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Medicine & Science in Sports & Exercise:
doi: 10.1249/MSS.0b013e318185d341
Clinical Sciences

Responses of Lean and Obese Boys to Repeated Summer Exercise in the Heat Bouts

DOUGHERTY, KELLY A.1; CHOW, MOSUK2; KENNEY, W. LARRY1

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Author Information

1Noll Laboratory, Department of Kinesiology, The Pennsylvania State University, University Park, PA; and 2Department of Statistics, The Pennsylvania State University, University Park, PA

Address for correspondence: Kelly A. Dougherty, The Pennsylvania State University, 229 Noll Laboratory, University Park, PA 16802-6900; E-mail: kellydoc35@aol.com.

Submitted for publication April 2008.

Accepted for publication July 2008.

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Abstract

Purpose: To determine the degree of natural acclimatization and artificially induced acclimation-related changes during repeated exercise in the heat bouts in seven lean and seven obese 9- to 12-yr-old boys during summer months.

Methods: Beginning at random times during the summer, subjects underwent a 70-min exercise (30% V˙O2max) in the heat exposure (38°C, 50% relative humidity) on six separate days.

Results: On day 1, obese children were less naturally acclimatized as indicated by significantly higher baseline core temperatures (Tc) (obese = 37.62 ± 0.06 vs lean = 37.41 ± 0.06; P < 0.004). By day 6 versus day 1, significant reductions in baseline Tc were evident in both groups (obese = 37.41 ± 0.04 vs lean = 37.18 ± 0.04; both P < 0.05). Baseline Tc in obese subjects by day 6 was similar to that of lean subjects on day 1. Daily reductions in exercise Tc were evident in both groups (final exercising Tc day 1 vs day 6: obese = 38.15 ± 0.05 vs 37.89 ± 0.05; lean = 38.17 ± 0.09 vs 37.72 ± 0.06°C; both P < 0.001), occurring at a significantly slower rate in obese subjects (final exercise Tc day 6 − day 1: obese vs lean = −0.26 ± 0.04 vs −0.45 ± 0.08°C; P < 0.05). Significant reductions in exercising heart rate (HR) occurred in the lean but not the obese subjects by day 6 (final exercising HR day 1 vs day 6: obese = 132 ± 3 vs 131 ± 3, P > 0.05; lean = 138 ± 3 vs 127 ± 3 bpm; P < 0.001).

Conclusions: During summer months, obese children are less naturally heat-acclimatized and subsequently acclimate at a slower rate.

Heat-acclimatization (induced in a natural environment) and acclimation (induced over a shorter period, often in a laboratory setting) result from repeated heat exposures that sufficiently increase body core temperature (Tc) and mean skin temperature and stimulate abundant sweating (38). In response, the body adapts through numerous physiological adjustments such as reductions in day-to-day exercise Tc, heart rate (HR), and mean skin temperature, an increase in sweating rate, an earlier onset of sweating with more dilute sweat, and an increase in plasma volume (32). In addition, decreases in morning baseline Tc have been observed after humid heat-acclimation (6). The induction of heat-acclimation during exercise is specific to the duration of exposure, environmental conditions, and intensity of exercise and can be attained in adults in 5-10 d through exercise in the heat exposures lasting 1-2 h each day (32).

Although both children and adults are able to acclimate to exercise in the heat, it has been reported that children acclimate at a slower rate (14) and attain a degree of acclimation that is somewhat lower (37) than adults. Only two studies (12,13) have investigated the responses of obese or overweight versus lean children to exercise in the heat, with both studies demonstrating no difference in heat tolerance (exercise time in the heat before a Tc of 39.4°C was reached). However, although all subjects in both studies underwent three exercise or heat-acclimation sessions before the heat tolerance trials, these data were not presented or discussed. Thus, neither the relative ability of obese versus lean children to acclimate to exercise in the heat nor a comparison of their rates of acclimation has been investigated.

