The average adult has two to three upper respiratory infections each year. We are exposed to bacteria all day long, but some people seem more susceptible to catching the bug. The following factors have all been associated with impaired immune function and increased risk of catching colds.

• old age
• cigarette smoking
• stress
• poor nutrition
• fatigue and lack of sleep
• overtraining

However, there are also some things that seem to protect us from picking up colds. One of those things seems to be moderate, consistent exercise. More and more research is finding a link between moderate, regular exercise and a strong immune system.

During moderate exercise immune cells circulate through the body more quickly, and are better able to kill bacteria and viruses. After the exercise ends, the immune system returns to normal within a few hours. However, consistent, regular exercise seems to make these changes a bit more long-lasting. According to professor David Nieman of Appalachian State University, when moderate exercise is repeated on a near-daily basis there is a cumulative effect that leads to a long-term immune response. His research showed that those who walk at 70-75 of their maximum heart rate for 40 minutes per day had half as many sick days due to colds or sore throats as those who don't exercise.

However, there is also evidence that too much intense exercise can reduce immunity. This research is finding that more than 90 minutes of high intensity endurance exercise can make athletes susceptible to illness for up to 72 hours after the exercise session. This is important information for those who compete in longer events such as marathons or triathlons.

How Much Helps; How Much Hurts?

However, intense exercise seems to cause a temporary decrease in immune system finction. Reserach has found that during intense physical exertion, the body produces certain hormones that temporarily lower immunity. Cortisol and adrenaline, known as the stress hormones, raise blood pressure and cholesterol levels and suppress the immune system. This effect has been linked to the increased susceptibility to infection in endurance athletes after extreme exercise (such as marathon running or ironman distance triathlon training).

If you are training for ultra-endurance events, a key component of your training should be including enough rest and recovery days to allow your immunity to recover.

If you are feeling run-down or have other symptoms of overtraining syndrome, such as increased resting heart rate, slower recovery heart rate, irritability or general heaviness and fatigue, you may need to tone down your workouts. You can also prevent overtraining by understanding why Less is More.

If you are already ill, you should be careful about exercising too intensely. Your immune sytem is already taxed by fighting your infection, and additional stress could undermine your recovery. In general, if you have mild cold symptoms, without fever, light or moderate exercise may help you feel a bit better and actually boost your immune system. Intense exercise will only make things worse and likely extend your illness. Read more about exercising with a cold or flu.

It's not only physical stress that increases the release of cortisol and adrenaline, psychological stress can also impair immunity and lead to an increase of cold and flu infections. Researchers at Ohio State followed individuals who had the stress of caring for a loved one with Alzheimer's disease and found that they experienced twice as many colds as non-caregivers. For such individuals there is clearly a physical benefit to moderate, regular exercise.

Moderate exercise is not the only way to avoid colds and other infections. For more tips, check out Ten Tips to Prevent Colds and the Flu.

Source: Acute exercise stimulates macrophage function: possible role of NF-kappaB pathways. Cell Biochemistry and Function.. 2006 Aug 14;

Nieman DC. Risk of Upper Respiratory Tract Infection in Athletes: An Epidemiologic and Immunologic Perspective. Journal of Athletic Training 1997 Oct.

From Elizabeth Quinn,
Your Guide to Sports Medicine.