Effects of Cold Environment Exposure and Cold Acclimatization on Exercise-Induced Salivary Cortisol Response

Abstract

Objective

Considering the adverse effects of exercise-induced cortisol secretion on health in athletes, it is important to determine the environmental and individual factors that contribute to the variations in exercise-induced cortisol secretion. In this study, the effects of cold environment exposure and cold acclimatization on exercise-induced salivary cortisol responses were investigated.

Methods

Short track skaters (n = 11), who usually practice under cold conditions, and inline skaters (n = 11), who usually practice under room temperature conditions, participated in a randomized crossover study. All participants cycled for 60 minutes at 65% $\dot{\text{V}}{\text{o}}_2\text{max}$ under cold (ambient temperature: 5 ± 1°C, relative humidity 41% ± 9%) and room temperature (ambient temperature: 21 ± 1°C, relative humidity 35% ± 5%) conditions. The participants had a 120-minute bed rest recovery phase at room temperature after both exercise bouts. Cortisol levels were measured in saliva samples collected pre-exercise and postexercise at 1 minute, 30 minutes, 60 minutes, and 120 minutes.

Results

Both short track and inline skaters exhibited clear cortisol responses to exercise under cold and room temperature conditions. The magnitude of the cortisol response, however, was different between skaters and conditions. The inline skaters exhibited significantly higher cortisol values under cold conditions than under room temperature conditions (7.6 nmol/L and 4.2 nmol/L, respectively). However, the short track skaters exhibited significantly higher cortisol values under cold conditions compared to room temperature conditions (8.7 nmol/L and 5.4 nmol/L, respectively).

Conclusions

The effects of cold environment exposure on exercise-induced cortisol response were different between skaters who usually practice under cold or room temperature conditions. These results can be interpreted as acclimatization to either cold or room temperature conditions attenuating the cortisol response, suggesting that acclimatization may be beneficial in reducing the exercise-induced cortisol response.

Introduction

A great number of studies have demonstrated changes in various hormones in response to exercise. ¹ One of the hormones that is remarkably responsive to exercise is cortisol, ² which is released from the adrenal cortex. Cortisol has been shown to increase after various types of activity such as treadmill, heavy resistance exercise, and ultraendurance exercise. ^{3 –}5 Cortisol exerts many actions in exercising humans by increasing the availability of metabolic substrates (glucose production in liver and protein degradation in muscle cells). ⁶ However, long-term elevation of cortisol, as has been observed in overtrained people, ⁷ has been implicated in immune suppression. ⁸ Therefore, it is important to determine the environmental and individual factors that contribute to the variations in exercise-induced cortisol secretion.

Although it is well known that intensity and duration of exercise influence the secretion of cortisol, ⁹ the effects of environmental factors such as heat and cold on exercise-induced cortisol changes are less understood. It was reported that concentrations of cortisol during exercise were higher in a hot environment.^1,10 Brenner et al ¹⁰ demonstrated that exercise in heat conditions (40°C) induced a larger cortisol response than exercise at room temperature did. It was also reported that cold conditions^11,12 or cold exposure ¹³ increased cortisol concentration. However, the cortisol response to exercise in cold conditions is unknown. Furthermore, past studies reported that long-term adaptation to cold reduced the activity of the autonomic nervous and endocrine systems.^11,14 There is, however, little evidence of the effect of either heat or cold acclimatization on the exercise-induced cortisol response.

The purpose of this study was to explore the effects of cold environmental exposure and cold acclimatization on exercise-induced salivary cortisol response. This study investigated the cortisol response to exercise under cold conditions (5°C) and room temperature conditions (20°C) and made a comparison between the cortisol response of participants who usually practice under cold conditions (short track skaters) and participants who usually practice under room temperature conditions (inline skaters).

Methods

Participants

The participants were male recreational skaters in either short track (n = 11) or inline skating (n = 11). All participants were recruited as volunteers from recreational club members. They had been skating 3 days per week for at least 1 year. They were all nonsmokers and did not have any health problems. The study protocol was approved by the Institutional Review Board of the Keimyung University. Informed consent was obtained from all study participants.

