Six out of 10 participants were women, and four out of 10 were aged between 18 and 39; 40% were middle aged, and one in four were aged 60 and older.
All the participants reported back on how frequently they took aerobic exercise and rated their fitness levels using a validated 10 point scoring system. They were also asked about lifestyle, diet and recent stressful events, as these can all affect immune system response. The number of days with cold symptoms varied considerably between winter and autumn, with an average of 13 days in the winter and 8 days in the autumn.
Being older, male, and married, seemed to reduce the frequency of colds, but after taking account of other influential factors, the most significant factors were perceived fitness and the amount of exercise taken.
In the US, an average adult can expect to have a cold two to four times a year, while children can catch between half a dozen and 10 colds a year, on average, all of which costs the US economy around $40 billion dollars.
Bouts of exercise spark a temporary rise in immune system cells circulating around the body, say the authors. Although these levels fall back within a few hours, each bout is likely to enhance surveillance of harmful viruses and bacteria, so reducing the number and severity of infections, such as the common cold.
Source: BMJ-British Medical Journal and Upper respiratory tract infection is reduced in physically fit and active adults. British Journal of Sports Medicine, 2010; DOI: 10.1136/bjsm.2010.077875
See also: Training In The Heat Even Helps Competing In Cool Temps and Is Exercise The Cure For Depression?