In adults, exposure to summer heat confers some degree of natural acclimatization. During both passive heat stress and exercise in the heat, Tc and HR are lower, and sweating is more profuse and dilute in summer compared with winter months (38). Due to this natural acclimatization, full artificially induced heat-acclimation in a warm environment occurs more rapidly (10). A high degree of fitness also hastens the acclimation process (31). Children indigenous to tropical climates display high sweating rates and a heat tolerance similar to adults during exercise in the heat (28,29). However, for both obese and lean children residing in more temperate climates, the degree of natural acclimatization incurred during the summer months and its impact on subsequent acclimation-related changes when exposed to regular exercise in the heat exposures are unknown.

As exercise is frequently prescribed as a main combatant of the pediatric obesity pandemic and as a greater number of children become more physically active, especially outdoors during the warm summer months, ensuring their physical well-being during every exercise session is important. The degree of heat-acclimatization and/or acclimation during exercise is one factor that could affect a child's physical performance, subjective comfort, and/or physical well-being during exercise in the heat. The smaller body surface area/mass ratio and the increased subcutaneous fat deposits of an obese versus lean individual may contribute to a slower rate of metabolic heat loss (30) and possible differences in the heat-acclimatization and/or heat-acclimation responses during exercise. Understanding the heat-acclimatization and/or heat-acclimation similarities and/or differences during exercise between lean and obese children has implications for scheduling youth sports games and practices, etc. If obese children are less naturally heat-acclimatized and/or display a slower rate of heat-acclimation during exercise, this might suggest that obese children require additional exercise bouts in the heat to achieve a degree of acclimation similar to that of lean children.

The purpose of this study was to determine the degree of initial natural acclimatization and subsequent artificially induced acclimation-related changes during repeated exercise in the heat bouts in seven lean and seven obese 9- to 12-yr-old boys during the warm summer months. It was hypothesized that obese children 1) would be less naturally acclimatized to the heat as shown by significantly higher baseline Tc and 2) would display a significantly slower time course for the classic markers of acclimation (e.g., day-to-day decreases in exercise Tc and HR, elevations in sweating rate) during repeated days of light-to-moderate intensity exercise in a warm, humid environment.

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METHODS

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Subjects.

This study was approved by the Institutional Review Board of The Pennsylvania State University. Seven lean and seven obese 9- to 12-yr-old boys volunteered to participate in this study. Lean and obese were defined as ≤20% and ≥25% body fat, respectively (18), as measured by whole-body dual energy x-ray absorptiometry scan (model QDR 4500W; Hologic, Waltham, MA). Each subject and his parent or guardian were advised of all experimental procedures and associated risks before verbal assent was given by the child, and a written informed consent was provided by the parent or guardian. Subjects were recruited via flyers distributed to schools in the Central Pennsylvania region. All subjects were healthy, normotensive, and not taking any medications that could affect their cardiovascular or thermoregulatory responses. Preliminary screening included blood chemistry analysis (CHEM-24, complete blood count and lipid profile; Quest Diagnostics) and resting 12-lead electrocardiogram. During a maximal graded exercise test on a treadmill, subjects began at a self-selected speed to elicit an HR of ∼140-150 bpm at 0% grade, followed by an increase in slope of 2% until two of the following four criteria were met: 1) a plateau in oxygen uptake (V˙O2) defined as an increase of ≤2.0 mL·kg−1·min−1; 2) an HR>195 bpm; 3) a respiratory exchange ratio >1.0; or 4) subjective indicators of fatigue such as hyperpnea, facial flushing, unsteady gait, or refusal of the child to exercise further (11,23). Subjects completed a physical exam during which a clinician determined pubertal status according to the criteria of Tanner (35). Subject characteristics are presented in Table 1.

