The number of investigations examining respiratory tract infections in athletes has increased in the last decade (4,7,21–22). One model describing the relationship between exercise and upper respiratory tract infections (URTI) has been proposed, in the form of a “J” curve (23). This model suggests that moderate exercise has lower risk of infection, but when athletes engage in heavy competitive matches and intensified training periods, the risk of infection increases greatly leading to diminished immunosurveillance. Recently, Martin et al. (15) provided evidence that supports an increase in susceptibility to URTI with chronic stressors in mice and this is consistent with the observational data on humans (4,22).
Some studies suggest an inverse relationship between salivary immunoglobulin A (IgA) and URTI (7,22), so that IgA monitoring may be used for assessing athlete risk status for illness (21). Recently, Moreira et al. (19) reported significant relationships between stress tolerance, training loads, URTI illness, and salivary IgA in elite basketball players during 4 weeks of training. Furthermore, in the second week, greater training loads and increased reports of URTI were accompanied by increases in cortisol and decreases in IgA. Cunniffe et al. (4) also provided evidence that salivary IgA monitoring with salivary lysozyme may help to assess URTI risk status in elite rugby players. The authors proposed that the stress-induced increases in cortisol might contribute to reductions in mucosal immunity, which could predispose players to increased risk of illness.
There is evidence that athletes undergo intensified stress when exposed to repeated matches or races (14). Libicz et al. (14) found that daily repeated triathlon races had a cumulative effect on the resting levels of salivary IgA. Moreover, other studies (1,17,24) have demonstrated that multiple stressors such as sleep deprivation, dietary, and psychological challenges can amplify exercise-induced alterations in mucosal immunity. These findings reinforce the idea that exposure to elevated psychological stressors can lower salivary IgA levels (5). Intuitively, sporting situations that combine high levels of physiological and psychological stress are more likely to reduce mucosal immunity, thereby exposing athletes to a greater risk of infection. Thus, the regular measurement of stress-related markers, in conjunction with training load and URTI occurrences, would be useful for monitoring and managing athletes in these situations.
It has been suggested that the younger the individual, the less effective the immune defense (2). However, few studies have investigated the association between mucosal immunity and physical activity in youth. Klentrou et al. (13) did find that those adolescents who spent less time in sporting activities reported more URTI than their more active counterparts. To complicate matters, Gleeson et al. (8) found significant fluctuations in the IgA levels of youth, peaking at 5 years of age and remaining relatively stable until 9 years of age. Despite adult evidence supporting a link between depressed salivary IgA levels and risk of URTI among athletes, no strong evidence is available in adolescent athletes. Timmons (26) pointed that more work is required to investigate changes in mucosal immunity in adolescents and its association with risk of URTI.
Young elite athletes are frequently involved in competitions involving multiple matches within a few days. In addition, these athletes are often under regimented daily schedules and sharing accommodation for extended periods of time, which adds to the psychological stressors imposed. Further, it is not uncommon for young athletes to be under constant pressure to perform, especially if a competition represents a major opportunity for becoming a professional athlete. This scenario is typical of young athletes in many sports and presents challenges for researchers, trainers, and coaches wanting to maximize athlete performance, while minimizing athlete injuries and illnesses. Therefore, this study examined the effect of a 20-day period of competition on salivary cortisol, mucosal immunity, and URTI in young male soccer players. It was hypothesized that this situation would lead to temporal increases in cortisol levels and impairment in mucosal immune function, thereby leading to greater risk for URTI.
Experimental Approach to the Problem
Research on adults suggests that intensive periods of training and competition can increase cortisol levels, leading to impairment in mucosal immune function and greater risk for URTI. However, there is a lack of research describing these effects and interactions in elite young athletes. A group of young male soccer players were monitored for various endocrine and immune parameters across a 20-day period during the main Brazilian soccer championship for under-19 players. The monitoring of young athletes responses during an actual competition improved the internal validity of the study. Saliva samples were collected in the morning of each match and analyzed for cortisol and IgA. The players were also tested for symptoms of URTI, as classified by Fahlman and Engels (7) and Moreira et al. (17). To assess match effort, a rating of perceived exertion (RPE) was also obtained from the soccer players. The outcome measures were chosen based on their ease of implementation, subject compliance, and practicalities within the sporting environment.
