Walkers 'ward off colds and flu'
Marathon runners may be as unhealthy as couch potatoes when it comes to catching a cold, it has been claimed.
On the other hand, a regular brisk walk can keep winter colds and flu at bay.
Moderate exercise strengthens the body's defences against nose and throat infections such as the common cold, flu and sinusitis, according to expert Professor Mike Gleeson. But too much exercise might be as bad as too little for the immune system.
Different levels of exercise significantly increase or decrease the chances of catching an upper respiratory tract infection (URTI), said Prof Gleeson, from the University of Loughborough.
Physical activity helps determine individual susceptibility to infection along with other factors such as genetics, stress, nutrition and sleep, he told the Association for Science Education conference at the University of Liverpool.
Prof Gleeson said: "If you have a tendency to be a couch potato then you probably have an average risk of catching an infection - typically 2-3 URTIs per year.
"Research shows that those undertaking regular moderate exercise (eg a daily brisk walk) can reduce their chance of catching a respiratory infection, such as a cold, by up to almost a third.
"Conversely, in periods following prolonged strenuous exercise, the likelihood of an individual becoming ill actually increases. In the weeks following a marathon, studies have reported a 2-6 fold increase in the risk of developing an upper respiratory infection."
Immune system cells called Natural Killer (NK) are important weapons in the fight against viral infections, said Prof Gleeson, who was speaking on behalf of the Society for General Microbiology and the British Society for Immunology. NK cells recognise cells invaded by viruses and force them to commit suicide.
Moderate exercise boosts the performance of NK cells, whereas stressful endurance activity such as running a marathon reduces it. The changes are tightly regulated by stress hormones and other immune cells, said Prof Gleeson.