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Relationship between change in core temperature and change in cortisol and TNFα during exercise
Journal of Thermal Biology 35 (2010) 348–353
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Relationship between change in core temperature and change in cortisol and TNFa during exercise Peter A. Hosick a,n, Mark P. Berry a, Robert G. McMurray a,b, Erica S. Cooper a, A.C. Hackney a,b a b
Department of Exericse and Sport Science, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA Department of Nutrition, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 30 March 2010 Accepted 8 July 2010
The combined thermal load created by exercise and a hot environment is associated with an exaggerated core temperature response. The elevated core temperature is believed to increase the total stress of the exercise. Increased stress during exercise has been associated with increased levels of cortisol. The association of cortisol with increased inflammatory responses following exercise in the heat is equivocal. Thus, the purpose of the current investigation was to explore the relationship between increases in rectal temperature (Tre) and TNFa and cortisol. To induce Tre changes, 8 male subjects (mean 7 SD, age ¼ 23.6 7 2 yr, VO2max ¼ 52.8 7 3.7 mL/kg/min, BMI ¼24.2 7 1.9) participated in two 40 min trials of cycle ergometry at 65% of VO2peak immersed to chest level in cool (25 1C) and warm (38.5 1C) water. Tre was monitored throughout each trial, with blood samples taken immediately pre and post of each trial. Neither cortisol nor TNFa changed significantly during exercise in the cool water; however, in the warm trial, both cortisol and TNFa significantly increased (p o 0.004). Concordance correlations (Rc) between D cortisol and D TNFa indicated a strong but non-significant correlation (Rc ¼0.833, p¼ 0.135). In conclusion, changes in core temperature may be impacting the relationship between exercise induced changes in cortisol and TNFa. Therefore, acute moderateintensity exercise (40 min or less) in warm water impacts the stress and inflammatory response. Understanding this is important because exercise load may need to be adjusted in warm and hot environments to avoid the negative effects of elevated stress and inflammation response. Published by Elsevier Ltd.
Keywords: Heat exposure Environmental stress Immersion Hormones
1. Introduction Significant research has been done exploring the stress response of exercise in extreme environments. In general, well trained subjects, who are normally hydrated, are able to tolerate moderate levels of exercise in hot temperatures with only minor effects on the stress induced glucocorticoid response (Collins et al., 1969; Francesconi, 1988; Francesconi et al., 1978). However, when similar moderate-intensity exercise induces a rise in core body temperature above a particular threshold point, an elevated glucocorticoid response can occur (Collins et al., 1969). Additionally, increases in core temperature can result in increases in both immune and hormonal markers of inflammation (Whipp and Wasserman, 1969; Nielsen and Davies, 1976; Shephard, 1988; Abbas et al., 2007). Tumor necrosis factor alpha (TNFa), a cytokine released mainly from monocytes and macrophages, is believed to be an important part of this immune reaction because it is a principal mediator of the acute inflammatory response (Abbas
n
Corresponding author. Tel.: +1 919 962 2986; fax: +1 919 962 0489 E-mail address: [email protected] (P.A. Hosick).
0306-4565/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.jtherbio.2010.07.003
et al., 2007). Cortisol, another important component of the immune response, is released in response to acute phase reactants (Berczi, 1998), as well as the stress of exercise (Shephard, 1998). Furthermore, research has shown that glucocorticoids, such as cortisol, can potentiate the inflammatory response including TNFa (Frank et al., 2010). Together, cortisol and TNFa may be used as an indication of the physical stress and inflammatory response of exercise that accompany the associated increase in core temperature. Increased inflammatory cytokines and hormonal stress markers during exercise in a hot environment are widely acknowledged (Cross et al., 1996; Pedersen et al., 1998; Radomski et al., 1998; Rhind et al., 2004; Starkie et al., 2005; Laing et al., 2008; Peake et al., 2008). However, the findings of these studies are nonconsistent with respect to the cortisol or TNFa response. For example, Peake et al. (2008) found core temperatures elevated by about 2 1C during exercise in the heat, yet circulating levels of cortisol were slightly reduced. Conversely, other studies of exercise in the heat have found increased cortisol responses (Cross et al., 1996; Radomski et al., 1998; Rhind et al., 2004; Laing et al., 2008; Kappel et al., 1991). Kappel et al. (1991) used passive sitting in hot water and demonstrated that TNFa was unchanged
P.A. Hosick et al. / Journal of Thermal Biology 35 (2010) 348–353
until rectal temperatures (Tre) reached 39.5 1C. However, exercise studies resulting in less of a change in core temperature noted small but significant increases in TNFa (Rhind et al., 2004; Peake et al., 2008). Furthermore, we found only a single citation concerned with the interaction of TNFa and cortisol response to exercise in the heat (Rhind et al., 2004). That study found a significant correlation between the change in core temperature and both cortisol and TNFa, yet the relationship between cortisol and TNFa was not significant. Understanding the response of hormonal and inflammatory markers of inflammation can provide insight to the overall inflammatory response to exercise in the heat. Therefore, the purpose of the current investigation was to explore how changes in core temperature relate to changes in TNFa and cortisol during exercise. The goal was to better characterize the relationship between inflammatory and hormonal responses to exercise that result in significant changes in core temperature. Furthermore, the study was designed to determine if there is a relationship between the change in cortisol and TNFa as a result of differing levels of core temperature change. To accomplish this, we exercised subjects using a water immersion technique (Cross et al., 1996; Rhind et al., 2004; Morlock and Dressendorfer, 1974; McMurray and Horvath, 1976) to control the rise in core body temperature, while maintaining equal exercise intensity between trials. Our hypothesis was that both TNFa and cortisol would increase in relation to the increases in core temperature. This information will help both coaches and fitness professionals better prescribe exercise and rest ratios in situations when increased core temperature is likely to happen.
2. Materials and methods 2.1. Participants Eight recreationally active, healthy men age 23.6 71.6 (SD) yr participated in the study. Based on a repeated measures design, eight subjects provided sufficient statistical power with an a ¼0.05 and b ¼0.90 to detect 95 nmol/L differences in cortisol and 0.4 ng/mL change in TNFa. Their physical characteristics were as follows: height 1.8070.10 m, mass 78.376.1 kg, and VO2peak (water cycle ergometry) 52.873.7 ml/kg/min. All subjects engaged in at least 30 min of moderate-to-vigorous physical activity for a minimum of 4 days per week. Participants were all nonsmokers, had no history of major medical or heat-related illness, were infection free within 3 weeks prior to participation, and were not using anti-inflammatory drugs (i.e. NSAIDs). The protocol was approved by the University of North Carolina at Chapel Hill Internal Review Board. 2.2. Instrumentation Participants heights and body masses were determined using a stadiometer (Perspectives Enterprises, Portage, MI, USA) and calibrated mechanical scale (Detecto, Webb City, MO, USA), respectively. All exercises were performed in a 1790 L water tank in which temperature was controlled and maintained at 70.2 1C of the starting temperature throughout each trial. Exercise was performed on a modified cycle ergometer with the friction belt removed from the flywheel and replaced with small fins to increase drag forces. A similar ergometer modification has been previously used and validated (Morlock and Dressendorfer, 1974; McMurray and Horvath, 1976). Oxygen uptake was measured using a Parvo Medics TrueMax 2400 Metabolic System (Parvo Medics, Salt Lake City, UT). Heart rate (HR) was monitored using a polar telemetry system (Polar Electro Inc., Lake Success, NY, USA).