Procedure

In preliminary testing, a graded maximal exercise test was used for determination of

$\dot{\text{V}}{\text{o}}_2\text{max}$

under room temperature conditions. The participants started cycling at 60 rpm at 0.5 kilopond (kp) for 2 minutes on the bicycle ergometer. Thereafter, the intensity increased 0.5 kp every 2 minutes until the participants became exhausted. We estimated the values of

$\dot{\text{V}}{\text{o}}_2\text{max}$

using a standardized method in which a point when oxygen consumption will level off or plateau during a graded maximal exercise test was determined for each participant, as previously described. ¹⁵

In experimental testing, a randomized crossover study was conducted. All participants cycled for 60 minutes at the skaters’ predetermined kilopond of 65% of

$\dot{\text{V}}{\text{o}}_2\text{max}$

under cold (ambient temperature: 5 ± 1°C, relative humidity 41% ± 9%) and room temperature (ambient temperature: 21 ± 1°C, relative humidity 35% ± 5%) conditions. Two experimental trials (cold and room temperature conditions) were randomized in a counterbalanced order and separated by 1 week to ensure complete recovery. The experimental trials were also conducted between 2:00 and 8:00 pm to minimize the effects of circadian rhythm. Except for the last 48 hours before each trial, the participants completed their regular exercise program and usual daily activity without any interruption during the study period. They were also asked to refrain from eating meals within 2 hours before exercise testing. In each trial, the participants had 60 minutes of bed rest under cold or room temperature conditions before submaximal exercise testing. Thereafter, they performed 5-minute warm-up cycling at an intensity of 30% to 45% of

$\dot{\text{V}}{\text{o}}_2\text{max}$

, immediately followed by 60 minutes at the skaters’ predetermined kilopond of 65% of

$\dot{\text{V}}{\text{o}}_2\text{max}$

under cold or room temperature conditions. The participants had a 120-minute bed rest recovery phase at room temperature after both exercise bouts. All participants wore standardized clothes during experimental sessions. Fluid ingestions were not allowed during and after exercise. Saliva samples were collected 5 times for each condition: pre-exercise (S1) and postexercise at 1 minute (S2), 30 minutes (S3), 60 minutes (S4), and 120 minutes (S5). The participants were asked to collect their saliva using Salivette (Sersted, Vümbrecht, Germany). ¹⁶ Obtained saliva samples were separated from the cotton in a centrifugal machine at 3000 rpm and frozen at −80°C until the assay.

Cortisol Assay

The concentration of cortisol in saliva was determined by an enzyme immunoassay by using the Cortisol EIA Kit (Salimetrics LLC, Pennsylvania, USA). Cortisol assays were conducted according to the manual supplied by the manufacturer. The interassay and intra-assay variations that were reported by the manufacturer were 6.9% and 6.2%, respectively.

Statistic Analysis

Characteristics of the participants (age, height, weight, %fat,

$\dot{\text{V}}{\text{o}}_2\text{max}$

, heart rate, ratings of perceived exertion and workloads on

$\dot{\text{V}}{\text{o}}_2\text{max}$

, years of experience) were compared using independent t tests. Cortisol data were analyzed using a linear mixed model. ¹⁷ Analyses were performed using compound symmetry, treating experimental condition (cold, room temperature), sampling time (S1, S2, S3, S4, S5), and group (inline skater, short track skater) as fixed effects and individual intercept as random effect. Also, years of experience was included in the analyses and treated as a fixed effect because short track skaters had more years of experience than inline skaters did. Posthoc comparisons were carried out using the Bonferroni adjustment. In cases of detected significant interactions, linear mixed models were separately performed for each group. An α-error below 5% was considered statistically significant. Statistical calculations were performed with the SPSS statistical package 15.0 for Windows (SPSS Software Inc, Tokyo, Japan).

