat glands

through other regulatory systems) means (Taylor 2014). Such variability may result in variable

changes in local sweating with heat acclimation, particularly when only one small area of skin is

examined. The current findings also highlight that sweating adaptations become more apparent

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when the drive for sweating is high, such as during Ex3 of the current study. As such, future

studies examining potential sweating adaptations to heat acclimation should consider

measurements of whole-body sweating (e.g. as assessed by changes in body mass, technical

absorbent method) and ensure that the stimulus for sweating is great enough to allow for

measurable improvements in sweat rate.

The increase in sweating with heat acclimation has been thought to be relatively greater

in the limbs (arm, forearm, legs) as opposed to the torso (chest and back) (Hofler 1968;

Magalhaes et al. 2010; Regan et al. 1996; Shvart et al. 1979). However, Regan et al. (1996) only

examined two local sites (forehead and forearm), Shvartz et al. (1979) collected data from three

local sites (chest, back, and forearm) without comparing the relative increase between

measurements, and Hofler (1968) derived limb, trunk, head, arm, and leg sweating responses

from the combination of multiple local sites at each region. A more recent study (Patterson et al.

2004) refuted the redistribution theory as a similar increase in sweat rate was measured on the

thigh and forehead (both 31%) as well as on the forearm (81%), chest (73%) and upper back

(64%). Given that we only observed an increase in forearm sweat rate with heat acclimation; it

could be argued that the traditional heat acclimation protocol employed resulted in greater

peripheral sweating. However, we did not measure sweat rate from other peripheral regions, nor

did we include a greater number of more “central” ones. Our findings therefore provide limited

insight into the redistribution theory of sweat rate with heat acclimation.

The findings of the current study are consistent with prior observations which measured

local sweat rates (Cotter et al. 1997; Fox et al. 1963; Nadel et al. 1974; Patterson et al. 2004;

Peter and Wyndham 1966; Roberts et al. 1977; Shvartz et al. 1979) such that the absolute onset

threshold for whole-body sweat rate is reduced with heat acclimation, and this general trend was

observed during each exercise bout and at all local measurement sites (see Table 2). Similarly,

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heat acclimation led to an increase in the thermosensitivity of whole-body sweat rate during each

exercise bout, a general trend that was observed at all local skin sites (see Table 2). Finally, the

greater mid-anterior forearm sweat rate after 14 days of heat acclimation was mainly attributed to

an increase in sweat gland output as opposed to an increase in the number of heat activated sweat

glands (see Table 2). It should be noted that the modified iodine-paper technique has an

intrasubject coefficient variation of 11 ± 10%, which could explain some of the variability

observed in the current study (Gagnon et al. 2012). Nonetheless, these sweating adaptations are

consistent with previous studies which report similar local sweating adaptations following heat

acclimation.

Considerations

An important consideration of the current study is that an uncompensable or nearly

uncompensable heat stress condition was achieved during Ex1, progressing to a fully

uncompensable condition during the final exercise bout. This experimental design allowed us to

measure an ~18% increase in the body’s maximal or near maximal whole-body sweat rate after

14 days of heat acclimation. However, heat acclimation led to an increase in dry heat gain, which

increased the combined metabolic and environmental heat load by ~6% in the post heat

acclimation sessions (Poirier et al. 2015) thereby accounting for part of the increase in whole-

body and forearm sweat rate. Furthermore, a fixed intensity protocol (50% of V� O�� –

traditional heat acclimation protocol) was used in the current study as we wanted to examine the

improvements in whole-body sweating that could be achieved with a protocol that has been

extensively employed in the literature. While we recognize that this model may lead to a

progressive reduction in intensity over the time course of the heat acclimation period, we believe

it was necessary to assess how this traditional acclimation model improves whole-body sweat

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rates and how this relates to improvements in local sweat rates. Future projects should assess the

relationship between whole-body and local sweat rates with other models such as the isothermic

clamping technique.

Perspective

In view of the fact that the availability of direct calorimeters is limited and costly,

researchers must rely upon measurement of local sweat rate and/or changes in body mass when

assessing the influence of various factors (such as heat acclimation) on the body’s physiological

capacity to dissipate heat. The present study provides valuable insight into the changes in local

and whole-body sweating during heat acclimation which will help guide the interpretation of

previous research which have yielded disparate findings. Moreover, it will assist in the design

and development of future studies by enabling a more comprehensive understanding of the

potential limitations that result from regional variations in local sweating. Specifically, by

identifying regions that best reflect changes occurring on the whole-body level, it will be

possible to better assess the effect of a heat acclimation program on the sweating response.