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Core body temperature (Tre) was monitored by a YSI rectal probe (YSI, Dayton, OH, USA) connected to a calibrated digital thermistor thermometer (Cole-Parmer, Vernon Hill, IL, USA). 2.3. Protocol The participants visited the laboratory on three separate occasions. The first visit was for screening, obtaining informed consent, and measuring peak oxygen uptake (VO2peak). During the second and third visits participants completed 40 min exercise trials while immersed in either 25 or 38.5 1C water. Exercise during water immersion has previously been shown to be a stable and reliable way of controlling a persons’ external environment (Cross et al., 1996; Rhind et al., 2004; Morlock and Dressendorfer, 1974; McMurray and Horvath, 1976). Since the goal of the study was to show differences in immune and hormonal markers of stress in response to elevated core temperature during exercise, the temperatures were chosen accordingly; the neutral trail, 25 1C was chosen because it allows subjects to exercise comfortably but prevents significant rises in core temperature. In the warm trial, 38 1C was chosen because it simulates exercise in a hot environment, but without exercise will not induce huge changes in core temperature (Cross et al., 1996). 2.3.1. Participant screening On arrival to our facility participants were given time to read and sign the informed consent form. A medical screening and physical examination were then completed to ensure all subjects were healthy enough to participate in the study. Next, heights and body masses were measured. Participants then donned a swimsuit and affixed a heart rate monitor around their chests. Participants were then given time to warm up on land, using a standard cycle ergometer, and stretch prior to the graded maximal exercise test. Following the warm up, participants entered the water tank. The water temperature was set at 25 1C for all VO2peak tests. Handle bar and seat height were adjusted for each participant and recorded for consistency during the subsequent exercise trials. Subjects attached a 10–12 kg belt around their waist to help maintain their seated position on the ergometer throughout the test. The water level was adjusted to the participants xyphoid process. The participants then inserted the mouthpiece from the metabolic system and rested for several minutes, during which reasting VO2 measurement was gathered. After the resting VO2 sample was gathered, the VO2peak exercise test commenced. Participants began pedaling at 40 revolutions per minute (rpm). After 5 min at that speed, the pedal rate increased to 50 rpm, subjects pedaled at that rate for 3 min. Thereafter, pedal rate was increased by 5 rpm every 3 min. Continuous VO2 measurements were gathered throughout the test, while HR was obtained min-by-min, and rate of perceived exertion (RPE) measurements (Borg 6–20 scale) were collected at the end of each stage. The test was terminated when subjects reached volitional fatigue or the administrator determined that the subject was unable to maintain the appropriate cadence on the cycle ergometer. Following cessation of the test, participants performed an active recovery at a self-selected pace on the ergometer until HR was less than 120 beats per minute (bpm). From this trial, pedal rates eliciting 65% of the subjects’ VO2peak were computed using the Karvonen method. 2.3.2. Experimental trials Trials began at approximately 0730–0830 h for all subjects. The protocols for the two experimental trials were identical, with the only exception being, the water temperature in the
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tank: 25 70.2 1C (cool) or 38.5 70.2 1C (warm). The order of exposure was counter balanced among the subjects. The first experimental trial occurred 2–7 days following the VO2peak test, with subjects arriving at the laboratory at least 4 h post-prandial. Participants self-inserted the rectal thermometer approximately 10 cm into the rectum, placed a heart rate monitor around their chests, and changed in the same clothing worn during the VO2peak test. Next, subjects rested quietly for 20 min on land in an upright-seated position. At minute eighteen of the rest, baseline HR and Tre were recorded, followed by a 10 ml venous blood sample, which was drawn via sterile venipuncture of an anticubital vein using vacutainers with EDTA. The blood sample was immediately placed in ice until it was centrifuged. Participants then entered the water tank and attached the weight belt. With the participants hands placed on the cycle ergometer handlebars, the water level was adjusted to the level of the zyphoid process. After the participants were immersed, they began a 5 min warm-up period at a self-selected pace. Following the warm-up period, participants cycled at the pedal frequency that elicited 65% VO2peak. After 4 min of exercise, the participants’ VO2 was monitored for 1 min to ensure that they were working at the proper intensity (65% VO2peak), with pedal rate being adjusted, if necessary. Any adjustments made during their first experimental trial were mimicked exactly in the second trial. Subsequent VO2 collection occurred during the last 2 min of each 10-min interval over the 40 min cycle, or the last minute of exercise if the participant stopped prematurely. During the exercise, Tre, HR, and RPE were monitored min-by-min and recorded during the last 15 s of each 5-min interval. For safety reasons the trial was terminated if a subject’s Tre increased above 39.5 1C (103 1F; Human Subject Review Committee requirement). Participants were allowed to drink water ad libitum during the exercise session. At the conclusion of the 40-min exercise (or if the participants Tre reached 39.5 1C) the mouthpiece was removed, participants exited the water tank and dried off ( o1 min). The participants then sat upright in a chair while another 10 ml blood sample was obtained, employing identical methods to the earlier sample. All samples were centrifuged at 4 1C within 5 min of being drawn; the separated plasma was decanted and stored at 80 1C until analysis. Plasma TNFa concentrations were determined in duplicate, using ELISA technology, following the manufacturer recommendations for a high-sensitivity, enzyme-linked immunosorbent assay kit (R&D Systems, Minneapolis, MN, USA), with an intra-assay coefficient of variation (CV) of 8.2%. All samples were completed in the same TNFa kit using one plate. Cortisol concentrations were also determined from the plasma using
commercially available radioimmunoassay kits (Coat-A-Count, Siemens Healthcare, Los Angeles, CA, USA), with an intra-assay CV of 3.5%. All samples were completed in the same cortisol kit and completed on the same day. 2.4. Analytical methods Means and standard deviations were computed for all measures. Repeated measures, 2-way ANOVA was used to determine if significant change occurred in VO2, RER, HR, RPE, and Tre between trials and over time. Paired sample t-tests were run on the pre–post values for TNFa and cortisol with a bonferroni adjustment. Change scores (D) for Tre, TNFa, and cortisol were computed by subtracting baseline measures from post-exercise values. Using the change scores to estimate the fit of D TNFa and D cortisol, the concordance correlations coefficients (Rc) were computed (Vonesh et al., 1996). The Rc is an estimate of ‘goodness of fit’, similar to a correlation coefficient and is best suited for continuous data and mixed models (Vonesh et al., 1996). As with correlations, values range from 1.0 to 1, with a perfect correlation having a value of one and complete lack of correlation having a value of 0.0. Finally, multiple regression was used to explore the relationship between DTre (independent variable) and D TNFa and D cortisol (dependant variables). Since we were interested in the interaction of cortisol or TNFa with DTre during exercise, circulating concentrations of each are more likely to be more important than absolute amounts. Therefore, neither cortisol nor TNFa values were not corrected for plasma volume shift. All statistics were completed using SAS statistical programming software (SAS, Cary, NC, USA), significance was set a priori at an alpha of 0.05.