Twenty young male soccer players were initially recruited for this study, but only 14 were retained for analysis because they participated in all 7 matches during the under-19 soccer championship, either as starters or nonstarters (mean ± SD: age, 18.5 ± 0.4 years; height, 177.7 ± 4 cm; body mass, 72.4 ± 6 kg). The players volunteering for this study belonged to the team who eventually become the soccer champions, winning all the matches played. Before the study commenced, all subjects had the protocols explained to them and gave written informed consent. The experimental procedures received local ethics committee approval.
Training and Competition Procedures
During the 20-day experimental period, the study population played 1 soccer match every 3 days (either as starters or nonstarters) with 2 days of recovery from the previous match played. The games were played under full international rules with 2 × 45-minute halves, and a 15-minute period at halftime. In the recovery period, 2 training sessions were implemented, but only 1 per day to facilitate the physical and psychological recovery of players. These training sessions consisted of a general team warm-up, specific soccer drills, and technical and tactical exercises performed at moderate intensity. The team also completed a tactical session before, and a debriefing session after, each soccer game. A standard dietary plan was developed for the squad during the scheduled meal breaks. However, the dietary plan was not provided for each individual within the squad because of logistical and resource constraints. Water and sports drinks were also provided before, during, and after all the training sessions and games to help maintain hydration and energy levels. There were no reports of severe injuries during the competition.
Monitoring of Upper Respiratory Tract Infections
Subjects were required to complete a weekly log in which they documented any signs or symptoms of URTI including coughing, a runny nose, and nasal congestion, and the number of days that the symptoms occurred. They were also instructed to code the severity of the infection from 1 to 3, where a value of 1 indicated that infection had some impact on their daily activity and a value of 3 indicated that the infection led to a significant decrease in daily activities (7). To be classified as a URTI, the infection had to last 2 or more days, and all 3 symptoms (coughs, a runny nose, and nasal congestion) had to be present for the entire 2-day period. If the subjects and researchers were uncertain as to the exact nature of the illness, the team physician made the final diagnosis. This method of classifying URTI is consistent with that proposed by Fahlman and Engels (7) and used by Moreira et al. (17).
Salivary Hormone and Immune Assessments
Whole saliva samples were collected at rest, in the morning before each soccer match (7 matches separated by 3 days) and at the same time of day (09:00 AM) to account for diurnal variation (6). Subjects abstained from food and caffeine products for at least 2 hours before saliva collection. Initially, the subjects were required to rinse out their mouths with distilled water to clean the oral cavity. In a seated position and with the head tilted slightly forward, unstimulated saliva samples were then collected by passive drool into sterile 15-ml centrifuge tubes over a 5-minute period. The saliva samples were stored at −80°C until assayed for cortisol and IgA.
After thawing and centrifugation, the samples were tested in duplicate for IgA concentrations using an enzyme-linked immunosorbent assay (ELISA, IgA EIA kit; ALPCO Diagnostics, Salem, MA, USA), in accordance with previous work (16,17). The salivary IgA secretion rate (IgArate; micrograms per minute) was also calculated by multiplying the absolute IgA concentration by salivary flow rate (milliliters per minute). Salivary flow rate was determined by dividing the volume of saliva collected by the duration of the collection period. Salivary cortisol concentrations were also measured in duplicate using an ELISA (Cortisol–Direct Salivary EIA kit; ALPCO Diagnostics), in accordance with previous work (18). The average intra-assay coefficient of variation for the cortisol and IgA assays were both less than 8%.
Match Ratings of Perceived Exertion
To quantify the intensity of the soccer matches played, a RPE was recorded using Borg's 6–20 scale (6 being the lowest rating, 20 being the highest rating) (11), approximately 30 minutes after the completion of each game. Each player was simply asked, “How would you rate your level of exertion from this match?” and a chart was shown that outlined the full RPE scale with explanations for each level. These details were recorded by the same investigator on all occasions. The players were familiarized with this procedure during their regular training sessions and previous matches.
