If yes? When? What is the strength of the evidence? What mechanisms are involved (how does it make you sick)? What are the key points and what does it mean for me? Those are a few questions that came to my mind when I saw an article regarding immune functioning in elite athletes. The article, in ACSM's Certified News 2011(21:4) and written by Eric Christensen, provided the answers for some of those questions. I will share this with you now.
There is strong evidence to support the benefits of daily exercise. There is evidence that exercise improves the immune system. One way that it does this is by reducing blood levels of cortisol. Cortisol is a stress hormone and it suppresses the body's immune system. As I learned in the article, the immune system has two parts - the innate and the adaptive. The innate responds to new germs or foreign bodies, while the adaptive system has learned to recognize repeat threats and can attack them in fewer steps, so to speak.
This article and my post, refer to exercise and the innate immune system. That being said, I just had a thought - I am supposed to be keeping these posts simple. Therefore, you have to trust that I did due diligence in my preparation while I try to cut to the chase.
The measure of sickness in the studies that were reviewed for the article is URIs - or upper respiratory tract infections (colds, sinus congestion, coughing, low fevers).
Elite athletes do have higher rates* of URI than non athletes, but and this is a big but, they are also in situations where germs are more likely to be in the air (locker rooms, buses, etc).
Researchers found that the immune system which is usually improved from exercise, becomes depressed after high intensity and duration of exercise (but not necessarily the frequency of exercise). [the chemical markers that change in the athletes are ones associated with fighting disease, like neutrophils, lymphocytes, etc. The athlete has LESS of them in their blood stream immediately after the prolonged bout of exercise]
The criteria seems to include 1) 90 minutes of continuous exercise that is 2) moderate to high in intensity (competitive cycling, swimming, running) and that 3) occurs without fueling or eating.
In a study of runners, it was found that those with faster paces, i.e sub 8 min miles, who ran 45 or more miles a week, were more at risk. (I can assure you that I do not come close to either of those markers)
The studies that were used to assess this issue did not have verification* of URIs. It is said that some of the symptoms are similar to those experienced from over training. Thus a person in a study could report having an URI and the scientist 1) takes that as true and 2) equates it with level of activity, but the person may not have had an URI.
The above was two caveats rolled into one. The athlete may not be sick and athletes may not have as high a rate of URIs as has been reported because we do not know for sure that they had an URI.
Either way - for most of us - the problem is getting sick from too little working out not too much. Also - the diseases associated with lack of physical activity can be much more serious than a head cold.