3. Results All subjects completed the trial in cool water, but three of the eight subjects had to prematurely terminate the trial in the warm water. Two subjects stopped early due to volitional fatigue, one at 30 min, the other at 35 min into the trial. The third subject terminated early because Tre reached our predetermined safety threshold of 39.5 1C. At termination, all three of these subjects reported rating of perceived exertion of maximal or near maximal intensities. All subjects successfully maintained 65% VO2peak throughout each trial. No difference in oxygen uptake or RER was found between the two trials (Table 1). Resting HR was similar between the two trials; however, exercise HR in warm trial was significantly elevated (p o0.001) at every time point compared
Table 1 Mean7 SD in trial physiologic responses of the participants for cool and warm trials. Trial
Rest
10 min
20 min
30 min
End of ex.
VO2 (mL/kg/min)
Cool Warm
4.4 7 1.2 4.2 7 0.6
31.4 7 3.7 33.07 4.4
33.3 7 2.9 35.5 7 2.7
33.67 4.0 34.57 4.1
34.3 73.6 35.6 75.8
RER
Cool Warm
0.85 7 0.05 0.85 7 0.15
0.927 0.03 0.937 0.05
0.917 0.04 0.937 0.05
0.907 0.04 0.927 0.04
0.90 70.04 0.91 70.06
HR (bpm)
Cool Warm
66 7 5 66 7 5
135 7 22 166 7 26*
141 7 21 177 7 20*
1437 19 1817 18*
144 78 180 712*
RPE
Cool Warm
– –
137 2.0 147 2.7
137 2.3 157 2.5*
147 2.0 167 2.5*
14 71.9 16 72.1*
Tre (1C)
Cool Warm
36.9 7 0.4 36.8 7 0.4
37.07 0.5 37.1 7 0.3
37.3 7 0.6 37.9 7 0.4*
37.57 0.6 38.97 0.3*
37.5 70.7 39.2 70.3*
VO2 ¼ volume of oxygen consumed, RER ¼ respiratory exchange ratio, HR¼ heart rate, RPE¼ rating of perceived exertion, and Tre ¼ rectal temperature. n
Significantly different at p o0.05 from cool trial at the same time point.
P.A. Hosick et al. / Journal of Thermal Biology 35 (2010) 348–353
Cortisol (nmol/L)
800
*
600 400 200 0 3.5
*
TNFα (pg/mL)
3.0 2.5 2.0 1.5 1.0 0.5 0.0
Cool
Warm Baseline = □ Post - Exercise = ■
Fig. 1. Mean7 SEM for cortisol and TNFa response to exercise in cool and warm trials. *p o0.004 baseline vs. post exercise.
with the cool trial (Table 1). Rectal temperature and RPE were similar 10 min into both trials, but at minutes 20, 30, and the end of exercise both Tre and RPE in the warm trial were significantly elevated above the cool trial (p o0.01, Table 1). Plasma volume shifts during the exercise were computed for each trial. For the cool trial, the mean plasma volume loss during exercise was 4.874.4%. In the hot exercise trial, the mean plasma volume loss during exercise was 8.875.3%, which was significantly greater than in the cool trial (p o0.05). Neither cortisol nor TNFa values were corrected for plasma volume shift, as indicated in Methods section (Section 2). Results of the paired sample t-tests are indicated for the cool trial; neither cortisol nor TNFa changed significantly in response to the exercise; however, in the warm trial there were significant increases in both cortisol (po0.001) and TNFa (po0.01). Fig. 1 shows the response of cortisol (top panel) and TNFa (lower panel) to exercise for the two trials. To determine the relationship of D cortisol with D TNFa concordance correlation coefficients were computed with the results of the warm and cool trials combined (Fig. 4). The correlation between D TNFa and D cortisol showed a positive relationship, yet was not statistically significant (Rc ¼ 0.833, p ¼0.136). The results of the exploratory multiple regression on the impact of DTre on D TNFa and D cortisol approached significance (p¼0.059). Change in cortisol contributed significantly to the regression model, explaining 31% of the variation in DTre (p ¼0.025), while D TNFa contributed only to 4% of the variance (p ¼0.373).
4. Discussion While it is well understood that the increased energy expenditure of exercise results in an increased core temperature, especially in a hot environment (Brandenberger and Follenius, 1975), it is less clear how increased core temperature relates to the acute exercise inflammatory response. Previous research is not in agreement as to the relationship between the TNFa and
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cortisol response to exercise in the heat (Cross et al., 1996; Pedersen et al., 1998; Radomski et al., 1998; Rhind et al., 2004; Starkie et al., 2005; Laing et al., 2008; Peake et al., 2008; Walsh and Whitham, 2006). Thus, the purpose of this investigation was to explore the relationship between increases in TNFa and cortisol in response to differing levels of Tre, while maintaining a controlled exercise intensity. Our results suggest that the changes in TNFa and cortisol during exercise are only somewhat associated with changes in core temperature, with the relationship to core temperature being stronger for cortisol than TNFa, implying that, hormonal responses to increased core temperature appear to be one of the driving factors that initiates the process of an increased inflammatory response. Walsh and Whitman (2006) in their recent review, report a temperature dependency associated with immune alterations in exercise. They suggest that consistent responses should not be expected with Tre of o39.3 1C. In the present study the ending Tre in the warm trial was 39.2 70.2 1C, though this does not meet the suggested threshold mentioned above, four of the eight subjects did meet or exceed an ending Tre of 39.3 1C and all four of these subjects had elevations of TNFa. However, the four subjects who did not exceed this threshold in the same trial also had increases in TNFa. Thus, our results suggest that the response is quite individual and there is perhaps no generalizable single threshold. In general, the exercise responses presented herein are similar to those in other literature (Cross et al., 1996; Rhind et al., 2004; Starkie et al., 2005) and our resting cortisol concentrations, although somewhat high, are well within normal ranges for the time of day of our testing (Kratz and Lewandrowski, 1998). The pre-exercise TNFa values for the warm trial are also slightly elevated from what has been previously reported (Starkie et al., 2005), but are not indicative of any inflammatory state. Nonetheless, these slight differences may, in part, explain some of the findings of the present investigation. The response of cortisol was significant in the warm trial but not the cool trial. In the warm trial all eight subjects exhibited an increase in cortisol concentrations, but only two subjects increased in cortisol during the cool trial. Interestingly, an intensity threshold of approximately 50% of maximal capacity has been proposed for cortisol (Hill et al., 2008). Thus, one may expect cortisol to be significantly raised from rest in both trials, as intensity was 65% of VO2peak, yet this was not the case. The temperature of the water may have prevented any rise in cortisol in the cool trial; as Rhind et al. (2004) suggest, the lack of a cortisol response is due to the lack of a core temperature change, although as Hill et al. (2008) point out this result may be found due to lack of hemoconcentration effect as well as lack of adequate HPA axis stimulation. With respect to hemoconcentration, the plasma volume shift in the hot trial was 9% and the change in serum cortisol concentration was 20%. In the cool trial there was a plasma volume shift of 5% and serum cortisol concentration decreased slightly, suggesting that cortisol may have actually decreased during the cool trial. A study by Armstrong et al. (1989) showed subjects acclimate to consecutive increases in body temperature, by mechanisms other than alterations in fluid balance. Acclimation of this nature is likely in the present subjects, due to their level of fitness and regular training regiments. Similar arguments for plasma volume shift affecting TNFa can be made, where the changes amounted to 16% increase in the hot trial and no change in the cool trial. Therefore, we believe our findings, in either trial, were not caused by changes in hemoconcentration, but represent alterations due to changes in core temperature. Figs. 2–4 graphically describe the variation of D cortisol and D TNFa with respect to trial. Change in cortisol can be seen in Fig. 2 with a range from 300 to + 400 nmol/L in the cool trial,
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500
Δ Cortisol (nmol/L)
400 300 200 100 -0.5
0 0.0 -100
0.5
1.0
1.5
2.0
2.5
3.0
3.5
= cool trial = warm trial
-200 -300 -400 Δ Temperature (°C)
Fig. 2. Effect of rectal temperature on the change in serum cortisol levels in human subjects exercising at two different water temperatures.
