During the 1980s a few studies reported a high prevalence of asthma in athletes taking part in the Olympics (7,33). These studies stimulated further research of athletes (29), which also included control groups (10,11,13,17,18,34). The new results agreed with those of the previous studies. An interpretation of these results is that physical training is a risk factor for asthma. However, because of a number of study limitations the role of physical exercise in the development of asthma is still poorly understood. For instance, the majority of studies were based upon athletes engaged mainly in endurance sports such as cross-country skiing and running (10,11,13,17,31). Only one study included athletes in all sports, but this study did not have a control group (12). Another study used self-selection and may be biased toward over-reporting of asthma and bronchial hyper responsiveness (BHR) (14). Furthermore, the sampling procedures of control groups were often methodologically insufficient.
Asthma is considered to be a complex disease for which development is determined by the interaction between host susceptibility and environmental exposures (3). If asthma represents the clinical manifestation of a number of different pathogenic processes, then asthma among top athletes may reflect a particular asthma phenotype. More specifically, exercise could be the main exposure to the development of asthma and an exercise-environment interaction could contribute. For instance, it has been suggested that the inflammatory process of the airways of skiers with asthma may differ in degree or nature from that seen in usual asthma (30). During the last decades sport has developed to be a full-time occupation. The occupational exposures can be extreme, depending on the type, intensity, and duration of exercise associated with the specific type of sport. Thus, depending on the type of sport, athletes may be differentially exposed to risk factors. Asthma in athletes (athletes-asthma) may be considered as occupational asthma. Little is known about the occurrence of asthma in athletes compared with that in the general population or about the differential risks for asthma conferred by different types of sports.
The main objectives of the present study were to estimate the prevalence of self-reports of asthma and wheezing among Norwegian elite athletes compared with that in the general population and to estimate the associations between asthma and types of sports, exercise, and team level.
MATERIALS AND METHODS
The present study is based on a cross-sectional survey of all Norwegian elite athletes on the national junior and senior teams in 1997 (N = 1620) and a survey of a random sample of the general male and female population (N = 1680). The general population was matched to the athletes for age and sex and was obtained through the Norwegian Population Register.
The data were collected as part of a study of risk factors for the development of eating disorders. Among the items were several questions about health. The results presented here are based on the items for asthma and respiratory symptoms derived from the questions of the International Study of Asthma and Allergies in Childhood (ISAAC) (1). Items were also included to assess in detail the history of participation in sports, type of sport and exercise, and team level.
Variables.
The main outcome were self-reports of current asthma, which was measured using the following question: “In the last 12 months, have you ever had asthma?” Other outcomes were self-reports of wheezing during exercise and at rest. These included: “Have you had wheezing or whistling in the chest during exercise in the last 12 months?” and “In the last 12 months, have you had wheezing or whistling in the chest when you do not exercise?” A question inquiring about use of asthma medication was also included.
The main exposure variable was being a top athlete on the national junior and senior team. Other exposure variables were type of sports, hours of training per week, and years of specialization. The athletes were grouped into seven categories based on training and performance characteristics of the sports. The categories were technical, esthetics, weight-class, weight and motor, strength, ball-team, and endurance sports. Training hours reflect the approximate number of hours spent training per week throughout the year during the period of training and were categorized into three levels: 10 h, 11–20 h, and >20 h. Years of specialization refer to the number of years actively taking part in the main type of sport. Scores for this variable were classified into three categories: 5 yr, 6–10 yr, and >10 yr. Team level was categorized into junior, senior recruits, and senior team plus other teams.
Statistical analysis.
The prevalence of self-reported asthma and wheezing were compared between athletes and the general population using crude (c) and adjusted (a) odds ratio (OR) with 95% confidence intervals (95% CI). The aOR was estimated using logistic regression to measure the association between the exposure variables and the outcomes adjusting for sex, age, height, and weight (Model I). This model was expanded (Model II) by classifying the elite athletes according to the types of sports in which they engaged. All estimates used the general population as the reference category. Next, differential risks associated with the specific sports were examined by analyzing only the data from the elite athletes and using technical sports as the reference category. Technical sports were chosen as the reference category because they require the lowest level of endurance among the sports studied here. Crude and aOR was estimated for the associations between current asthma, sex, type of sports, and training hours per week. The aOR was adjusted for age, years of specialization, and team level.
The response rate was 77.7% (1259/1620) among athletes and 71.6% (1203/1680) in the general population. The present results are based on analyses for which there was complete data for all variables included in the multivariate models. The resulting sample included 86% (1082/1259) of the athletes and 86.3% (1038/1203) of the general population.