Conclusion

The current study examined if local sweat rates measured at three commonly employed

skin sites reflect improvements in whole-body sweat rate as determined via direct calorimetry

following a 14-day period of traditional heat acclimation. There was a high degree of inter-

individual variability, as well as marked regional differences in the effect of heat acclimation on

local sweat rate measurements. Overall, local sweat rate measurements did not consistently

reflect the consistent improvements observed at the whole-body level. These results suggest that

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measurements of local sweat rate do not always reflect changes in whole-body sweat rate

following a 14-day traditional heat acclimation protocol.

ACKNOWLEDGEMENTS

The authors thank all the participants who volunteered for this study, as well as the

members of the Human and Environmental Physiology Research Unit for their contributions

during the project. This research was supported by the Natural Sciences and Engineering

Research Council of Canada (Discovery Grants Program, RGPIN-06313-2014; Discovery Grants

Program - Accelerator Supplements, RGPAS-462252-2014) (All grants held by Dr. Glen P.

Kenny). Dr. Glen P. Kenny is supported by a University of Ottawa Research Chair Award. Mr.

Martin P. Poirier is supported by a Natural Science and Engineering Research Council Alexander

Graham Bell Graduate Scholarship (CGS-D). The authors declare that they have no competing

interests.

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Table 1. Summary of studies examining changes in local and whole-body sweating during heat acclimation.

Note: WBSR= whole-body sweat rate; LSR= local sweat rate; RH = relative humidity; V� O��= maximal oxygen consumption; ↑= increase; and ↔ = no change.

Study Heat Acclimation Protocol Sweating Measurements Results

Buono et al. (2009) 120 min exercise in the heat (35˚C and 75% RH) for 8 days WSBR

LSR: forearm

↑ WBSR

↑ LSR on forearm

Cotter et al. (1997) 70 min exercise in the heat (39.5˚C and 59% RH) for 6 days

Isothermic clamping (1.4˚C above baseline)

WBSR

LSR: foot, leg, thigh, hand, forearm, arm, upper back, and forehead

↔ WBSR ↔ LSR at all sites

Fox et al. (1963) 2-hour heat exposure (35-40˚C, 24 to 100% RH) for 12 days WBSR ↑ WBSR

Gisolfi et al. (1973) 100-min exercise in the heat (48.9˚C and 23.5% RH) for 8 days

5.6 km/hr on a treadmill WBSR ↔ WBSR

Havenith et al. (1986) 2-hour exercise in the heat (40˚C and 20% RH) for 7 days

Isothermic clamping (core temperature of 38.3 ˚C) WBSR ↔ WBSR

Hessemer et al. (1986) 60 min of passive heat exposure (55-60˚C and 20% RH) for 5 days LSR: chest ↔ LSR

Inoue et al. (1999) 90 min of exercise at 35% of V� O�� in the heat (43˚C and 30% RH)

for 8 days

WBSR LSR: chest, upper back, forearm, and

thigh

↔ WBSR

↔ LSR at all sites

Lee et al. (2010) 60 min of lower body immersion (43˚C) for 10 days LSR: forearm ↑ LSR on forearm

Magalhaes et al. (2010) 60 min exercise in the heat (40˚C and 45% RH) for 11 days

Isothermic clamping (1 ˚C above baseline)

WBSR

LSR: chest, forehead, forearm, arm, and

thigh

↑ WBSR

↑ LSR on arm, forearm, and thigh

↔ LSR on chest and forehead

Nielsen et al. (1993) 90 min of exercise at 50% of V� O�� in the heat (40-42 ˚C and 10-15%

RH) for 9 to 12 days WBSR ↑ WBSR

Nadel et al. (1974) 60 min exercise of 50% of V� O�� in the heat (45˚C) for 10 days LSR: chest ↑LSR on the chest

Patterson et al. (2004) 90 min exercise in the heat (40˚C and 60% RH) for 3 weeks

Isothermic clamping (target of 38.5˚C esophageal temperature)

WBSR LSR: chest, forehead, upper back,

forearm, and thigh

↑ WBSR ↑LSR at all sites

Regan et al. (1977) 60 min exercise in the heat (38.2˚C and 39.7% RH) for 10 days

Isothermic clamping (1 ˚C above baseline)

WBSR

LSR: forehead and forearm

↔ WBSR

↑LSR at both sites

Roberts et al. (1977) 60 min of exercise at 50% of V� O�� in the heat (35˚C and 81% RH)

for 10 days LSR: chest ↑LSR on the chest

Shvartz et al. (1979) 120 min of exercise at 50% of V� O�� in the heat (39.8˚C and 50%

RH)

WBSR

LSR: chest, thigh, and upper arm

↑ WBSR

↑LSR at all sites

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Table 2. Onset threshold, thermosensitivity, number of active sweat glands, and the sweat output per gland during exercise at fixed rates of metabolic heat

production equal to 300, 350, and 400 W·m-2 prior to (Pre HA) and following (Post HA) a 14-day traditional heat acclimation (HA) protocol.