0.8 0.6
Δ TNFα (pg/mL)
0.4 0.2 -0.5
0.0 0.0 -0.2
0.5
1.0
1.5
2.0
2.5
3.0
3.5
=cool trial =warm trial
-0.4 -0.6 -0.8 -1.0 -1.2
5. Conclusion
Δ Temperature (°C) Fig. 3. Effect of rectal temperature on the change in serum cortisol level in human subjects exercising at two different water temperatures.
0.8 0.6 Δ TNFα (pg/mL)
0.4 0.2 -350
-250
-150
0.0 -50 -0.2
50
150
-0.4 -0.6
relationship between Tre and TNFa in a group of 10 men. However, a cause/effect relationship cannot be determined from our results. Multiple regression analysis was completed to determine which measure, D TNFa or D cortisol, was more related to DTre. Results of the multiple regression show that the change in Tre was more related to changes in cortisol than changes in TNFa. This agrees with the findings of Jimenez et al. (2007), which suggests the cortisol is elevated to prevent pro-inflammatory overshoot as Tre increases, therefore, cortisol more closely follows the DTre. Additionally, because of the inter-relationship between TNFa and cortisol in our study, and because cortisol appears to be controlling this relationship, it seems plausible that increases in cortisol could be contributing to the TNFa response. However, D cortisol, while significantly contributing, explained only 31% of the variation in DTre; thus, we hesitate emphasizing this result. One major limitation of this investigation was the small sample size of eight subjects. However, our cortisol and TNFa change data have a moderate effect size (r ¼0.37 for TNFa and 0.66 for cortisol) and a Cohen’s d value of 1.04–0.94, which is considered ‘good’. In support, the exercise changes observed in both TNFa and cortisol were very similar to those in previous studies (Cross et al., 1996; Pedersen et al., 1998; Starkie et al., 2005; Laing et al., 2008; Peake et al., 2008). Also, despite the abbreviated exercise time of a few subjects, their Tre, TNFa, and cortisol were comparable to those of subjects who completed the 40 min of exercise warm trial. Finally, this sample size is comparable with those in previously published studies of a similar nature, which have used between seven and ten subjects (Cross et al., 1996; Rhind et al., 2004; Starkie et al., 2005; Peake et al., 2008). Thus, we feel our results are relevant.
250
350
450
=cool trial = warm trial
-0.8 -1.0 Δ Cortisol (nmol/L) Fig. 4. Change in serum TNFa vs. change in serum cortisol level in human subjects exercising at two different water temperatures. Rc ¼ 0.833, p ¼ 0.135.
In summary, we have shown a relationship between TNFa and cortisol in response to different levels of DTre. Our data suggest that during exercise at 65% of VO2peak both TNFa and cortisol increase when core temperatures are elevated approximately 1 1C above rest. Furthermore, any changes in core temperature during exercise appear to have more of an influence on cortisol than TNFa. Acute moderate-intensity exercise (40 min or less) in a warm environment appears to have a greater impact on the hormonal response in comparison with the immune response. This finding is of importance to individuals, coaches, and fitness professionals attempting to understand ideal training load and design in warm or hot environments. Future work may benefit by looking at longer durations, more extreme ambient temperatures, and greater changes in Tre, as would occur during exercise training sessions. Also, additional biomarkers of the stress and immune response need to be explored that can provide a more complete understanding of the immune response to exercise in the heat. Finally, this investigation hopefully stimulates further research into factors associated with inter-subject variability in response to heat load.
Conflict of interest and a range of + 40 to + 290 nmol/L in the hot trial. All subjects increased their cortisol in the hot trial, while only 2 subjects showed increased cortisol in the cool trail. This may be related to the effect on DTre, since workload between the trials was equal. Therefore, when core temperature is significantly increased, the cortisol response is more consistent. A similar, but less clear pattern follows for D TNFa (Fig. 3). These patterns tend to agree with the findings of Rhind et al. (2004), which show a moderate
The authors report no conflict of interest related to this article.
Acknowledgments This study was supported by a grant from the Smith Graduate Fund at the University of North Carolina. We would like to thank
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Mr. J.J. Duke and Mr. Michael Anderson for their assistance with this study. References Abbas, A.K., Lichtman, A.H., Pillai, S., 2007. Cellular and Molecular Immunology sixth ed. W.B. Saunders Company, Philaedelphia, pp. 273–278. Armstrong, L.E., Francesconi, R.P., Kraemer, W.J., Leva, N., De Luca, J.P., Hubbard, R.W., 1989. Plasma cortisol, renin, and aldostrone during and intense heat accumulation program. Int. J. Sports Med. 10, 38–42. Berczi, I., 1998. Neurohormonal host defense in endotoxin shock. Ann. N.Y. Acad. Sci. 840, 787–802. Brandenberger, G., Follenius, M., 1975. Influence of timing and intensity of musclar exercise on temporal patterns of plasma cortisol levels. J. Clin. Endocrinol. Metab. 4, 845–849. Collins, K.J., Few, J.D., Forward, T.J., Giec, L.A., 1969. Stimulation of adrenal glucocorticoid secretion in man by raising body temperature. J. Physiol. 202, 644–648. Cross, M.C., Radomski, M.W., VanHelder, W.P., Rhind, S.G., Shephard, R.J., 1996. Endurance exercise with and without a thermal clamp: effects on leukocytes and leukocyte subsets. J. Appl. Physiol. 81, 822–829. Francesconi, R.P., 1988. Endocrinological responses to exercise in stressful environments. Exer. Sports Sci. Rev. 16, 255–284. Francesconi, R.P., Maher, J., Mason, J.W., Bynum, G.D., 1978. Hormonal responses of sedentary and exercising men to recurrent heat exposure. Aviat. Space Environ. Med. 49, 1102–1106. Frank, M.G., Miguel, Z.D., Watkins, L.R., Maier, S.F., 2010. Prior exposure to glucocorticoids sensitizes the neuroinflammatory and peripheral inflammatory responses to E. coli lipopolysaccharide. Brain Behav Immun. 24, 19–30. Hill, E.E., Zack, E., Battaglini, C., Viru, M., Viru, A., Hackney, A.C., 2008. Exercise and circulating cortisol levels: the intensity threshold effect. J. Endocrinol. Inv. 31, 587–591. Jimenez, C., Melin, B., Savourey, G., Launay, J.C., Alonso, A., Mathieu, J., 2007. Effects of passive hyperthermia versus exercise-induced hyperthermia on immune responses: hormonal implications. Eur. Cytokine Netw. 1, 154–161.