Table 1
Table 1
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A minimum of 8 h before each test, subjects swallowed an ingestible temperature sensor (CorTemp; HQ Inc., Palmetto, FL) for the measurement of Tc. The sensor is a single-use, pill-shaped electronic device that contains a telemetry system, a microbattery, and a quartz crystal whose frequency of vibration is linearly related to temperature. Each temperature sensor was calibrated by the manufacturer, which provides a serial number that is programmed into a handheld recorder (CT2000), ensuring an accuracy of 0.1°C. Each pill was used within 6 months from the date it was shipped by the manufacturer. During steady-state exercise in a warm environment, the temperature and the response time of the ingestible temperature sensor fall between that of rectal and esophageal temperatures (22).

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Testing procedures.

Subjects were asked to refrain from caffeine consumption on each day of the experiment and reported to the laboratory at least 2 h after a meal. After providing a urine sample, the subject was instrumented with a Polar® HR monitor, belt, and pouch to attach the handheld recorder (CT2000) to the subject for continuous Tc measurement and was weighed (Seca 770, accuracy ± 50 g) wearing only shorts (all subsequent weights were taken wearing shorts only).

A total of six 70-min acclimation sessions were completed (one session per day) by each subject on separate days. During each trial, two subjects were test concurrently, one lean and one obese, to control for early or late summer seasonal and time-of-day variations. Testing began in the beginning of June and concluded by early September. Local weather for this period averages from 22 to 28°C (71 to 82°F; National Oceanic and Atmospheric Administration, 2007). For all experimental trials, the time between each scheduled test day was no more than 2 d. Subjects were encouraged to stay well hydrated the day before each trial. For all experimental trials, subjects wore shorts, socks, and sneakers. Because experiments were conducted in the summer months, subjects were partially heat-acclimatized due to routine outdoor activities (38). Thus, baseline Tc in lean and obese children were measured and compared to assess the degree of natural acclimatization. Subsequently, each subject completed six acclimation sessions to compare physiological responses between lean and obese boys during repeated exercise in the heat bouts from the partially heat-acclimatized state. Attainment of acclimation was defined by a similar final Tc for two consecutive sessions and by a leveling off of Tc within the last exercise bout (all subjects completed six trials). A schematic of the experimental design is diagrammed in Figure 1.

FIGURE 1-Schematic o...
FIGURE 1-Schematic o...
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During each session, subjects exercised at 30% of maximal aerobic capacity (V˙O2max) alternating between a treadmill (Precor USA C962) and a cycle ergometer (Monark Ergomedic 818E) for three 20-min bouts interspersed with 5-min rests. Environmental conditions were held constant at 38°C, 50% relative humidity (RH). During school recess and spontaneous playtime, children spend most the time participating in light-to-moderate intensity activities (26,27); thus, this exercise intensity was chosen because it reasonably simulates an intensity typical of a child during spontaneous physical exertion and of heat-acclimation studies. Body weight was measured during each rest period, and the subject was given water to maintain body weight by replacing most water lost through sweat. The experiment ended when the subject either completed the protocol, if the Tc exceeded 39°C, if the subject experienced adverse signs (nausea, dizziness, etc.), or if the subject desired to stop. After exiting the chamber after the experiment, a postexperiment urine sample was obtained.

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Measurements.

All HR and Tc data were measured continually through the protocol and were stored as 1-min averages using computer software (Labview) in conjunction with a data-acquisition system (National Instruments, Austin, TX). Blood pressure by brachial auscultation (sphygmomanometry) was measured 10 min into each exercise bout. To ensure that each subject was working at the desired workload, expired air was measured 10 min into the second exercise bout for 5 min for the determination of V˙O2 (TrueOne 2400 Metabolic Measurement System; ParvoMedics, Salt Lake City, UT). Urine volume was measured with a graduated cylinder, and urine color was determined by holding each specimen container next to a validated color scale (3) in a well-lit room. The eight-color scale ranges from 1 (very pale yellow) to 8 (brownish green). Urine osmolality (freezing point depression, Advanced DigiMatic Osmometer Model 3D2) and specific gravity (Refractometer, Atago A300CL) were determined in triplicate. Sweating rate was calculated from the net change in body weight corrected for fluid consumption and urine was excreted.

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Subjective ratings.

