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“Regular moderate exercise training stimulates a recirculation of cells from the innate immune system and decreases upper respiratory tract infection (URTI), while sustained and intense exercise induces immune dysregulation, inflammation, oxidative stress and increases URTI risk.” David Nieman, DrPH, Appalachian State University, Kannapolis, N. C., and colleagues

Exercise Boosts Resistance to Colds

A Two-Edged Sword 

Conventional wisdom says the person who finds the cure for the common cold will not only be able to move to an island, he or she will be able to buy the island. The fact that colds are caused by more than 200 different viruses makes a vaccine or cure extremely elusive. But conventional wisdom can be wrong. Protection may have been under our noses all along—and the only cost is sweat equity. Our best defense against colds may be regular exercise.

Researchers at Appalachian State University in North Carolina have found that regular exercise and the perception of being fit are associated with a substantial reduction in upper respiratory tract infection. As in many things, moderation may be the key. (The study, lead by David Nieman, DrPH, was published online November 1, 2010, in the British Journal of Sports Medicine.)

The Study

The study followed 1,002 adults (male & female, 18 to 85) over two 12-week periods; half participated in the fall and half in the winter. Those who said they exercised (details below) at least five days a week had 43% fewer days with upper respiratory tract infection (URTI) than those who exercised no more than one day a week (4.41 days versus 8.18). Similarly, those who rated themselves as highly fit had 46% fewer days with a respiratory infection than those who reported low fitness (4.89 days versus 8.60). Even when they did get sick, frequent exercisers and the most fit suffered less severe symptoms, by 32% to 41% between high versus low exercisers and perceived fitness.

One feature that sets this study apart from earlier research is that validated methods were used to track respiratory infections and related symptoms. David Nieman and his colleagues chronicled the effects of the common cold using the Wisconsin Upper Respiratory Symptoms Survey, a reliable and valid daily logging system.

The participants reported how many days per week they exercised enough to at least moderately increase breathing and heart rate and make them sweat for at least 20 minutes. They were also asked to rate their physical fitness on a 10-point Likert scale. They were divided into three groups (8-10 high, 6-7 middle, and 1-5 low) based on their response.

Although the underlying mechanism remains unclear, the researcher believe that exercise affects the body’s immune response. These scientists and others believe exercise can both enhance and undermine immune function.

The Mechanism

As you’d expect, it’s a bit technical. “Each aerobic exercise bout causes a transient increase in the recirculation of immunoglobulins, and neutrophils and natural killer cells, two cells involved in innate immune defenses. Animal data indicate that lung macrophages [devour invaders] play an important role in mediating the beneficial effects of moderate exercise on lowered susceptibility to infection,” Nieman and his colleagues wrote.

“Stress hormones, which can suppress immunity, and pro- and anti-inflammatory cytokines, indicative of intense metabolic activity, are not elevated during moderate aerobic exercise,” they continued.

“Although the immune system returns to pre-exercise levels within a few hours after the exercise session is over, each session may improve immunosurveillance against pathogens that reduce overall upper respiratory tract infection incidence and symptomatology.” 

Got that? Even though the immune system returns to normal levels within a few hours, each workout provides a shield of protection against infection. In short, exercise increases the odds of dodging the onslaught of cold bugs.

On the other hand, very intense or prolonged exercise may provide a window of opportunity for viruses to do their thing. “Sustained and intense exercise induces immune dysregulation, inflammation, oxidative stress and increases URTI risk,” the researchers wrote in their report.

The Dark Side

It is well-known that marathon runners often come down with colds or worst in the weeks and sometimes months after the race. This is especially true of ultra-marathoners. In a five year study of 350 athletes in the 160 kilometer (about 96 miles) Western States Endurance Run, Dr. Nieman found that one out of four runners reported sickness during the 2-week period after the race. (Medicina Sportiva, November 16, 2009)

Dr. Marybeth Crane, a sports medicine podiatrist board certified in foot surgery and wound care--and a runner--summarized research on runners on her blog:

“A recent survey of 30 different studies of runners and decreased immune function that may lead to increased upper respiratory infections revealed little agreement from the experts. Yes, they all agree that moderate activity may enhance immune function, but they describe this as brisk walking for 30 to 45 minutes a day. What runner does that little activity? Most studies also agreed that high-intensity exercise temporarily impairs the immune competence. Hence the increased incidence of upper respiratory infections in marathon runner and especially ultra-marathon runners.

“Athletes, when compared with their couch potato colleagues, experience higher rate of upper respiratory infections especially in the few weeks after intense training and races. In non-athletes, increasing physical activity is associated with a decreased risk of upper respiratory infections.”

*  *  *

 It’s a good bet that compromised immune function also afflicts bodybuilders who spend hours in the gym 5 or 6 days a week.

The immune system problem is another reason to recommend brief, hard, and infrequent training—weights and aerobics—combined with walking and staying active between workouts. It also invites consideration of the one-set-after-warm up approach.

(For what it’s worth, I rarely catch cold and when I do the duration is usually short and the symptoms mild.)

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