Whole-body sweat production was measured by direct calorimetry. SGO, sweat gland output. *Significantly different then Pre HA, p≤0.05. Values are mean ± SD.

Onset Threshold

(°C)

Thermosensitivity

(Whole-body: W·m-2

·°C-1

, local: mg·min-1

·cm-2

·°C-1

)

300 W·m-2

350 W·m-2

400 W·m-2

300 W·m-2

350 W·m-2

400 W·m-2

Pre HA Post HA Pre HA Post HA Pre HA Post HA Pre HA Post HA Pre HA Post HA Pre HA Post HA

Whole-body

36.63 36.42 36.84 36.74 37.02 36.83 512 612 528 708 520 731

±0.18 ±0.12* ±0.24 ±0.18 ±0.24 ±0.23* ±130 ±129* ±243 ±169* ±298 ±402*

Forearm

36.71 36.59 37.07 36.86 37.22 36.98 1.33 2.03 1.24 1.92 1.02 1.74

±0.37 ±0.41 ±0.42 ±0.31* ±0.39 ±0.33* ±0.40 ±0.71* ±0.77 ±0.75* ±0.73 ±0.87*

Back

36.72 36.55 37.04 36.88 37.25 36.97 1.41 1.77 1.42 2.13 1.17 1.49

±0.39 ±0.37* ±0.41 ±0.31* ±0.34 ±0.32* ±0.32 ±0.76 ±0.61 ±1.19 ±0.66 ±0.58

Chest

36.67 36.54 37.04 36.93 37.22 36.98 1.41 1.64 1.54 1.91 1.22 1.57

±0.38 ±0.35* ±0.40 ±0.35 ±0.39 ±0.30* ±0.53 ±0.75 ±0.68 ±0.88 ±0.62 ±0.82*

Active glands (per cm2) Sweat gland output ( ug•gland

-1) 300 W·m

-2 350 W·m

-2 400 W·m

-2 300 W·m

-2 350 W·m

-2 400 W·m

-2

Pre HA Post HA Pre HA Post HA Pre HA Post HA Pre HA Post HA Pre HA Post HA Pre HA Post HA

Forearm 90 92 93 96 94 97 10.87 11.86 12.28 14.05 13.33 15.15

±20 ±18 ±18 ±19 ±18 ±19 ±4.23 ±4.74 ±4.52 ±5.61* ±4.76 ±5.95

Back 57 56 59 58 60 58 19.17 19.94 21.77 23.19 23.48 25.48

±10 ±9 ±9 ±8 ±9 ±8 ±5.38 ±6.81 ±5.97 ±5.17 ±5.54 ±6.00

Chest 51 52 52 55 55 56 22.38 21.05 25.99 23.06 27.47 24.24

±8 ±9 ±8 ±8 ±8 ±6 ±6.92 ±7.21 ±8.10 ±6.76 ±8.36 ±6.23

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FIGURES CAPTIONS.

Figure 1. Mean (±SD) changes in mean body temperature prior to (Pre HA) and following

(Post HA) 14 days of traditional heat acclimation (HA) during sessions in the calorimeter (Cal)

and thermal chamber (TC). *Significantly greater than Pre HA values, p≤0.05.

Figure 2. Mean (±SD) changes in whole-body sweat rate (panel A) and local sweat rates on the

forearm (panel B), upper back (panel C), and chest (panel D) prior to (Pre HA, white circles) and

after (Post HA, black circles) 14-days of traditional heat acclimation (HA). Whole-body sweat

rate was measured in the calorimeter while the local sweat rates were measured in the thermal

chamber. *Significantly greater than Pre HA values, p≤0.05. SR, sweat rate.

Figure 3. Individual (black circles) and mean (white circles) changes in whole-body and local

sweat rates on the forearm, upper back, and chest at the end of each exercise bout prior to (Pre

HA) and following (Post HA) 14-days of traditional heat acclimation (HA). Whole-body sweat

rate was measured in the calorimeter while the local sweat rates were measured in the thermal

chamber.*Significantly greater than Pre HA, p≤0.05.

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Figure 1.

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Figure 2.

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Figure 3.

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Documents

Draft - University of Toronto T-Space€¦ · Draft 1 1 Local vs. whole-body sweating adaptations following 14 days of traditional 2 heat acclimation 3 Martin P. Poirier1, Daniel