During the preliminary screening, the Physical Activity subscale of the Physical Self-Description Questionnaire-which has been validated for use in adolescents (19,20,21)-was completed to subjectively determine how "active" each subject perceived himself to be on a daily and/or weekly basis. Each item is a simple declarative statement, all positively worded, and subjects respond on a six-point true-false response scale where 1 = "false," 2 = "mostly false," 3 = "more false than true," 4 = "more true than false," 5 = "mostly true," and 6 = "true". The statements to which the subjects responded were, "Several times a week I exercise or play hard enough to breathe hard (to be out of breath)," "I often do exercise or activities that make me breathe hard," "I get exercise or do sports activities 3 or 4 times a week that make me breathe hard and last at least 30 minutes," "I do physically active things (like jogging, dancing, bicycling, aerobics, gym, or swimming) at least three times a week," "I do lots of sports, dance, gym, or other physical activities," and "I do sports, exercise, dance, or other activities almost every other day." During each experiment, ratings of perceived exertion (RPE; Borg scale) (5), and thermal sensation (TS), using a 0-8 scale in which 0 = unbearable cold, 4 = thermoneutral, and 8 = unbearably hot (40), were measured 10 min into each exercise bout.

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Statistical analyses.

A repeated-measures ANCOVA was used to fit a model to the data by SAS PROC MIXED. This linear mixed model took into account the correlated nature of the repeated measures. Group was treated as a fixed effect, and subjects were treated as random effects. The independent variables were group, time, and day (where appropriate), and the dependent variable was the measured physiological response. When making multiple comparisons, Bonferroni adjustments were used. Results were considered significant at P < 0.05.

During exercise in the heat, whereas metabolic heat production is a reflection of absolute intensity, heat loss mechanisms are a function of relative intensity. Thus, the heat storage and the subsequent rise in Tc are dependent to some degree upon both absolute and relative intensities (17). In the present study, work at the same relative intensity was the logical choice to investigate differences in heat loss mechanisms between lean and obese boys. However, to investigate the impact of absolute versus relative intensity, four lean and three obese subjects repeated the first heat-acclimation trial, which matched the absolute and relative workloads of the lean and obese groups (i.e., decreasing the workload for the lean group to match the obese group and increasing the workload for the obese group to match the lean group). The time between the completion of the sixth heat-acclimation trial and the repeat heat-acclimation trial was >2 months.

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RESULTS

Responses to the Physical Activity subscale of the Physical Self-Description Questionnaire are presented in Figure 2. Compared with lean subjects, obese subjects perceived themselves to be significantly less active (P < 0.03) as determined by significantly lower ratings to the following questions: "Several times a week I exercise or play hard enough to breath hard (to be out of breath)" (P < 0.03); "I get exercise or do sports activities 3 or 4 times a week that make me breath hard and last at least 30 minutes" (P < 0.02); and "I do lots of sports, dance, gym, or other physical activities" (P < 0.003).

FIGURE 2-Responses t...
FIGURE 2-Responses t...
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Six lean and two obese subjects were classified as prepubertal (Tanner stage 1), five obese subjects were classified as midpubertal (Tanner stage 2 to stage 4), and one lean subject was classified as late pubertal (Tanner stage 5). As expected, obese subjects weighed more and had a higher body surface area, a lower body surface area to mass ratio, a higher percent body fat, and a lower V˙O2max (all P < 0.05; Table 1). Body fatness ranged from 14% to 20% in the lean subjects and from 28% to 45% in the obese subjects. The measured exercise intensity ranged from 27.6% ± 0.5% to 35.3% ± 1.0% for the lean subjects and from 27.5% ± 1.2% to 35.5% ± 0.5% for the obese subjects across trials (P > 0.05).

Baseline, final exercise, and change in Tc per trial by day of acclimation are presented in Table 2. On day 1, obese subjects were less naturally acclimatized as indicated by a significantly higher baseline Tc (P < 0.004). By day 6 compared with day 1, significant reductions in baseline Tc were evident in both groups (both P < 0.05), occurring at a similar rate (baseline Tc day 6 day 1; P > 0.05). Obese subjects continued to have significantly higher baseline Tc on days 2 through 6 (all P < 0.05). Baseline Tc in obese subjects by day 6 was similar to that of lean subjects on day 1 (P > 0.05).