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Kappel, M., Diamont, M., Hansen, M.B., Klokker, M., Pederson, B.K., 1991. Effects of in vitro hyperthermia on the proliferative response of blood mononuclear cell subsets, and detection of interleukins 1 and 6, tumour necrosis factor-alpha and interferon-gamma. Immunology 73, 304–308. Kratz, A., Lewandrowski, K.B., 1998. Normal reference laboratory values. N. Eur. J.Med. 339, 1063–1073. Laing, S.J., Jackson, A.R., Walters, R., Lloyd-Jones, E., Whitham, M., Maassen, N., Walsh, N.P., 2008. Human blood neutrophil responses to prolonged exercise with and without a thermal clamp. J. Appl. Physiol. 10, 20–26. Morlock, J.F., Dressendorfer, R.H., 1974. Modification of a standard bicycle ergometer for underwater use. Undersea Biomed. Res. 1, 335–342. McMurray, R.G., Horvath, S.M., 1976. Thermoregulation in swimmers and runners. J. Appl. Physiol. 4, 1086–1092. Nielsen, B., Davies, C.T., 1976. Temperature regulation during exercise in water and air. Acta. Physiol. Scand. 98, 500–508. Pedersen, B.K., Ostrowski, K., Rohde, T., Bruunsgaard, H., 1998. The cytokine response to strenuous exercise. Can. J. Physiol. Pharmacol. 76, 505–511. Peake, J., Peiffer, J.J., Abbiss, C.R., Nosaka, K., Okutsu, M., Laursen, P.B., Suzuki, K., 2008. Body temperature and its effect on leukocyte mobilization, cytokines and markers of neutrophil activation during and after exercise. Eur. J. Appl. Physiol. 10, 391–401. Radomski, M.W., Cross, M., Buguet, A., 1998. Exercise-induced hyperthermia and hormonal responses to exercise. Can. J. Physiol. Pharmacol. 76, 547–552. Rhind, S.G., Gannon, G.A., Shephard, R.J., Buguet, A., Shek, P.N., Radomski, M.W., 2004. Cytokine induction during exertional hyperthermia is abolished by core temperature clamping: neuroendocrine regulatory mechanisms. Int. J. Hyperthermia. 20, 503–516. Shephard, R.J., 1998. Immune changes induced by exercise in an adverse environment. Can. J. Physiol. Pharmacol. 76, 539–546. Starkie, R.L., Hargreaves, M., Rolland, J., Febbraio, M.A., 2005. Heat stress, cytokines, and the immune response to exercise. Brain Behav. Immun. 19, 404–412. Vonesh, E.F., Chinchill, V.M., Pu, K., 1996. Goodness-of-fit in generalized nonlinear mixed effects models. Biometrics 52, 572–587. Whipp, B.J., Wasserman, K., 1969. Efficiency of muscular work. J Appl Physiol 26, 644–648. Walsh, N.P., Whitham, M., 2006. Exercising in environmental extremes: a greater threat to immune function? Sports Med. 36 941–976.
Contents lists available at ScienceDirect
Journal of Thermal Biology journal homepage: www.elsevier.com/locate/jtherbio
Relationship between change in core temperature and change in cortisol and TNFa during exercise Peter A. Hosick a,n, Mark P. Berry a, Robert G. McMurray a,b, Erica S. Cooper a, A.C. Hackney a,b a b
Department of Exericse and Sport Science, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA Department of Nutrition, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 30 March 2010 Accepted 8 July 2010
The combined thermal load created by exercise and a hot environment is associated with an exaggerated core temperature response. The elevated core temperature is believed to increase the total stress of the exercise. Increased stress during exercise has been associated with increased levels of cortisol. The association of cortisol with increased inflammatory responses following exercise in the heat is equivocal. Thus, the purpose of the current investigation was to explore the relationship between increases in rectal temperature (Tre) and TNFa and cortisol. To induce Tre changes, 8 male subjects (mean 7 SD, age ¼ 23.6 7 2 yr, VO2max ¼ 52.8 7 3.7 mL/kg/min, BMI ¼24.2 7 1.9) participated in two 40 min trials of cycle ergometry at 65% of VO2peak immersed to chest level in cool (25 1C) and warm (38.5 1C) water. Tre was monitored throughout each trial, with blood samples taken immediately pre and post of each trial. Neither cortisol nor TNFa changed significantly during exercise in the cool water; however, in the warm trial, both cortisol and TNFa significantly increased (p o 0.004). Concordance correlations (Rc) between D cortisol and D TNFa indicated a strong but non-significant correlation (Rc ¼0.833, p¼ 0.135). In conclusion, changes in core temperature may be impacting the relationship between exercise induced changes in cortisol and TNFa. Therefore, acute moderateintensity exercise (40 min or less) in warm water impacts the stress and inflammatory response. Understanding this is important because exercise load may need to be adjusted in warm and hot environments to avoid the negative effects of elevated stress and inflammation response. Published by Elsevier Ltd.
Keywords: Heat exposure Environmental stress Immersion Hormones
1. Introduction Significant research has been done exploring the stress response of exercise in extreme environments. In general, well trained subjects, who are normally hydrated, are able to tolerate moderate levels of exercise in hot temperatures with only minor effects on the stress induced glucocorticoid response (Collins et al., 1969; Francesconi, 1988; Francesconi et al., 1978). However, when similar moderate-intensity exercise induces a rise in core body temperature above a particular threshold point, an elevated glucocorticoid response can occur (Collins et al., 1969). Additionally, increases in core temperature can result in increases in both immune and hormonal markers of inflammation (Whipp and Wasserman, 1969; Nielsen and Davies, 1976; Shephard, 1988; Abbas et al., 2007). Tumor necrosis factor alpha (TNFa), a cytokine released mainly from monocytes and macrophages, is believed to be an important part of this immune reaction because it is a principal mediator of the acute inflammatory response (Abbas
n
Corresponding author. Tel.: +1 919 962 2986; fax: +1 919 962 0489 E-mail address: [email protected] (P.A. Hosick).
0306-4565/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.jtherbio.2010.07.003
et al., 2007). Cortisol, another important component of the immune response, is released in response to acute phase reactants (Berczi, 1998), as well as the stress of exercise (Shephard, 1998). Furthermore, research has shown that glucocorticoids, such as cortisol, can potentiate the inflammatory response including TNFa (Frank et al., 2010). Together, cortisol and TNFa may be used as an indication of the physical stress and inflammatory response of exercise that accompany the associated increase in core temperature. Increased inflammatory cytokines and hormonal stress markers during exercise in a hot environment are widely acknowledged (Cross et al., 1996; Pedersen et al., 1998; Radomski et al., 1998; Rhind et al., 2004; Starkie et al., 2005; Laing et al., 2008; Peake et al., 2008). However, the findings of these studies are nonconsistent with respect to the cortisol or TNFa response. For example, Peake et al. (2008) found core temperatures elevated by about 2 1C during exercise in the heat, yet circulating levels of cortisol were slightly reduced. Conversely, other studies of exercise in the heat have found increased cortisol responses (Cross et al., 1996; Radomski et al., 1998; Rhind et al., 2004; Laing et al., 2008; Kappel et al., 1991). Kappel et al. (1991) used passive sitting in hot water and demonstrated that TNFa was unchanged
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until rectal temperatures (Tre) reached 39.5 1C. However, exercise studies resulting in less of a change in core temperature noted small but significant increases in TNFa (Rhind et al., 2004; Peake et al., 2008). Furthermore, we found only a single citation concerned with the interaction of TNFa and cortisol response to exercise in the heat (Rhind et al., 2004). That study found a significant correlation between the change in core temperature and both cortisol and TNFa, yet the relationship between cortisol and TNFa was not significant. Understanding the response of hormonal and inflammatory markers of inflammation can provide insight to the overall inflammatory response to exercise in the heat. Therefore, the purpose of the current investigation was to explore how changes in core temperature relate to changes in TNFa and cortisol during exercise. The goal was to better characterize the relationship between inflammatory and hormonal responses to exercise that result in significant changes in core temperature. Furthermore, the study was designed to determine if there is a relationship between the change in cortisol and TNFa as a result of differing levels of core temperature change. To accomplish this, we exercised subjects using a water immersion technique (Cross et al., 1996; Rhind et al., 2004; Morlock and Dressendorfer, 1974; McMurray and Horvath, 1976) to control the rise in core body temperature, while maintaining equal exercise intensity between trials. Our hypothesis was that both TNFa and cortisol would increase in relation to the increases in core temperature. This information will help both coaches and fitness professionals better prescribe exercise and rest ratios in situations when increased core temperature is likely to happen.