Results

The characteristics of the participants are shown in Table 1. No significant differences in each parameter were found between groups. The trial under cold conditions was not completed by 1 short track skater, and the trials under room temperature conditions were not completed by 2 inline skaters and 2 short track skaters.

Table 1

Characteristics of participants^*

The cortisol data for each condition are shown in Table 2. The linear mixed model revealed a significant main effect of sampling time (F [4.00/155.32] = 8.59, P < .01). Posthoc comparison indicated that cortisol values of S2 and S3 were higher than those of S1 and S5. Also, an experimental condition by group interaction was significant (F [1.00/164.79] = 11.10, P < .01). The interaction is illustrated in the Figure. The inline skaters exhibited higher cortisol values under cold conditions by an average of 80% compared with cortisol values under room temperature conditions (F [1.00/83.62] = 4.01, P < .05). However, short track skaters exhibited higher cortisol values under room temperature conditions by an average of 60% compared with cortisol values under cold conditions (F [1.00/81.41] = 9.92, P < .01).

Table 2

Means (standard deviations) of cortisol concentration (nmol/L) in each condition^*

Figure 1

Estimated cortisol values in each condition adjusting for years of experience. Bars are standard error of the mean. **P <.01 *P < .05.

Discussion

The purpose of this study was to explore the effect of the cold environmental exposure and cold acclimatization on exercise-induced salivary cortisol response. As shown in the Figure, the inline skaters exhibited a larger cortisol response under cold conditions than under room temperature conditions (7.6 nmol/L and 4.2 nmol/L, respectively). However, the short track skaters exhibited higher cortisol values under room temperature conditions than under cold conditions (8.7 nmol/L and 5.4 nmol/L, respectively). These results indicated that the effect of cold environmental exposure on exercise-induced cortisol response was not the same for inline and short track skaters and that the degree of cold acclimatization of the individuals determined the magnitude of the exercise-induced cortisol response. Past studies reported that cold exposure increased cortisol levels, and such increases were not observed in people who were acclimatized to cold. ^{10 –}12 This study demonstrated that these findings could be relevant to the case of exercise-induced cortisol response in a cold environment. The effect of cold exposure on exercise-induced cortisol response was not observed in people who were acclimatized to exercising in cold. Moreover, this study demonstrated that the opposite was true: the people who were acclimatized to exercising under cold but not under room temperature conditions exhibited a larger cortisol response under room temperature conditions. Taken together, these results indicated that the participants exhibited a smaller cortisol response under the conditions to which they were acclimatized and a larger cortisol response under the conditions to which they were not acclimatized. Viru et al ⁹ demonstrated a large interindividual variability in changes of cortisol concentration during exercise. Differences in the acclimatization to exercising under cold or room temperature conditions could be one of the factors that explain the large interindividual variability in cortisol concentration changes during exercise. Such acclimatizations are thought to be adaptive because a larger and frequent cortisol response to regular exercise might impair the body's function, such as immunity, and acclimatized and reduced cortisol secretion would be helpful for avoiding such damages.

These results should be interpreted in the context of some limitations. First, this study was carried out with a small sample. Second, in this study, the short track skaters had more years of experience than had the inline skaters. The differences in the years of experience could influence the results of this study, although the effects of the years of experience were statistically controlled. Third, the participants of cold conditions had recovery periods at room temperature. Temperature differences between cold and room temperature might affect the cortisol response. Fourth, this study could not deny that other factors contributed to the differences in cortisol response between groups and conditions. For example, there is a possibility that psychological stress, such as anxiety, caused by the unfamiliar conditions contributed to the differences in cortisol response. Also, daily diet intakes such as carbohydrate or hydration state could influence the cortisol response.^20,21

In conclusion, this study demonstrated that exercising under cold conditions caused a larger cortisol response in people who usually exercised under room temperature conditions and that the effects of the environmental conditions (cold, room temperature) on cortisol response were opposite in people who usually practice in cold conditions. Acclimatizations to exercising in either a cold or room temperature condition could contribute to the differences in the cortisol responses. Additional study could investigate the dynamic relationships between environmental factors and cortisol response to exercise.