Table 2
Table 2
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Compared with day 1, significant reductions in exercising Tc throughout the protocol were evident by day 6 in both groups (Fig. 3; both P < 0.001), occurring at a significantly slower rate (final exercise Tc day 6 day 1) in obese versus lean subjects (Table 2; P < 0.05). The change in Tc per trial (ending − beginning Tc) was significantly lower in lean subjects on days 5 and 6 compared with day 1 (Table 2; P < 0.05) but not in obese subjects (P > 0.05). For both groups, there were no significant differences in Tc between days 5 and 6 (final Tc during exercise day 5 vs day 6: obese = 37.96 ± 0.05 vs 37.89 ± 0.05; lean = 37.78 ± 0.07 vs 37.72 ± 0.06°C; P > 0.05), suggesting an attainment of heat-acclimation according to the operationally defined criteria of a similar final Tc for two consecutive sessions and a clear plateau in Tc during the last exercise bout (Fig. 3).

FIGURE 3-Time course...
FIGURE 3-Time course...
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The Bland-Altman approach to measuring agreements for repeated measures was used to determine the agreement of Tc between the first (relative exercise intensity) and the repeated (absolute exercise intensity) heat-acclimation trials using ± 0.3°C as the physiological threshold for assessment. This threshold takes into account the anticipated SD for Tc measurement in boys of this age (4). The mean difference between the two trials was 0.01°C, and the SD of the difference between the two trials was 0.08°C. The 95% limits of agreement were −0.1503 to 0.1621. Therefore, when matched for absolute and relative exercise intensity, the difference in Tc was within acceptable limits and was considered marginal. This suggests that other factors independent of exercise intensity contribute to significant differences observed in the present study.

A significant reduction in HR from day 1 to day 6 occurred in the lean (P < 0.001) but not the obese subjects (Fig. 4). The change in HR per trial (ending − beginning Tc) was significantly different between groups within each trial (P < 0.01) but not between days within each group (change in HR day 1 vs day 6: obese = 26 ± 4 vs 31 ± 6; lean = 39 ± 5 vs 37 ± 3). Obese subjects had a significantly lower relative (mL·m−2·h−1) but not absolute (mL·h−1) sweating rate compared with lean subjects across all days (Table 3; P < 0.01). No urine variable was significantly different between groups.

Figure 4
Figure 4
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Table 3
Table 3
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Subjective responses to the exercise in the heat bouts on days 1 through 6 are presented in Table 4. At all time points on days 3 through 6, RPE were significantly higher in obese subjects. In lean subjects, a significant reduction in RPE occurred at 35 and 60 min on day 6 compared with day 1 and in TS at all time points on days 3 through 6.

Table 4
Table 4
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DISCUSSION

The main findings from this study are that during the summer months, obese (compared with lean) 9- to 12-yr-old boys 1) are less naturally heat-acclimatized as indicated by significantly higher baseline Tc and 2) display a significantly slower rate of decrease in exercise Tc and less of an elevation in sweating rate during repeated bouts of light-to-moderate exercise at a similar relative intensity (30% V˙O2max) in a warm, humid environment. After 6 d of artificial heat-acclimation, baseline Tc in obese children reached that of lean children on day 1, whereas by day 6 lean children acclimated to a new baseline Tc. Because obese children started at a higher baseline Tc and acclimate at a slower rate, this suggests that they require additional exercise in the heat bouts to achieve a degree of acclimation similar to that of lean children. Therefore, obese children may require more attention and close monitoring to ensure their safety during exercise in the heat.

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Beneficial effects of natural acclimatization.