2. Materials and methods 2.1. Participants Eight recreationally active, healthy men age 23.6 71.6 (SD) yr participated in the study. Based on a repeated measures design, eight subjects provided sufficient statistical power with an a ¼0.05 and b ¼0.90 to detect 95 nmol/L differences in cortisol and 0.4 ng/mL change in TNFa. Their physical characteristics were as follows: height 1.8070.10 m, mass 78.376.1 kg, and VO2peak (water cycle ergometry) 52.873.7 ml/kg/min. All subjects engaged in at least 30 min of moderate-to-vigorous physical activity for a minimum of 4 days per week. Participants were all nonsmokers, had no history of major medical or heat-related illness, were infection free within 3 weeks prior to participation, and were not using anti-inflammatory drugs (i.e. NSAIDs). The protocol was approved by the University of North Carolina at Chapel Hill Internal Review Board. 2.2. Instrumentation Participants heights and body masses were determined using a stadiometer (Perspectives Enterprises, Portage, MI, USA) and calibrated mechanical scale (Detecto, Webb City, MO, USA), respectively. All exercises were performed in a 1790 L water tank in which temperature was controlled and maintained at 70.2 1C of the starting temperature throughout each trial. Exercise was performed on a modified cycle ergometer with the friction belt removed from the flywheel and replaced with small fins to increase drag forces. A similar ergometer modification has been previously used and validated (Morlock and Dressendorfer, 1974; McMurray and Horvath, 1976). Oxygen uptake was measured using a Parvo Medics TrueMax 2400 Metabolic System (Parvo Medics, Salt Lake City, UT). Heart rate (HR) was monitored using a polar telemetry system (Polar Electro Inc., Lake Success, NY, USA).
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Core body temperature (Tre) was monitored by a YSI rectal probe (YSI, Dayton, OH, USA) connected to a calibrated digital thermistor thermometer (Cole-Parmer, Vernon Hill, IL, USA). 2.3. Protocol The participants visited the laboratory on three separate occasions. The first visit was for screening, obtaining informed consent, and measuring peak oxygen uptake (VO2peak). During the second and third visits participants completed 40 min exercise trials while immersed in either 25 or 38.5 1C water. Exercise during water immersion has previously been shown to be a stable and reliable way of controlling a persons’ external environment (Cross et al., 1996; Rhind et al., 2004; Morlock and Dressendorfer, 1974; McMurray and Horvath, 1976). Since the goal of the study was to show differences in immune and hormonal markers of stress in response to elevated core temperature during exercise, the temperatures were chosen accordingly; the neutral trail, 25 1C was chosen because it allows subjects to exercise comfortably but prevents significant rises in core temperature. In the warm trial, 38 1C was chosen because it simulates exercise in a hot environment, but without exercise will not induce huge changes in core temperature (Cross et al., 1996). 2.3.1. Participant screening On arrival to our facility participants were given time to read and sign the informed consent form. A medical screening and physical examination were then completed to ensure all subjects were healthy enough to participate in the study. Next, heights and body masses were measured. Participants then donned a swimsuit and affixed a heart rate monitor around their chests. Participants were then given time to warm up on land, using a standard cycle ergometer, and stretch prior to the graded maximal exercise test. Following the warm up, participants entered the water tank. The water temperature was set at 25 1C for all VO2peak tests. Handle bar and seat height were adjusted for each participant and recorded for consistency during the subsequent exercise trials. Subjects attached a 10–12 kg belt around their waist to help maintain their seated position on the ergometer throughout the test. The water level was adjusted to the participants xyphoid process. The participants then inserted the mouthpiece from the metabolic system and rested for several minutes, during which reasting VO2 measurement was gathered. After the resting VO2 sample was gathered, the VO2peak exercise test commenced. Participants began pedaling at 40 revolutions per minute (rpm). After 5 min at that speed, the pedal rate increased to 50 rpm, subjects pedaled at that rate for 3 min. Thereafter, pedal rate was increased by 5 rpm every 3 min. Continuous VO2 measurements were gathered throughout the test, while HR was obtained min-by-min, and rate of perceived exertion (RPE) measurements (Borg 6–20 scale) were collected at the end of each stage. The test was terminated when subjects reached volitional fatigue or the administrator determined that the subject was unable to maintain the appropriate cadence on the cycle ergometer. Following cessation of the test, participants performed an active recovery at a self-selected pace on the ergometer until HR was less than 120 beats per minute (bpm). From this trial, pedal rates eliciting 65% of the subjects’ VO2peak were computed using the Karvonen method. 2.3.2. Experimental trials Trials began at approximately 0730–0830 h for all subjects. The protocols for the two experimental trials were identical, with the only exception being, the water temperature in the
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tank: 25 70.2 1C (cool) or 38.5 70.2 1C (warm). The order of exposure was counter balanced among the subjects. The first experimental trial occurred 2–7 days following the VO2peak test, with subjects arriving at the laboratory at least 4 h post-prandial. Participants self-inserted the rectal thermometer approximately 10 cm into the rectum, placed a heart rate monitor around their chests, and changed in the same clothing worn during the VO2peak test. Next, subjects rested quietly for 20 min on land in an upright-seated position. At minute eighteen of the rest, baseline HR and Tre were recorded, followed by a 10 ml venous blood sample, which was drawn via sterile venipuncture of an anticubital vein using vacutainers with EDTA. The blood sample was immediately placed in ice until it was centrifuged. Participants then entered the water tank and attached the weight belt. With the participants hands placed on the cycle ergometer handlebars, the water level was adjusted to the level of the zyphoid process. After the participants were immersed, they began a 5 min warm-up period at a self-selected pace. Following the warm-up period, participants cycled at the pedal frequency that elicited 65% VO2peak. After 4 min of exercise, the participants’ VO2 was monitored for 1 min to ensure that they were working at the proper intensity (65% VO2peak), with pedal rate being adjusted, if necessary. Any