It is not uncommon to feel the symptoms of an upper respiratory tract infection (URTI), especially a sore throat, after periods of exercise. This is especially true for high-performance athletes. While I am far from a high-performance athlete, I often feel sore throat and malaise when I am in periods of peak training. This, for me in the Chicago area, is usually in the summer during our short, but wonderful, triathlon season and not during our usual cold and flu season. When I experience these symptoms, I usually ascribe them to pre-race nerves. But, what is the real evidence for a relationship between URTIs, or related symptoms, and exercise? Is performance in athletic events affected?
The first consideration when exploring the relationship between URTIs and exercise is whether symptoms of URTIs (upper-respiratory symptoms or URS) truly represent infections. As it turns out, there are only limited studies on this topic. Furthermore, physicians, like me, may not be very accurate in differentiating between infections and other causes of URS. The diagnosis of URTI is often based on clinical history and physical exam, so true laboratory confirmation of an infection may be the exception rather than the rule. However, the few studies that have pursued such laboratory testing have found that about 5% of URS in high-performance athletes arise from bacterial infections, while viral infections appear to account for 30-40%. The pathogens that have been identified in these studies are typical of URTIs in the general population, and include rhinovirus, influenzae, parainfluenzae, adenovirus, coronovirus, metapneumovirus, epstein barr virus, mycoplasma pneumoniae, streptococus pneumonia, and staphylococus pyogenes. In further studies, it has appeared that, in athletes with URS, infections account for about one third of cases, non-infectious medical causes account for another third (these include treatable conditions such as asthma, allergy, autoimmune disorders, vocal cord dysfunction, and unresolved non-respiratory infections), and an “unknown etiology” accounts for the final third. Unproven, speculative, ideas about the causes of URS for athletes with an “unknown etiology” include drying of the airways, psychological impacts of exercise, and the migration to the airways of inflammatory cytokines that had been generated during damage to muscles.
There have been numerous studies examining the association between changes in immune parameters and the risk of URTIs in athletic and non-active populations. The only immune measures that have shown a consistent relationship between URTIs and level of exercise has been the concentration and rate of secretion of salivary IgA. Antibodies are divided into different subgroups, including IgM, IgG, IgE, and IgA. The role of IgA is primarily to provide a first line of defense on internal surfaces of the body (such as the mouth, lungs, and intestines) that come into contact with pathogens. It has been demonstrated that prolonged high-intensity exercise can reduce levels of salivary IgA whereas moderate exercise can lead to increases in salivary IgA. This appears to lead to increased susceptibility to URTIs in high-performance endurance athletes undertaking intensive training, but reduced susceptibility to URTIs in people undertaking moderate regular exercise. However, there is very little evidence to support the commonly-held belief that elite athletes experience more URTIs overall. In fact, most studies have shown that the rates of URTIs are similar in elite athletes to the general population. Also of interest is that episodes of URS in elite athletes do not follow the usual seasonal patterns of URTI, but, rather, occur during or around competitions. For swimmers, URS occur more frequently during high intensity training and the taper before competitions, but in long distance running, URS appear to occur more frequently after competitions. The limited data about the effects of URS on athletic performance suggests that there can be decrements in performance in athletes with URS.
There appears to be a subset of elite athletes experiencing recurrent URS, associated with long-term fatigue and poor performance. A high percentage of these individuals have been found to have primary infection with herpes group viruses (such as cytomegalovirus and Epstein-Barr virus (EBV)) or to have reactivation of EBV. Reactivation of EBV has also been demonstrated in endurance athletes with URS and this may help explain why many athletes with URS have a short duration of symptoms (since symptoms associated with reactivation of virus would be of shorter duration than symptoms associated with an initial, or primary, infection). However, in a study examining the prophylactic use of an antiviral treatment in elite runners, the expression of EBV was reduced (in other words, the virus appeared to be suppressed), but it was not effective in reducing the frequency of episodes of URS. This, once more, points to the importance of considering non-infectious cases of URS.
Cytokines, which are proteins that act as regulators all over the body, are also involved in the relationship between exercise and URS. They are likely to play an important role in modulating post-exercise changes in immune function, which can increase the risk of infection or increase symptoms of inflammation (such as URS). In athletes prone to frequent URS, there has been shown to be an underlying genetic predisposition to increased expression of the pro-inflammatory cytokine, interleukin-6. This finding suggests that URS in these athletes may be due to a non-infection-driven inflammatory response.
The bottom line is that many athletes experience URS, especially in periods surrounding very intense exercise. These symptoms may or may not represent infection. However, if you are an athlete with URS, especially in a critical phase of your training and racing schedule, it may be helpful to see your health-care provider to determine if your symptoms represent an infection (which may be treatable) or not (which may, still, respond to appropriate treatment). For athletes with recurrent URS associated with long-term fatigue and poor performance, more thorough clinical investigation may be warranted, including testing for possible involvement of the herpes group viruses.
Position statement. Part one: Immune function and exercise. Walsh, NP, Gleeson, M, Shephard, RJ et al. Exerc Immunol Rev. 2011; 17:6-63.