Adults exposed to warm summer weather attain some degree of natural acclimatization. Tc and HR are lower and sweating is more profuse and dilute in the summer compared with winter months during both a passive heat stress and exercise in the heat (38). Experimental heat-acclimation occurs at a faster rate in both acclimatized (10) and more fit (31) adults. Physically fit adults during exercise in the heat display traits similar to that of heat-acclimatized adults (25). Pandolf et al. (24) showed that V˙O2max before acclimation was directly related to the rate of drop in exercising Tc over the course of heat-acclimation. Responses such as lower Tc and HR and higher sweating rates during exercise in the heat are similar for adults residing in tropical climates and those who are artificially acclimated compared with unacclimated controls (38).

In children, very little is known regarding the beneficial effects of natural acclimatization. Children indigenous to tropical climates display high sweat rates and heat tolerance similar to adults during exercise in the heat (28,29). Although the American Academy of Pediatric guidelines (1) state that children should not perform physical activity if the wet-bulb globe temperature (WBGT) is greater than 29°C, heat-acclimatized 11- to 14-yr old girl athletes are able to tolerate exercise in conditions of higher heat and humidity (WBGT = 31.9 ± 1.5°C) (9). Girls of similar aerobic capacity, hydration status, and degree of heat-acclimatization as adult women display a stable HR, stroke index, and cardiac index while cycling at 60% V˙O2max until fatigue in a hot and humid environment (WBGT = 29.9 ± 0.2°C) (29). Collectively, these studies suggest that children who are naturally acclimatized to tropical climates are able to effectively tolerate exercise in the heat. The present study indicates that both lean and obese children residing in more temperate climates who vary in their degree of acclimatization to the heat during the summer months are also able to tolerate exercise in the heat. On day 1, the obese boys were less naturally acclimatized as indicated by a significantly higher baseline Tc compared with lean boys. Although the Physical Activity subscale of the Physical Self-Description Questionnaire in the present study did not differentiate between indoor and outdoor activity, it is likely that the higher baseline Tc on day 1 in the obese subjects was due to less outdoor physical activity and, thus, less natural exercise in the heat exposure compared with the lean subjects.

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Heat-acclimation.

In adults, significant reduction in resting Tc after acclimation to humid heat for 7 d has been observed (6). The present study demonstrates that children show similar physiological adaptations to 6 d of humid heat-acclimation. The reduction in baseline Tc in adults ranged from−0.1 to−0.5°C (6), whereas children tested here also fell within this range (reduction in baseline Tc: obese = −0.21 ± 0.06; lean = −0.23 ± 0.04). Interestingly, baseline Tc in the obese children on day 6 was similar to that of lean children on day 1, whereas lean children by day 6 acclimated to a new baseline Tc. This suggests that with regard to heat-acclimation during the summer months, the obese children in our sample were approximately six acclimation days behind the lean children. Therefore, for obese versus lean children, additional heat exposures may be needed to match the degree of acclimation, even during warm summer months.

Heat-acclimation is most effectively induced through a combination of repeated exercise in the heat bouts and is essential to minimize the associated thermal and cardiovascular stress. In adults, Buskirk et al. (8) reported that during a 10-d exercise in the heat-acclimation protocol (temperatures = 46°C dry bulb; 27°C wet bulb), overweight women compared with their lean counterparts were repeatedly unable to complete three 20-min walks interspersed with 20-min rests. In contrast, both lean and obese men tolerated the exercise in the heat exposures well and were able to complete all experimental trials without incident. Lean 8- to 10-yr-old children were able to tolerate and complete repeated (7 d) acclimation bouts (temperatures = 43°C dry bulb; 24°C wet bulb), resulting in reduced cardiovascular and thermal strain (15). Only two studies (12,13) have investigated the response of an obese or overweight versus lean child to exercise in the heat. Both studies reported no difference in heat tolerance (exercise time in the heat before a Tc of 39.4°C was reached) between lean and obese 9- to 12-yr-old children. However, although all subjects were able to complete three acclimation sessions (temperature range = 32-50°C dry bulb; 18-27°C wet bulb) before the heat tolerance trials, no acclimation data were provided. The present study indicates that although both groups were able to incur acclimation-related changes, obese children display a significantly slower rate of decrease in exercise Tc and less of an elevation in sweating rate during repeated bouts of exercise in a warm, humid environment. Although both groups began the study with different degrees of natural acclimatization, this still suggests that during the summer months, additional exercise in the heat-acclimation bouts may be necessary in order for obese children to obtain a degree of acclimation similar to that of lean children.