adjustments made during their first experimental trial were mimicked exactly in the second trial. Subsequent VO2 collection occurred during the last 2 min of each 10-min interval over the 40 min cycle, or the last minute of exercise if the participant stopped prematurely. During the exercise, Tre, HR, and RPE were monitored min-by-min and recorded during the last 15 s of each 5-min interval. For safety reasons the trial was terminated if a subject’s Tre increased above 39.5 1C (103 1F; Human Subject Review Committee requirement). Participants were allowed to drink water ad libitum during the exercise session. At the conclusion of the 40-min exercise (or if the participants Tre reached 39.5 1C) the mouthpiece was removed, participants exited the water tank and dried off ( o1 min). The participants then sat upright in a chair while another 10 ml blood sample was obtained, employing identical methods to the earlier sample. All samples were centrifuged at 4 1C within 5 min of being drawn; the separated plasma was decanted and stored at 80 1C until analysis. Plasma TNFa concentrations were determined in duplicate, using ELISA technology, following the manufacturer recommendations for a high-sensitivity, enzyme-linked immunosorbent assay kit (R&D Systems, Minneapolis, MN, USA), with an intra-assay coefficient of variation (CV) of 8.2%. All samples were completed in the same TNFa kit using one plate. Cortisol concentrations were also determined from the plasma using
commercially available radioimmunoassay kits (Coat-A-Count, Siemens Healthcare, Los Angeles, CA, USA), with an intra-assay CV of 3.5%. All samples were completed in the same cortisol kit and completed on the same day. 2.4. Analytical methods Means and standard deviations were computed for all measures. Repeated measures, 2-way ANOVA was used to determine if significant change occurred in VO2, RER, HR, RPE, and Tre between trials and over time. Paired sample t-tests were run on the pre–post values for TNFa and cortisol with a bonferroni adjustment. Change scores (D) for Tre, TNFa, and cortisol were computed by subtracting baseline measures from post-exercise values. Using the change scores to estimate the fit of D TNFa and D cortisol, the concordance correlations coefficients (Rc) were computed (Vonesh et al., 1996). The Rc is an estimate of ‘goodness of fit’, similar to a correlation coefficient and is best suited for continuous data and mixed models (Vonesh et al., 1996). As with correlations, values range from 1.0 to 1, with a perfect correlation having a value of one and complete lack of correlation having a value of 0.0. Finally, multiple regression was used to explore the relationship between DTre (independent variable) and D TNFa and D cortisol (dependant variables). Since we were interested in the interaction of cortisol or TNFa with DTre during exercise, circulating concentrations of each are more likely to be more important than absolute amounts. Therefore, neither cortisol nor TNFa values were not corrected for plasma volume shift. All statistics were completed using SAS statistical programming software (SAS, Cary, NC, USA), significance was set a priori at an alpha of 0.05.
3. Results All subjects completed the trial in cool water, but three of the eight subjects had to prematurely terminate the trial in the warm water. Two subjects stopped early due to volitional fatigue, one at 30 min, the other at 35 min into the trial. The third subject terminated early because Tre reached our predetermined safety threshold of 39.5 1C. At termination, all three of these subjects reported rating of perceived exertion of maximal or near maximal intensities. All subjects successfully maintained 65% VO2peak throughout each trial. No difference in oxygen uptake or RER was found between the two trials (Table 1). Resting HR was similar between the two trials; however, exercise HR in warm trial was significantly elevated (p o0.001) at every time point compared
Table 1 Mean7 SD in trial physiologic responses of the participants for cool and warm trials. Trial
Rest
10 min
20 min
30 min
End of ex.
VO2 (mL/kg/min)
Cool Warm
4.4 7 1.2 4.2 7 0.6
31.4 7 3.7 33.07 4.4
33.3 7 2.9 35.5 7 2.7
33.67 4.0 34.57 4.1
34.3 73.6 35.6 75.8
RER
Cool Warm
0.85 7 0.05 0.85 7 0.15
0.927 0.03 0.937 0.05
0.917 0.04 0.937 0.05
0.907 0.04 0.927 0.04
0.90 70.04 0.91 70.06
HR (bpm)
Cool Warm
66 7 5 66 7 5
135 7 22 166 7 26*
141 7 21 177 7 20*
1437 19 1817 18*
144 78 180 712*
RPE
Cool Warm
– –
137 2.0 147 2.7
137 2.3 157 2.5*
147 2.0 167 2.5*
14 71.9 16 72.1*
Tre (1C)
Cool Warm
36.9 7 0.4 36.8 7 0.4
37.07 0.5 37.1 7 0.3
37.3 7 0.6 37.9 7 0.4*
37.57 0.6 38.97 0.3*
37.5 70.7 39.2 70.3*
VO2 ¼ volume of oxygen consumed, RER ¼ respiratory exchange ratio, HR¼ heart rate, RPE¼ rating of perceived exertion, and Tre ¼ rectal temperature. n
Significantly different at p o0.05 from cool trial at the same time point.
P.A. Hosick et al. / Journal of Thermal Biology 35 (2010) 348–353
Cortisol (nmol/L)
800
*
600 400 200 0 3.5
*
TNFα (pg/mL)
3.0 2.5 2.0 1.5 1.0 0.5 0.0
Cool
Warm Baseline = □ Post - Exercise = ■
Fig. 1. Mean7 SEM for cortisol and TNFa response to exercise in cool and warm trials. *p o0.004 baseline vs. post exercise.
with the cool trial (Table 1). Rectal temperature and RPE were similar 10 min into both trials, but at minutes 20, 30, and the end of exercise both Tre and RPE in the warm trial were significantly elevated above the cool trial (p o0.01, Table 1). Plasma volume shifts during the exercise were computed for each trial. For the cool trial, the mean plasma volume loss during exercise was 4.874.4%. In the hot exercise trial, the mean plasma volume loss during exercise was 8.875.3%, which was significantly greater than in the cool trial (p o0.05). Neither cortisol nor TNFa values were corrected for plasma volume shift, as indicated in Methods section (Section 2). Results of the paired sample t-tests are indicated for the cool trial; neither cortisol nor TNFa changed significantly in response to the exercise; however, in the warm trial there were significant increases in both cortisol (po0.001) and TNFa (po0.01). Fig. 1 shows the response of cortisol (top panel) and TNFa (lower panel) to exercise for the two trials. To determine the relationship of D cortisol with D TNFa concordance correlation coefficients were computed with the results of the warm and cool trials combined (Fig. 4). The correlation between D TNFa and D cortisol showed a positive relationship, yet was not statistically significant (Rc ¼ 0.833, p ¼0.136). The results of the exploratory multiple regression on the impact of DTre on D TNFa and D cortisol approached significance (p¼0.059). Change in cortisol contributed significantly to the regression model, explaining 31% of the variation in DTre (p ¼0.025), while D TNFa contributed only to 4% of the variance (p ¼0.373).
