In a recent study, researchers followed over a thousand adults (18-85 years old) for 12 weeks during the fall and winter seasons. Over this time, the participants had to report two measures: any symptoms of upper respiratory tract infection (such as a cold), and how much they exercised.
While running in the cold winter air might sound like a counterproductive measure to prevent colds, the researchers found that participants who reported being physically active (aerobic exercise) five days a week or more experienced significantly less cold and flu symptoms (a 43% reduction in number of days with an illness). This relationship held true event when several factors were controlled for, such as dietary habits (eating lots of fruits and veggies) and stress levels.
Why might exercise prevent colds? While we don't have a clear cut answer to this question, animal studies suggest a few leads. When you exercise, you increase the circulation of cells that are important for immunity and that are involved in fighting off the bad guys. More specifically, exercise has been shown to boost macrophages (cells that eat up invaders) in your lungs. In addition, exercise can lower the levels of immunity-compromising stress hormones.
Is there anything exercise can't do? Now I need researchers to study how one can be motivated to exercise when they are sitting in a comfy chair by the fire with a mug of chai tea and a pile of work to do and it's below zero outside. Tell me something I don't know, right?
Reference: Upper respiratory tract infection is reduced in physically fit and active adults. (2010) Nieman DC et al. [Epud ahead of print].