Numerous factors may account for the slower rate of heat-acclimation during exercise in obese versus lean children in the present study. Due to the increase in subcutaneous body fat deposits, a larger obese individual, with a smaller body surface area/mass ratio, loses metabolic heat generated during exercise at a slower rate than a smaller lean individual (30) thus resulting in greater heat storage. Because adipose tissue has a lower specific heat of stored lipid (0.40 kcal·kg−1·°C−1 adipose tissue vs 0.82 kcal·kg−1·°C−1 entire human body), storing the same amount of heat would induce a greater rise in temperature in adipose versus lean tissue (7). Thus, the combination of a smaller body surface area/mass ratio and greater subcutaneous fat deposits may result in greater heat storage in an obese compared with lean child. Previous research has suggested that the degree of heat-acclimatization is related to body heat storage: the greater the amount of heat stored in the body, the higher the degree of heat-acclimatization (34). Others have suggested that there may be a "ceiling effect"; or an optimal rate of heat storage above or below which a slower rate of acclimatization will occur (15). Although heat storage was not calculated in the present study, it is possible that the "ceiling effect" combined with the possible impaired heat dissipation mechanisms in obese versus lean individuals discussed below contributed to the slower rate of heat-acclimation during exercise in obese versus lean subjects in the present study.

Increases in body surface area cause inverse changes in sweat gland density but not in the total number because the number of eccrine sweat glands in an individual does not change after 2 yr (16). Thus, the capacity for evaporative cooling in an obese child may be reduced. In the warm and humid environment of the present study, the evaporation of sweat was the primary means of heat dissipation and likely depends on the optimal sweating rate for a given unit of metabolic heat production and surface area. The lower sweating rates per body surface area in the obese versus lean subjects in the present study may have been insufficient to maintain the evaporative heat loss necessary to match metabolic heat production, resulting in greater heat storage. However, previous research in children has reported no difference in sweating rate per body surface area in 9- to 12-yr-old lean versus obese boys during four exercise in the heat tolerance tests at a similar absolute intensity after partial heat-acclimation (13), which is at odds with the present study. The same study also reported higher evaporative rates per kilogram of weight in lean versus obese boys. Due to the differing environmental conditions and exercise intensities (absolute vs relative) between studies, it is difficult to explain the above discrepancies and addition research is warranted.

One hallmark of heat-acclimation is a reduction in day-to-day exercising HR. It is likely that not one but a combination of several mechanisms contributes to this improvement in cardiovascular function, including expansion of plasma volume, increase in venous tone from cutaneous and noncutaneous beds, and a reduction in Tc (38). In the present study, significant reductions in exercising HR by day 6 occurred in the lean but not the obese children. Because no prior study had addressed changes in cardiovascular function during heat-acclimation in obese children, it is difficult to speculate reasons for their lack of change in HR. However, previous findings in obese adults may provide some insight. Plasma volume expansion is most likely mediated via the influx of protein from cutaneous interstitial space to vascular compartments (33). Forearm blood flow during exercise in the heat is attenuated in obese compared with lean adults (36). Thus, obese subjects may be less able to flush proteins into the vascular compartments, resulting in a lower amount of fluid shifting from the intra- to the extracellular compartments, less plasma volume expansion, and subsequently less of a reduction in HR during repeated exercise in the heat bouts compared with their lean counterparts. In addition, the cardiovascular system may be compromised in an obese adult, as demonstrated by left ventricular hypertrophy accompanied by systolic or diastolic dysfunction and increased cardiac output and stroke volume both at rest and during exercise (7). In adults during acclimatization in a hot, humid environment, HR is significantly correlated with both stroke volume and Tc, suggesting that both an increase in stroke volume and decrease in Tc independently are associated with the decrease in HR (39). Thus, if stroke volume was significantly higher on day 1 in obese subjects, then they may have less reserve to further increase stroke volume which would result in an attenuated decrease in HR.

