The more physically fit and active you are, the less likely you are to suffer colds in the winter months.
That's the conclusion of US researchers, who studied about 1000 adults and found those who exercised the most were least likely to suffer from colds in the winter months.
The researchers, from the Appalachian State University and the University of North Carolina, have published their results online in the British Journal of Sports Medicine. They followed a group of 1002 healthy adults aged from 18 to 85 years, over a 12 week period during the US autumn and winter seasons in 2008.
At the beginning of the study, the subjects were examined, and questioned on their diet and lifestyle, including how much exercise they did and how fit they perceived themselves to be.
Then, every day over 12 weeks, each participant reported any symptom of respiratory illness they experienced (such as sneezes, coughs, fever or other symptoms) and its severity, according to a standardised scale called the Wisconsin Upper Respiratory Symptom Survey.
Over the 12 weeks, the subjects reported experiencing symptoms of an upper respiratory tract illness (URTI) on average for 13 days in the winter and 8 days in the autumn.
But those who were fit and exercised frequently were much less likely to develop a cold, and when they did, it was much less severe.
Those in the top quarter for fitness levels (who did five or more days of exercise a week) experienced 43 % fewer days with URTI symptoms than those in the lowest 25 % of fitness levels (who did one day or less of exercise).
And when they did get cold symptoms, the symptoms were less severe. URTI symptoms were 32 % less severe in the top 25 % of exercisers compared to the bottom 25 %.
(The researchers adjusted for various other factors that can affect immune response such as mental stress, lack of sleep, poor nutrient status, and old age.)
Previous studies have also shown this relationship between fitness and reduced incidence and/or severity of URTI symptoms, the researchers say.
Exercise appears to reduce URTI incidence anywhere from 18 to 67 per cent they say, depending on the study.
The precise nature of the link between exercise and increased immunity remains a mystery, but it could be that each bout of exercise causes a transient increase in immune system activity, increasing the numbers of white blood cells and immunoglobulin in the blood, which acts to reduce a person¹s susceptibility to disease the researchers suggest.
Associate Professor Stephen Turner from the Department of Microbiology & Immunology at the University of Melbourne says he's not surprised at the findings.
"As a general rule the healthier you are, the easier you'll find it is to fight off infections", he says.
The effect may be hormone-mediated, he says.
"We know that people who exercise regularly have lower levels of stress hormones in the blood, and there's a definite link between low levels of stress hormones and improved immunity", he says.
He suggests it may also simply be that when people are exercising, they are more likely to be outdoors and less likely to pick up colds from other people.
Although it's impossible for most people to avoid catching colds altogether, the findings so suggest that regular exercise can reduce a person's chances of catching a cold, and/or reducing its severity if they do catch it.
The US study was part-funded by grants from Coca-Cola and a drug company, Quercegen Pharmaceuticals.