The distribution of the data was analyzed by the Shapiro-Wilk test. The Mauchly's test of sphericity was performed to test the null hypothesis that the error covariance matrix of the orthonormalized transformed dependent variables was proportional to an identity matrix. An analysis of variance (ANOVA) with repeated measures was used to compare the 7 moments (matches 1, 2, 3, 4, 5, 6, and 7) for each dependent variable (salivary cortisol, salivary IgA, and match RPE). The Tukey's post hoc test was used when necessary. In the case of violation of the assumption of sphericity, the significance was established by using the Greenhouse-Geisser correction. The level of significance was set at p ≤ 0.05.
Upper respiratory tract infection occurrences are reported as the percentage of subjects reporting symptoms (i.e., prevalence), and the total number of days for each period with URTI symptoms (i.e., incidence). The periods of consideration started from the first saliva collection to the next and so forth (i.e., matches 1–2, 2–3, 3–4, 4–5, 5–6, and 6–7). Therefore, 6 periods were retained for analyses. The URTI were also tested using ANOVA and Tukey's post hoc comparisons. Spearman rank correlation coefficients were used to examine the relationships between the variances in URTI and salivary IgA across each period.
A significant main effect was identified for match RPE over time (p < 0.01). As seen in Figure 1, post hoc testing revealed a significant increase in player RPE in the soccer matches 4 (180%), 5 (183%), 6 (185%), and 7 (202%), when compared with match 1 (p < 0.05).
Repeated analysis indicated that the resting concentrations of salivary cortisol did not change significantly before any match played across the experimental period (main effect p > 0.05, Figure 2).
Significant main effects were identified for the reported changes in URTI and salivary IgA (p < 0.01). Post hoc testing highlighted significant (p < 0.05) increases in URTI across period 2 (between matches 2 and 3) and period 6 (between matches 6 and 7), as compared with the first period (between matches 1 and 2). Significant (p < 0.05) relative decrements in salivary IgA levels were seen at the same time points (−15% for period 2 and −12% for period 6) (Figure 3). Overall, 67% of the subjects tested experienced at least 1 URTI over the investigated period. The percentage of subjects reporting URTI symptoms varied throughout, ranging from 17 to 35% between each soccer match.
On an individual level, significant correlations were seen between the reported incidence of URTI and the decreases in salivary IgA concentrations during match 2 (r = −0.60; p < 0.05) and match 6 (r = −0.65; p < 0.05). There were no significant correlations between these variables in the other matches played.
To our knowledge, this is the first investigation that has mapped the baseline dynamics of salivary cortisol, IgA, and URTI occurrences in elite young soccer players across a major competition. The main finding was the negative trend between salivary IgA concentrations (which decreased) and the incidence of URTI episodes (which increased) in matches 2 and 6. The correlational analyses of individual data supported these results. Interestingly, no significant changes in salivary cortisol concentrations were seen across the competition.
The RPE is a well-accepted marker of internal training load (11). As the soccer championship progressed, match RPE also increased, being significantly higher in the last 4 matches played. These RPE responses are consistent with another investigation (10), which demonstrated an increase in athlete anxiety levels and psychological stress during competitive matches. It seems reasonable to suggest that as competition progresses with more difficult games and opponents, along with greater psychological stress and some level of accumulative fatigue, that athlete RPE values would increase in a linear manner. Psychological state has also been shown to influence the perceived ratings of athletes to the internal load of exercise (12,20) and greater emotional stress seems to be inherent within the competitive environment (29). Nevertheless, the similarities between the changes in RPE scores and each soccer match (during the latter stages of the competition) reinforces the idea that, during short-term competition, young athletes are subjected to increasing levels of perceived stress over time.
Cortisol responsiveness presents a known hallmark of the adaptive mechanism of the human hypothalamic-pituitary-adrenal (HPA) axis to stress (28,30). Despite the observed increases in perceived match effort and an expected increase in player stress levels as the championship progressed, resting salivary cortisol levels remained unchanged throughout the study. Still, the baseline cortisol responses of athletes across competition are unclear with reports of changes (4) or no changes (25). It is possible that the cortisol levels of subjects were already elevated before study commencement and, thus, less responsive to the competition situation. The lack of preexisting data to establish this is a limitation of the present study, as is the absence of cortisol measures taken during different times of day (e.g., pre- and postmatch). Individual responsiveness to the stressors of competitive sport offers another possible explanation for some of the variability reported.
