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Effects of exercise on immune function and risk of infection

We all suffer from colds at some time but recent research indicates that a person’s level of physical activity influences their risk of respiratory tract infections such as a cold, most likely by affecting immune function. Moderate levels of regular exercise seem to reduce our susceptibility to illness compared with an inactive lifestyle but long hard bouts of exercise and periods of intensified training put athletes at increased risk of colds and flu.


Upper respiratory tract infections (URTIs) are the most common ones that people get and include the common cold, sinusitis and tonsillitis. Most are due to an infection with a virus. The average adult has two to four URTIs each year and young children have twice as many. We are constantly exposed to the viruses that cause these infections, but some people seem more susceptible to catching URTIs than others. Every day our immune system protects us from an army of pathogenic microbes that bombard the body. Immune function is influenced by an individual’s genetic make-up as well other external factors such as stress, poor nutrition, lack of sleep, the normal ageing process, lack of exercise or overtraining. These factors can suppress the immune system, making a person more vulnerable to infection.

What is the effect of exercise?

Exercise can have both a positive and negative effect on the functioning of the immune system and can influence a person’s vulnerability to infection. Researchers have found a link between moderate regular exercise and reduced frequency of URTIs compared with an inactive state and also with excessive amounts of exercise and an increased risk of URTIs. A one year study of over 500 adults found that participating in 1-2 hours of moderate exercise per day was associated with a one third reduction in the risk of getting a URTI compared with individuals that had an inactive lifestyle (Nieman et al. 2011). Other studies have shown that people who exercise 2 or more days a week have half as many days off school or work due to colds or flu as those who don’t exercise.

Is more exercise better?

More is not always better in terms of exercise volume as other studies have reported a 2 to 6 fold increase in risk in developing an URTI in the weeks following marathon (42.2 km) and ultra-marathon (90 km) races. This is due, in part, to increased levels of stress hormones like adrenaline and cortisol that suppress white blood cell functions. After strenuous exercise, athletes enter a brief period of time in which they experience weakened immune resistance and are more susceptible to viral and bacterial infections, in particular URTIs. Post-exercise immune function depression is most pronounced when the exercise is continuous, prolonged (>90 minutes), of moderate to high intensity (55-75% of aerobic capacity), and performed without food intake (Gleeson et al. 2013). Another problem for athletes is that their exposure to pathogenic (disease causing) microorganisms in the environment may be higher than normal due to increased rate and depth of breathing during exercise (increasing exposure of the lungs to airborne pathogens), exposure to large crowds and frequent foreign travel. Some of the reported sore throats may not be due to infectious agents but to non-infectious airway inflammation caused by allergies or inhalation of pollutants and cold dry air.

When are athletes most susceptible?

A common perception is that exposure to cold wet weather can increase the likelihood of catching the common cold but the available evidence does not support this. Most people are more susceptible to colds in winter (which is possibly due to reduced vitamin D status at this time) but numerous studies on athletes indicate that they tend to be most susceptible to picking up infections at times close to competition. This usually follows a period of intensive training and added mental stress with the anxiety of wanting to perform well. The worry for athletes is that even a mild infection can impair their ability to perform at the highest level. Preventing infections is therefore very important to them and they can help themselves by ensuring good personal hygiene, good nutrition and minimizing other life stresses.

These illness prevention strategies are discussed in future articles in the following blogs:

Strategies to reduce illness risk in athletes



  1. Gleeson M, Bishop NC and Walsh NP (2013) Exercise Immunology. London: Routledge (Taylor and Francis). ISBN 978-0-415-50725-7 (Hb); 978-0-415-50726-4 (Pb); 978-0-203-12641-7 (Ebook).

  2. Nieman DC, Henson DA, Austin MD and Sha W (2011) Upper respiratory tract infection is reduced in physically fit and active adults. British Journal of Sports Medicine 45:987-992.


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