How Does Running Affect Your Immune System? The Research on Illness, Part II

Last week, we looked at some research on running while sick.

Surprisingly, we found evidence that running when you’ve come down with a minor illness like a cold isn’t such a bad idea.

In fact, it doesn’t seem to change the course of your recovery at all!

But what about getting sick? Does exercise keep you healthier? Or does the wear-and-tear on your body from training make your immune system less effective and you more vulnerable to sickness? That’s the topic we’ll look at today.

The research on the effects of running on the immune system

The first study we’ll examine was published by L. Spence and coworkers in 2007.¹

His study looked at the incidence of upper-respiratory infections (e.g. a cold) in three groups of subjects over a five-month period: Elite athletes, recreational athletes, and sedentary control subjects.  When the subjects displayed symptoms of a cold, like a runny nose or a sore throat, the researchers swabbed their mouth and nose to search for viruses.

While they were only modestly successful in actually identifying the viruses that caused the infections in their subjects, the researchers’ data nevertheless showed that the rate of infection was higher both in the elite athletes and in the sedentary subjects when compared to the recreational athletes.

Spence, et al., admitted that the findings of their study were limited because they could not identify the cause of illness in the majority of the cases, but regardless, their findings are important, as it hints that more exercise may not always be better when it comes to preventing illness.

The more significant impact of intense training

A larger study using only runners was published several years earlier by G.W. Heath and co-workers at the Centers for Disease Control and Prevention.²

In this study, 530 runners kept a monthly log of their training and health for 12 months.  By using the running logs of the subjects, the researchers were able to deduce some of the risk factors for having a higher frequency of upper respiratory infection.  On the whole, the subjects averaged 1.2 upper respiratory infections per year—about one every 10 months.

But the incidence rate varied significantly; people who lived alone, for example, had a much higher incidence rate than those who did not (for unknown reasons). And more relevant to the topic at hand:

Mileage was also correlated with infection risk, with runners logging higher yearly mileage incurring more upper respiratory infections.

This provides further evidence that a higher training load might increase your risk of getting sick.

The mechanism of action identified

A review study by Laurel T Mackinnon of the University of Queensland in Australia attempted to explain why moderate exercise seems to lower your risk of infection while heavier loads seem to increase it.³

Looking at several biological trials studying the effects of an exercise load on the cellular response of the immune system, she demonstrated that moderate exercise has either no effect on or has a small stimulating effect on a range of immune functions, from immunoglobulin levels to “killer cell” activity.

On the other hand, it appears that heavy training impairs the function of these same mechanisms.

In particular, it seems that long sustained periods of intense training over many days or weeks has a deleterious effect on the immune system.

Mackinnon emphasizes that athletes in hard training are in no way immunodeficient, and are still fully capable of fighting off infections and keeping their body healthy.

However, the small but measurable drop in immune function during intense training makes them more vulnerable, statistically speaking, to infectious disease.  This explains the findings of Spence, et al., and Heath, et al.

Light-to-moderate training seems to provide a small protective boost to your immune system.  But as your training becomes more serious, you become somewhat more vulnerable to minor illnesses like the common cold.

The training that constitutes “serious” according to the research consists of fewer miles than you might think. Heath, et al., showed an increased risk of upper-respiratory infection in subjects who ran an average of more than 20 miles per week annually vs. those who averaged less. 

The dearth of studies with high-mileage participants render the risks in athletes logging higher mileage (those training for a marathon, for example) less well-understood.

Final recommendations

Regardless, from what the available studies show, your risk of infection is indeed higher, but is probably nothing to worry excessively about.  Laurel Mackinnon recommends that athletes:

• Avoid over-training
• take adequate sleep and rest between workouts and after races
• eat a healthy diet (perhaps with a vitamin C supplement—a topic for another day)

There’s probably not a lot you can do in your daily life to lower your exposure to viruses, except perhaps washing your hands more often, so just be aware that you may come down with a cold more often when you are in heavy training — though when you do, try not to let it slow you down too much! As we saw last week, running easy during a mild or moderate cold doesn’t appear to make it last any longer or get any worse.