RESULTS
The mean age was 22.4 yr (95% CI 22.1–22.7) in athletes and 24.9 yr (95% CI 24.5–25.3) in the general population. The respective mean values for heights and weights were 1.77 m (95% CI 1.76–1.77) and 71.4 kg (95% CI 70.7–72.1) in athletes, and 1.74 m (95% CI 1.74–1.75) and 71.1 kg (95% CI 70.4–71.9) in the general population. Among the athletes, 55.9% were males and 44.1% females, and in the general population sample 51% were males and 49% were females.
The prevalence of asthma was significantly greater in athletes (10.0%) compared with that in the general population (6.9%) (Table 1). Otherwise, there was a tendency for the use of asthma medication and reports of wheezing during exercise to be more common among athletes. In contrast, reports of wheezing at rest were less common among the athletes.
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Table 1: The prevalence of self-reports of current asthma, use of asthma medication, and wheezing in Norwegian top athletes (N = 1082) and the general population (N = 1038).
Table 2 lists the reported prevalence of current asthma and wheezing in relation to type of sport and exercise level in Norwegian elite athletes. The prevalence of asthma was higher among female (12.6%) than male athletes (7.9%), cOR = 1.7 (1.1–2.5). Female athletes (20.1%) also reported more wheezing during exercise than did male athletes (15.8%), cOR = 1.3 (1.0–1.8). The prevalence of asthma was highest for strength and endurance sports. The prevalence of reports of wheezing during exercise was also highest for strength and endurance sports, whereas for reports of wheezing at rest the prevalence was highest among athletes in esthetics and weight class sports. Reports of asthma increased with increasing number of training hours per week. Furthermore, a larger proportion of female (10.5%) than male (8.4%) athletes used asthma medication (not in table). Use of asthma medication was also more common among athletes in strength (14.3%) and endurance sports (14.1%) compared with those in technical sports (2.7%).
Table 2: Self-reports of current asthma and wheezing in relation to sex and training characteristics Norwegian elite athletes (N = 1082).
In the general population there were no sex differences for self-reports of current asthma; the prevalences were 6.6% in males and 7.3% in females (not in table). The prevalence of wheezing during exercise tended to be lower among females (14.1%) than among males (18.0%), cOR = 0.7 (0.5–1.0), whereas the prevalence of wheezing at rest was similar for males (9.7%) and females (8.9%). Seventy percent (750/1038) of the general population reported participation in regular physical activity. The majority, about 90%, of those who exercised, exercised 5 h or less per week.
After adjusting for age, height, and weight, the risk of asthma remained greater among athletes than in the general population (Table 3, Model I). In contrast, athletes tended to report fewer symptoms at rest (aOR 0.7 (0.5–1.0)). Next, analyses classifying the elite athletes according to the type of sports they engaged in and comparing them with the general population revealed that the highest risk for asthma was among athletes participating in sports requiring strength (aOR = 3.5 (1.6–7.6)) and endurance (aOR = 2.2 (1.4–3.5)) (Table 3, Model II). There was no consistent association between reports of wheezing either during exercise or at rest and any types of sports.
Table 3: Adjusted odds ratio (aOR) for the association between self-reports of current asthma and wheezing and being an elite athlete (Model I) and specific types of sports (Model II) using males and the general population as the reference category.
Stratified analyses based only upon data on athletes and adjusting for potential confounding factors revealed that asthma remained more common among female than male athletes (Table 4). The association between asthma and type of sports was strongest for strength and endurance sports. Extreme training, defined as more than 20 h·wk−1, also tended to be associated with asthma when compared with levels of training less than 10 h·wk−1 (aOR 1.9 (1.0–4.1)). Estimates of the association between training hours per week and asthma within each type of sport revealed a fairly linear increase in the prevalence of asthma associated with the number of hours of training per week, particularly among athletes in endurance sports (N = 213).
Table 4: Crude (c) and adjusted odds ratio (aOR) for the association between current asthma, sex, type of sports, and training hours per week adjusting for age, years of specialization, and team level in Norwegian elite athletes (N = 1082).
DISCUSSION
We studied the risk for asthma and wheezing among elite athletes and a representative sample from the general population. The prevalence of asthma is greater among athletes compared with that in the general population and is higher in female than in male athletes. Furthermore, asthma is more frequent in strength and endurance sports compared with other types of sports. The prevalence of asthma also increases with increasing hours of training per week. Self-reports of wheezing are not consistently associated with the items of sports.
Our results support those from previous studies reporting an increased prevalence of asthma among athletes compared with that in the general population (10,11,13,17,18,34). One explanation of our findings is that there is a real increase in the morbidity among athletes compared with that in the general population. The finding that the number of hours of training per week is associated with asthma supports the idea that the amount of exercise is associated with the risk of athletes’ asthma.