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Subjective ratings.

In lightly clothed adults, repeated exercise in the heat-acclimation bouts at a given relative intensity decrease RPE and TS, possibly reflecting a decrease in physiological (Tc and HR) strain (2). Very little is known regarding how perceived physical effort and/or thermal comfort change in response to an exercise in the heat-acclimation protocol in children. Bar-Or and Inbar (4) found a significant reduction in RPE after a 5-day exercise in the heat-acclimation protocol in 8- to 10-yr-old lean boys. Findings from the present study, which demonstrates in lean children a significant reduction in RPE at min 35 and 60 on day 6 compared with day 1, supports this previous research. The significant reduction in TS at all time points on days 3 through 6 compared with day 1 in lean children also suggests improved thermal comfort. In response to the heat-acclimation protocol in the present study, neither RPE nor TS (except for day 6 min 35) significantly decreased in obese subjects. This might suggest that obese children require additional exercise/heat-acclimation bouts to achieve a similar degree of improvement in effort perception and thermal comfort as lean children (although it is possible that the obese children may not be able to achieve this due to their obese condition). In addition, the present study indicates that obese compared with lean children have significantly higher RPE values during repeated exercise in the heat bouts at all time points on days 3 through 6. It is difficult to postulate factors that may have contributed to the higher RPE values (i.e., increases in ventilation, metabolic rate, HR, mean skin temperature, Tc, acidity, etc.) and to differentiate the magnitude of their impact. It is interesting to note that the obese children had significantly higher RPE values 10 min into the exercise bout. The significantly higher effort perception during exercise in the heat in obese versus lean children in the present study could mean that obese children may require enhanced encouragement and support while exercising in the heat.

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Limitations.

An ingestible temperature sensor was used to measure Tc in the present study. One of the limitations of this procedure is that the position of the pill in the intestinal tract cannot be confirmed. Therefore, it is possible that if the same position is not achieved in each test, this could influence Tc measurement. However, for all six heat-acclimation trials, before each experiment, the time at which the subject swallowed the ingestible temperature sensor was standardized (with the minimum being 8 h before each test). In addition, the time of day in which the subject was tested was standardized for all six heat-acclimation trials.

In summary, during the summer months, obese (compared with lean) 9- to 12-yr-old boys are less naturally heat-acclimatized as indicated by significantly higher baseline Tc. In addition, obese children display a significantly slower rate of decrease in exercise Tc and less of an elevation in sweating rate during repeated bouts of light-to-moderate exercise in a warm, humid environment compared with their lean counterparts. This suggests that obese children may require additional exercise bouts in the heat to achieve a degree of acclimation similar to that of lean children.

The authors thank the children for their participation in this study and the parents for supporting them. The technical assistance of Randy McCullough, Doug Johnson, and Jane Pierzga and the data collection assistance of Allison Palaio, Matt Kenney, John Jennings, Samantha Wollman, Dave Nhan and Kristin Wielkiewicz is greatly appreciated. We thank the General Clinical Research Center nursing staff for their medical support. Results of the present study do not constitute endorsement by the American College of Sports Medicine.

This study was supported by the National Institute of Health grants R01-AG-07004-14 (W. L. Kenney) and M01-RR-10732 (General Clinical Research Center), the Graduate Student Research Endowment from the College of Health and Human Development, The Pennsylvania State University (K. A. Dougherty), and the Carl V. Gisolfi Memorial Research Fund from the American College of Sports Medicine Foundation (K. A. Dougherty).

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Keywords:

ACCLIMATIZATION; ACCLIMATION; CHILDREN; THERMOREGULATION; CORE TEMPERATURE; BODY COMPOSITION

©2009The American College of Sports Medicine

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