4. Discussion While it is well understood that the increased energy expenditure of exercise results in an increased core temperature, especially in a hot environment (Brandenberger and Follenius, 1975), it is less clear how increased core temperature relates to the acute exercise inflammatory response. Previous research is not in agreement as to the relationship between the TNFa and
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cortisol response to exercise in the heat (Cross et al., 1996; Pedersen et al., 1998; Radomski et al., 1998; Rhind et al., 2004; Starkie et al., 2005; Laing et al., 2008; Peake et al., 2008; Walsh and Whitham, 2006). Thus, the purpose of this investigation was to explore the relationship between increases in TNFa and cortisol in response to differing levels of Tre, while maintaining a controlled exercise intensity. Our results suggest that the changes in TNFa and cortisol during exercise are only somewhat associated with changes in core temperature, with the relationship to core temperature being stronger for cortisol than TNFa, implying that, hormonal responses to increased core temperature appear to be one of the driving factors that initiates the process of an increased inflammatory response. Walsh and Whitman (2006) in their recent review, report a temperature dependency associated with immune alterations in exercise. They suggest that consistent responses should not be expected with Tre of o39.3 1C. In the present study the ending Tre in the warm trial was 39.2 70.2 1C, though this does not meet the suggested threshold mentioned above, four of the eight subjects did meet or exceed an ending Tre of 39.3 1C and all four of these subjects had elevations of TNFa. However, the four subjects who did not exceed this threshold in the same trial also had increases in TNFa. Thus, our results suggest that the response is quite individual and there is perhaps no generalizable single threshold. In general, the exercise responses presented herein are similar to those in other literature (Cross et al., 1996; Rhind et al., 2004; Starkie et al., 2005) and our resting cortisol concentrations, although somewhat high, are well within normal ranges for the time of day of our testing (Kratz and Lewandrowski, 1998). The pre-exercise TNFa values for the warm trial are also slightly elevated from what has been previously reported (Starkie et al., 2005), but are not indicative of any inflammatory state. Nonetheless, these slight differences may, in part, explain some of the findings of the present investigation. The response of cortisol was significant in the warm trial but not the cool trial. In the warm trial all eight subjects exhibited an increase in cortisol concentrations, but only two subjects increased in cortisol during the cool trial. Interestingly, an intensity threshold of approximately 50% of maximal capacity has been proposed for cortisol (Hill et al., 2008). Thus, one may expect cortisol to be significantly raised from rest in both trials, as intensity was 65% of VO2peak, yet this was not the case. The temperature of the water may have prevented any rise in cortisol in the cool trial; as Rhind et al. (2004) suggest, the lack of a cortisol response is due to the lack of a core temperature change, although as Hill et al. (2008) point out this result may be found due to lack of hemoconcentration effect as well as lack of adequate HPA axis stimulation. With respect to hemoconcentration, the plasma volume shift in the hot trial was 9% and the change in serum cortisol concentration was 20%. In the cool trial there was a plasma volume shift of 5% and serum cortisol concentration decreased slightly, suggesting that cortisol may have actually decreased during the cool trial. A study by Armstrong et al. (1989) showed subjects acclimate to consecutive increases in body temperature, by mechanisms other than alterations in fluid balance. Acclimation of this nature is likely in the present subjects, due to their level of fitness and regular training regiments. Similar arguments for plasma volume shift affecting TNFa can be made, where the changes amounted to 16% increase in the hot trial and no change in the cool trial. Therefore, we believe our findings, in either trial, were not caused by changes in hemoconcentration, but represent alterations due to changes in core temperature. Figs. 2–4 graphically describe the variation of D cortisol and D TNFa with respect to trial. Change in cortisol can be seen in Fig. 2 with a range from 300 to + 400 nmol/L in the cool trial,
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500
Δ Cortisol (nmol/L)
400 300 200 100 -0.5
0 0.0 -100
0.5
1.0
1.5
2.0
2.5
3.0
3.5
= cool trial = warm trial
-200 -300 -400 Δ Temperature (°C)
Fig. 2. Effect of rectal temperature on the change in serum cortisol levels in human subjects exercising at two different water temperatures.
0.8 0.6
Δ TNFα (pg/mL)
0.4 0.2 -0.5
0.0 0.0 -0.2
0.5
1.0
1.5
2.0
2.5
3.0
3.5
=cool trial =warm trial
-0.4 -0.6 -0.8 -1.0 -1.2
5. Conclusion
Δ Temperature (°C) Fig. 3. Effect of rectal temperature on the change in serum cortisol level in human subjects exercising at two different water temperatures.
0.8 0.6 Δ TNFα (pg/mL)
0.4 0.2 -350
-250
-150
0.0 -50 -0.2
50
150
-0.4 -0.6
relationship between Tre and TNFa in a group of 10 men. However, a cause/effect relationship cannot be determined from our results. Multiple regression analysis was completed to determine which measure, D TNFa or D cortisol, was more related to DTre. Results of the multiple regression show that the change in Tre was more related to changes in cortisol than changes in TNFa. This agrees with the findings of Jimenez et al. (2007), which suggests the cortisol is elevated to prevent pro-inflammatory overshoot as Tre increases, therefore, cortisol more closely follows the DTre. Additionally, because of the inter-relationship between TNFa and cortisol in our study, and because cortisol appears to be controlling this relationship, it seems plausible that increases in cortisol could be contributing to the TNFa response. However, D cortisol, while significantly contributing, explained only 31% of the variation in DTre; thus, we hesitate emphasizing this result. One major limitation of this investigation was the small sample size of eight subjects. However, our cortisol and TNFa change data have a moderate effect size (r ¼0.37 for TNFa and 0.66 for cortisol) and a Cohen’s d value of 1.04–0.94, which is considered ‘good’. In support, the exercise changes observed in both TNFa and cortisol were very similar to those in previous studies (Cross et al., 1996; Pedersen et al., 1998; Starkie et al., 2005; Laing et al., 2008; Peake et al., 2008). Also, despite the abbreviated exercise time of a few subjects, their Tre, TNFa, and cortisol were comparable to those of subjects who completed the 40 min of exercise warm trial. Finally, this sample size is comparable with those in previously published studies of a similar nature, which have used between seven and ten subjects (Cross et al., 1996; Rhind et al., 2004; Starkie et al., 2005; Peake et al., 2008). Thus, we feel our results are relevant.
250
350
450
=cool trial = warm trial
-0.8 -1.0 Δ Cortisol (nmol/L) Fig. 4. Change in serum TNFa vs. change in serum cortisol level in human subjects exercising at two different water temperatures. Rc ¼ 0.833, p ¼ 0.135.
In summary, we have shown a relationship between TNFa and cortisol in response to different levels of DTre. Our data suggest that during exercise at 65% of VO2peak both TNFa and cortisol increase when core temperatures are elevated approximately 1 1C above rest. Furthermore, any changes in core temperature during exercise appear to have more of an influence on cortisol than TNFa. Acute moderate-intensity exercise (40 min or less) in a warm environment appears to have a greater impact on the hormonal response in comparison with the immune response. This finding is of importance to individuals, coaches, and fitness professionals attempting to understand ideal training load and design in warm or hot environments. Future work may benefit by looking at longer durations, more extreme ambient temperatures, and greater changes in Tre, as would occur during exercise training sessions. Also, additional biomarkers of the stress and immune response need to be explored that can provide a more complete understanding of the immune response to exercise in the heat. Finally, this investigation hopefully stimulates further research into factors associated with inter-subject variability in response to heat load.
Conflict of interest and a range of + 40 to + 290 nmol/L in the hot trial. All subjects increased their cortisol in the hot trial, while only 2 subjects showed increased cortisol in the cool trail. This may be related to the effect on DTre, since workload between the trials was equal. Therefore, when core temperature is significantly increased, the cortisol response is more consistent. A similar, but less clear pattern follows for D TNFa (Fig. 3). These patterns tend to agree with the findings of Rhind et al. (2004), which show a moderate
The authors report no conflict of interest related to this article.
Acknowledgments This study was supported by a grant from the Smith Graduate Fund at the University of North Carolina. We would like to thank
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