Our results indicate that the decrements in salivary IgA in matches 2 and 6 could also explain the concomitant increase in URTI at the same time points. The congruence between these markers and the correlational evidence strongly supports a link between illness and mucosal immunity in young elite athletes in a valid ecological setting. These results are supported by other research. For example, Neville et al. (22) showed a significant reduction in salivary IgA 3 weeks before changes in URTI episodes, before returning to baseline 2 weeks after an URTI. Similarly, Carins and Booth (1) demonstrated that dietary restriction, body mass loss, and URTI were all negatively associated with salivary IgA levels and concluded that this measure of mucosal immunity has potential as a surrogate marker of stress encountered during intensive training.
Some studies have not verified the observed associations between salivary IgA and URTI. It was reported by Tiollier et al. (27) that despite increments in the frequency of URTI, after a 3-week period of military training followed by an intensive 5-day combat course, salivary IgA concentrations remained unchanged. Moreira et al. (17) also failed to find an association between the temporal changes in salivary IgA and URTI in their assessment of basketball players across a 17-day period. The authors of the latter study pointed out that, possibly, a more frequent saliva-sampling protocol would be required to establish a relationship between salivary IgA and URTI, which is in accordance with other research findings (9). Earlier work by Costa et al. (3) found that a high-carbohydrate diet, during a 6-day period of increased exercise workload, had a positive effect on salivary IgA levels in endurance athletes. Despite the absence of UTRI reporting in this study, the findings reported by Costa et al. (3) suggest that athlete susceptibility to respiratory illness may be influenced by the nutritional habits of a team before, or during, a competitive event.
A benefit of the present study is the frequent saliva-sampling protocol (baseline measures) used. The employment of this protocol within the real-world context might help to explain the equivocal findings concerning salivary IgA behavior and its association with URTI. In fact, most of the studies that did not demonstrate this association employed a less frequent sampling protocol (17,27). We also recognize the value of saliva as a noninvasive media for monitoring or testing various biological components (e.g., steroid hormones, immune markers, stress proteins), especially in younger populations where blood or urine collection is often undesirable, difficult, or unethical.
Despite adult evidence supporting a link between the depression in salivary IgA levels and the risk of URTI among athletes, to date, no strong evidence has been presented to clearly identify adolescent-adult differences in these responses. The results of the present study do suggest that adult and adolescent athletes may exhibit similar trends in their salivary IgA responses, particularly in competition settings involving multiple matches or trials. However, evidence directly comparing younger and older athletes within the same sport is lacking and so this issue should be addressed using cross-sectional and experimental studies. As highlighted by Timmons (26), more work is required to investigate changes in mucosal immunity in adolescents and its association with risk of URTI, especially in sporting situations where these young athletes are repeatedly exposed to high levels of physical and psychological stress.
In conclusion, we demonstrated that decrements in mucosal immunity, as measured by salivary IgA concentrations, during a short-term soccer competition might lead to greater incidence of URTI in young male soccer players. It may be speculated that a combination of physiological and psychological stressors imposed by competition and training (in a relatively short timeframe) are major factors contributing to these responses.
The current findings suggest that the monitoring of salivary IgA may provide a useful and noninvasive approach for predicting URTI occurrences in young athletes during short-term competitions, especially if rapid measurements are made. With this knowledge, strength and conditioning coaches can potentially manipulate daily training loads to attenuate the physical stressors imposed upon the athletes and thereby decrease the likelihood of URTI occurring. Ideally, data should be collated across different training phases (both with and without competition) to first establish normative values and trends for individual players. This research also supports the need for a more frequent saliva-sampling protocol to investigate the real association between salivary IgA and URTI in short-term competitions involving young athletes.
We would like to thank the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP grant 2008/10404-3) for funding this research. We also acknowledge all the soccer players and research support staff involved in this study for their input.
Author Contributions: A. L. Mortatti and A. Moreira contributed equally to this article.
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