Are you making yourself sick?
How many times have you looked outside on a cold, rainy, winter’s day and said to yourself if I go out training in this will I get sick?
To answer this question we need to look at how exercise affects the immune system. Exercise represents a physical stress which the body reacts to with a certain pattern of hormonal and immunological responses. Depending on the volume and intensity of the exercise will determine the pattern of the response and whether the immune system is stimulated or suppressed. However, it should not be forgotten that other factors such as nutrition, psychological stress and training status will also influence the immune response to a specific training session.
During a strenuous training session there is an increase in stress hormones such as adrenaline and cortisol. Adrenaline increases during exercise in response to both the intensity and duration of the exercise bout. Cortisol increases only in response to exercise of long duration (greater than 1 hour) and shows persistently high levels for hours after the cessation of exercise (Pedersen & Hoffman-Goetz, 2000). Both of these hormones have a direct effect on immune variables such as cytokines and lymphocytes which play a big part in the body’s defence against infection. Though lymphocytes increase in concentration during exercise, concentrations drop below pre-exercise levels after long duration exercise bouts (Pedersen, Bruunsgaard, Jensen, Krzywkowski, & Ostrowski, 1999). It therefore appears that after strenuous exercise there is an “open window” where the body is vulnerable to infection (Pedersen et al., 1999).


