Immune Response in Athletes
It has been demonstrated that exercise training in previously sedentary individuals has beneficial effects in terms of reducing the incidence of upper respiratory tract infections (URTI) (Nieman et al. 1990; Nieman, Hensen, et al. 1993). Nieman, Hensen, and colleagues (1993) reported a 50% reduction in URTI in women exercising 5 days per week compared with sedentary age-matched controls (figure 5.4). Exercise also enhances the perception of being healthy. A survey of 750 masters athletes (ranging in age from 40 to 81 years) showed that these active individuals considered themselves less vulnerable to viral infection than their sedentary counterparts (Shephard et al. 1995). An additional survey reported that 90% of nonelite runners who had been running marathons for an average of 12 years agreed with the statement that they "rarely got sick" (Nieman 2000).
Effects of moderate exercise training on number of days with URTI in previously sedentary women.
Data from Nieman et al. 1990.
In contrast to the proposed health benefits of training, other reports have suggested that athletes may be at a greater risk for URTI than other populations. Nieman and coauthors (1990) reported that 12.9% of the runners in a marathon experienced URTI symptoms during the week after the race in comparison with 2.2% of the control runners (runners who did not run the marathon). Similarly, Peters (1990) reported a 28.7% incidence of URTI in runners after a 35 mi (56K) race compared with a 12.9% incidence rate in controls. The higher incidence of URTI appears to occur during the 2 weeks after the event. The elevated risk for URTI after acute athletic events may depend on the distance of the race. During the weeks after races of 3 mi (5K), 6 mi (10K), and 13 mi (21K), Nieman, Johanssen, and Lee (1989) were unable to see any increased prevalence in URTI compared with the week before the race. In that same study, the researchers also noted no differences in the incidence of URTI in runners who ran an average of 26 mi (42 km) per week in comparison with runners averaging 7.5 mi (12 km) per week.
The greater incidence of URTI appears to occur in highly stressful events such as marathons or ultramarathons. However, investigations have also suggested that intense prolonged training increases the risk of URTI. Linde (1987) reported that a greater incidence of URTI was seen in elite orienteers (2.5 episodes per subject) compared with age-matched controls (1.7 episodes per subject) during a year of training. Other epidemiological studies have also implied that intense, prolonged training is associated with an increased risk for URTI (Heath, Macera, and Nieman 1992; Nieman et al. 1994). The increased risk for URTI appears to be limited to endurance athletes. No known studies have reported an increase in URTIs in strength or power athletes. However, the amount of research on this athletic population is considerably less than that on endurance athletes.
Evidence suggests that exercise training is beneficial for lowering the risk of infection. However, intense prolonged training in elite-level athletes performing long-distance endurance events (marathons or ultramarathons) may cause an elevated susceptibility to illness. Athletes in general do not appear to be immunosuppressed. The only illness to which they appear more susceptible is URTI (Mackinnon 1999). In the last few years, many studies cited in this chapter have indicated that several aspects of the immune system are affected by prolonged periods of intense training (reduced leukocyte counts, lower immunoglobulin concentrations, suppression of antimicrobial activity).
It has been proposed that athletes become more susceptible to infection in the days after intense exercise (Pedersen and Ullam 1994). If the athlete begins a training session before full recovery from the previous exercise session (during a period of potential immunosuppression), the risk of infection increases because the athlete is training at a lower baseline. If this pattern of beginning exercise sessions before complete recovery continues, a cumulative suppression of some aspects of immune function may be seen, causing a further elevation in the risk for infection.
Cox and colleagues compared the immune and inflammatory response in healthy and illness-prone runners (Cox et al. 2007, 2009). Male runners who had experienced no more than two episodes of URTI per year were classified as healthy, and runners who had experienced four or more episodes per year were classified as illness prone. Each subject was assessed by incremental endurance tests in the laboratory. In one test the runners performed a 30 min run at a speed corresponding to 65% of their V\od\O2max; in another they performed a 60 min run at the same intensity; and in a third, they performed six 3 min interval runs at a speed corresponding to 90% of their V\od\O2max. Resting and postexercise CRP concentrations were within normal ranges, and no differences were observed between the groups in any of the exercise protocols (Cox et al. 2009). However, the cytokine responses to the running protocols differed between the healthy and illness-prone athletes. Resting IL-8, IL-10, and IL-1ra concentrations were 19% to 38% lower in illness-prone subjects; postexercise IL-10 concentrations were 13% to 20% lower, and IL-1ra concentrations were 10% to 20% lower. In contrast,Â IL-6Â elevations were 84% to 185% higher in illness-prone subjects. The illness-prone distance runners showed evidence suggestive of impaired inflammatory regulation in the hours after exercise that may account for the greater frequency of upper respiratory symptoms experienced.