Exercise is known to induce a cascade of physiological responses, which vary depending on the type, intensity, and duration of exercise (20,36). Several theories support the concept that asthma is more common among athletes than in the general population. First, it is suggested that the occurrence of asthma among athletes who participate in endurance sports results from the repeated overstimulation of the mechanisms that protect against dry air-induced mucosal injury (8). If such injury occurs, it may lead to chronic inflammation. Repeated exposures to dry air may then contribute to the pathogenesis of athletes-asthma (8). Second, periods of excessive amounts of exercise may confer a greater than normal risk for upper respiratory tract infection (23). Extreme training could alter host susceptibility such that individuals become more vulnerable to develop infections (21,22,25), and recurrent infections in addition to exercise may contribute to the development of athletes-asthma. Several epidemiological studies suggest that athletes engaged in marathon type of events and/or heavy training are at increased risk of upper respiratory tract infection (15,23,24). Third, high levels of exercise also increase the exposures of the airways to environmental factors such as air pollutants that could increase the risk of developing asthma. This theory is supported by studies showing an increased bronchial responsiveness (BR) among swimmers during intense training (5) and in cross-country skiers during the winter season (10). Increased airway exposure to chlorine and cold air is proposed as an explanatory factor for these findings (6,10), which suggests that an exercise-environmental interaction contributes to the development of athletes’ asthma.
Our study did not demonstrate any long-term effect of exercise since the number of years actively taking part in sports was not associated with any of the outcomes. However, long-term effects of intensive exercise are difficult to explore with a cross-sectional study design. A follow-up study of former athletes in Finland also did not support the idea that exercise effects the occurrence of respiratory diseases later in life (16). However, the athletes included in that study represented Finland at least once in international competitions between 1920 and 1965, and their training regimes and careers are not representative of the athletes of the 1990s. Today, sport is a full-time occupation with high levels of physical training throughout the year during several years. To resolve the question of long-term effect of current exercise on later asthma requires follow-up studies following individuals from childhood through the years when they become athletes.
The difference in asthma prevalence by sex among athletes is consistent with other evidence suggesting that females are more likely than men to develop lung function deficit in adulthood (35). The finding of a high prevalence of asthma among athletes in strength sports needs further investigation, and we note that this group is small in the present study. Self-reports of wheezing were not consistently associated with any types of sports. The reason for this is unknown. It is likely that athletes report more symptoms during exercise than at rest. Exercise makes them aware of their symptoms. Wheezing characterizes, however, respiratory disorders other than asthma (28,32). It may be difficult to distinguish between reports of wheezing as a respiratory disorder and wheezing that reflects general breathlessness. We have tried to address this problem by assessing wheezing at rest and wheezing during exercise. Our results illustrate that epidemiological studies should investigate in more detail the association between respiratory illness and different types of self-reports of wheezing in different populations.
Another explanation of our findings may be that the results are influenced by different types of bias. One bias may arise if individuals with asthma were more likely to answer our questionnaire. This seems unlikely because the asthma items were part of a more extensive questionnaire. To check for selection bias within the data, analyses were conducted to compare those who had missing data with those who had complete questionnaires. The distribution of type of sports and hours of training per week were alike in these groups. An inaccurate recall could also effect the results if athletes and the general population differentially report asthma and wheezing. An increased awareness of respiratory illnesses among athletes may result in a higher prevalence of reported asthma among athletes compared with that in the general population. Furthermore, asthma may be underdiagnosed in the general population. However, several population-based studies report good correspondence between questionnaire and clinical assessment (4,19). The prevalence of asthma in the general population here was also similar to that of other Norwegian studies (2,9,26,27). We can, however, not exclude that the results are influenced by the fact that athletes have an increased awareness of respiratory illnesses, which partly may reflect an increased perception of dyspnea and not increased morbidity alone. The finding that a smaller proportion of the athletes use asthma medication than the proportion that report asthma reflect also that the prevalence of asthma among athletes may be over reported.
The present study does not include any objective measures of specific environmental risk factors related to each type of sport, and future studies should incorporate measures of exercise-specific environmental interactions to assess which factors may be important for the development of asthma. Longitudinal studies of the new generation of athletes are also needed to investigate possible long-term effects of elite sports on the development of athletes’ asthma. Studies should include physiological measures such as airflow variability, bronchial hyper responsiveness and atopy, or other possible markers of asthma and related phenotypes as health outcomes. Whether intense exercise triggers the same physiological chain of events that eventuates in other types of asthma is unknown.
In summary, our results indicate that asthma is more common among athletes than in the general population and may define a subgroup of asthma cases for whom etiology is related to extensive exercise.
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