People who are physically fit and active have fewer and milder colds, indicates research published online in the British Journal of Sports Medicine.

The US researchers base their findings on 1,000 adults up to the age of 85 whose respiratory health was tracked for 12 weeks during the autumn and winter of 2008.

Six out of 10 participants were women, and 4 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.

The number of days with symptoms among those who said they were physically active on five or more days of the week and felt fit was almost half (43% to 46% less) that of those who exercised on only one or fewer days of the week.

The severity of symptoms fell by 41% among those who felt the fittest and by 31% among those who were the most active.

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.

For more information on the BMJ-British Medical Journal, visit www.bmj.com.