“I am at this moment deaf in the ears, hoarse in the throat, red in the nose, green in the gills, damp in the eyes, twitchy in the joints, and fractious in temper from a most intolerant and oppressive cold.” Although your symptoms may vary, this is how Charles Dickens once described with painful and colorful accuracy his personal suffering and misery from a bad cold.
Despite all attempts to provide a medical solution for the common cold, little progress has been made, especially in regards to prevention (1,23). Upper respiratory tract infections (URTIs) such as the common cold are the most frequently occurring illnesses in humans worldwide, with an annual incidence of 2 to 3 for adults and 6 to 8 for children (13,14). More than 200 different viruses cause the common cold, and rhinoviruses and coronaviruses are the culprits 25% to 60% of the time. The viruses are spread by hand or aerosol contact with secretions from infected individuals, and the resulting acute respiratory illnesses impose a significant economic burden in terms of medical visits, school absenteeism, and work productivity (7).
Various demographic characteristics predict lower URTI rates including being married, older age, and male sex (14). Few lifestyle strategies for URTI prevention have received consistent scientific support other than proper hygiene (i.e., wash hands frequently and keep them away from the eyes and nose) (1,23). However, there is now growing awareness that near-daily physical activity has a strong influence in lowering the odds of getting sick with the common cold.
PHYSICAL ACTIVITY AND URTI RISK
A common belief among fitness enthusiasts is that regular exercise confers resistance against the common cold. In surveys, 80% to 90% of regular exercisers perceive themselves as less vulnerable to viral illnesses compared with sedentary peers (22,24). Epidemiological investigations and randomized controlled trials are largely supportive of this perception (2,3,8,9,12,18–21).
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Retrospective and prospective epidemiologic studies have measured URTI incidence in large groups of moderately active and sedentary individuals. Collectively, the epidemiologic studies consistently show reduced URTI rates in physically active or fit individuals. A 1-year epidemiological study of 547 adults showed a 23% reduction in URTI risk in those engaging in regular versus irregular moderate-to-vigorous physical activity (12). A cohort of 1,509 Swedish men and women ages 20 to 60 years were followed for 15 weeks during the winter/spring (9). Subjects in the upper tertile for physical activity experienced an 18% reduction in URTI risk, but this proportion improved to 42% among those with high perceived mental stress.
A group of 1,002 adults (ages 18 to 85 years, 60% female, 40% male) were followed for 12 weeks (half during the winter, half during the fall) while monitoring URTI symptoms and severity using the Wisconsin Upper Respiratory Symptom Survey (18). Subjects reported frequency of moderate-to-vigorous aerobic activity and rated their physical fitness level using a 10-point Likert scale. The number of days with URTI was 43% lower in subjects reporting an average of 5 or more days of aerobic exercise (20-minute bouts or longer) compared with those who were largely sedentary (≤1 day per week). This relationship occurred after adjustment for important confounders including age, education level, marital status, sex, body mass index, and perceived mental stress. The number of days with URTI was 46% lower when comparing subjects in the highest versus lowest tertile for perceived physical fitness, even after adjustment for confounders (Figure 1).
Figure 1: A group of 1,002 adults (ages 18 to 85 years, 60% female, 40% male) were followed for 12 weeks during the winter and fall seasons while monitoring URTI symptoms and severity. The number of days with URTI during the 12-week period was significantly reduced (46%) when comparing subjects in the high versus low fitness tertile (after control of various confounders). Of all lifestyle habits measured in this study (stress, diet, sleep), physical fitness emerged as the strongest predictor of URTI. Of various demographic factors, older age, low education, marital status, and male sex also were predictive of reduced number of days with URTI (data from reference 18).
Randomized experimental trials provide important data in support of the viewpoint that moderate physical activity reduces URTI symptomatology. In a randomized, controlled study of 36 women (mean age, 35 years), subjects walked briskly for 45 minutes, 5 days a week, and experienced one half the days with URTI symptoms (5.1 vs. 10.8) during the 15-week period compared with that of the sedentary control group (21).
The effect of exercise training (five 45-minute walking sessions/wk at 60% to 75% maximum heart rate) and/or moderate energy restriction (1,200 kcal/d to 1,300 kcal/d) on URTI was studied in obese women (N = 91) randomized to one of four groups: control, exercise, diet, and exercise and diet (20). Energy restriction had no significant effect on URTI incidence, and subjects from the two exercise groups were contrasted with subjects from the two nonexercise groups. The number of days with URTI for subjects in the exercise groups was reduced 40% relative to the nonexercise groups (5.6 vs. 9.4), similar to the level of nonobese, physically active controls (N = 30, 4.8 days with URTI). In another study, 30 sedentary elderly women (mean age, 73 years) were assigned to walking or sedentary groups (19). The exercise group walked 30 to 40 minutes, 5 days per week, for 12 weeks at 60% heart rate reserve. Incidence of URTI in the walking groups was 21% compared with 50% in the callisthenic control group during the fall season.
A 1-year randomized study of 115 overweight, postmenopausal women showed that regular moderate exercise (166 minutes per week, ~4 days per week) lowered URTI risk compared with controls (who engaged in a stretching program) (3). In the final 3 months of the study, the risk of colds in the control group was more than threefold that of the exercisers.
PHYSICAL ACTIVITY ENHANCES IMMUNE SURVEILLANCE
During moderate exercise, several positive changes occur in the immune system (6,15–17). Moderate exercise increases the recirculation of neutrophils and natural killer cells, two cells that play a critical role in innate or “front line” immune defenses. Animal data indicate that lung macrophages play an important role in mediating the beneficial effects of moderate exercise on lowered susceptibility to infection (15). Stress hormones, which can suppress immunity, and pro- and anti-inflammatory cytokines, indicative of intense metabolic activity, are not elevated during moderate exercise. Although the immune system returns to pre-exercise levels within a few hours after the exercise session is over, each session represents a boost in immune surveillance that reduces the risk of infection over the long term. Several studies indicate that both acute and chronic moderate exercise training improves the body’s antibody response to the influenza vaccine (6,10,11). In general, both human and animal studies indicate that moderate exercise favorably influences overall immune function and the capacity to boost the antibody titer to a vaccine.
A TOTAL LIFESTYLE APPROACH
The data on the relationship between moderate exercise, enhanced immunity, and lowered risk of acute respiratory illnesses are consistent with guidelines urging the general public to engage in near-daily brisk walking. The magnitude of reduction in URTI risk with regular moderate physical activity exceeds levels reported for most medications and supplements, but more research is needed with large groups of study participants followed for long time periods (1,18,23).
Regular physical activity should be combined with other lifestyle strategies to more effectively reduce URTI risk. These strategies include stress management, regular sleep, avoidance of malnutrition, high intake of fruit, and proper hygiene (1,2,4,5,23). URTI is caused by multiple and diverse pathogens, making it unlikely that a unifying vaccine will be developed (1,23). Lifestyle strategies are receiving increased attention by investigators and public health officials, and a comprehensive lifestyle approach that includes physical activity has high potential to make the common cold less common.
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