You may have noticed that as a runner, you take fewer sick days than your co-workers. You might also be the only one of your group of friends who doesn't catch the cold that is going around. This isn't just your imagination. Numerous studies have demonstrated that regular exercise improves how well your immune system works. By understanding and applying the science of the immune system, and the role that exercise plays in keeping you healthy, you can optimize your health for years to come.
What the Research Tells Us
The primary research finding that runners need to understand has to do with what is called the "J-shaped hypothesis." In studies that compare the incidence of upper respiratory tract infections (UTRI) to the intensity of training, athletes who engage in a moderate level of exercise are found to be less likely to develop a runny nose or scratchy throat associated with a URTI than their sedentary counterparts. In terms of moderate exercise, a recent study followed a group of physically active adults over the fall and winter months. The researchers found those adults who exercised more than five days a week at a low to moderate intensity experienced cold symptoms for half the number of days than their less active counterparts. Furthermore, the severity of symptoms was lower in the people who were the fittest. This research provides yet another example of the benefits of exercise for the general population. Increase your activity levels to a moderate amount, about five or six hours each week, and you can expect to experience fewer illnesses over the course of the year. This is partly due to the fact that exercise improves the flow of fluids in your lymphatic system, which means that viruses, bacteria and toxins are filtered from your blood and lymph more effectively.
The research also indicates that as the intensity of an athlete's training increases, the risk that an athlete will contract an illness also rises. So training camps and stressful races can compromise your immune system for a short period of time. By looking at the stresses of high-intensity training, which causes an extended period of high stress for the body, researchers have been able to determine that it suppresses some of the critical components of the immune system. For example, it appears that intense exercise decreases the production of natural killer cell activity (an immune system cell that defends the body against invaders) which exposes us to great risk of infection. Research has also discovered that intense physical exertion may decrease the body's production of immunoglobulin, also known as antibodies, in our digestive and respiratory tracts. Antibodies are proteins produced by our white blood cells that identify specific pathogens that enter the body. If their production is depleted, our susceptibility to illness increases.
The findings indicate that the paradox of the immune system and exercise is that exercise can actually limit your body's ability to cope with illness in the short term. When you exercise, your body releases epinephrine and cortisol, two stress-related hormones which give your body the additional energy it needs to meet the challenges of the exertion. The difficulty is that these hormones reduce the activity of T-cells and macrophages, cells which engulf and digest bacteria and other pathogens. Research has also shown that following exercise, the level of B cell antibodies in your body, which are responsible for recognizing bacteria and viruses, drops and the reactive oxygen from neutrophils that normally kills microbial pathogens is less active. This "open window" effect means that for up to a few hours following a particularly intense workout or race, you are especially susceptible to infections.
Improve Your Performance
To optimize the long-term benefits of exercise and reduce the short-term risk of illness:
• Establish a consistent and regular habit of exercise that lasts throughout your life with an emphasis on consistency
• Get adequate sleep so that your body has time to recover
• When you are planning an intense workout or race, make sure you plan for appropriate rest in the following 24 hours
• Pay special attention to washing your hands after your runs or training sessions. That is when you're most at risk for infection by viruses, bacteria, microbes, and other pathogens
• Stay away from caffeine when you're sick – caffeine may suppress the immune system
Nutrition and Psychology
In terms of nutrition, there are a few specific tips that can help you boost your immune function. To help protect yourself against free radical-induced impairment of the immune system after intense exercise, I suggest you take an antioxidant supplement containing vitamin C, vitamin E and beta-carotene. Also consider increasing your general consumption of fruits and vegetables, nuts and whole grains – all of which are high in antioxidants. You also need to make sure that you consume adequate levels of the complex carbohydrates found in whole grains, brown rice and other foods that are high in fibre because white blood cells (the body's immune system cells) use carbohydrates as fuel. You should also consider a zinc supplement because zinc may improve immune function. Remember that for supplements, more is not necessarily better. Taking zinc or any other vitamin or mineral above a certain threshold may actually be damaging.
Mentally, a critical thing to consider about the relationship between exercise and illness are social connections. Research from Carnegie Mellon University in Pittsburgh reported that people with good social bonds (close relationships with friends and family) are less susceptible than others to the common cold. If you want to avoid illness, spend more time with your friends and family. Preferably while out for a run!
Nieman, David C. (2007). "Marathon Training and Immune Function," Sports Medicine 37: 412-415.
2 Gleeson M. and D.B. Payne, (2000). "Special Feature for the Olympics: Effects of Exercise on the Immune System: Exercise Effects on Mucosal Immunity," Immunology & Cell Biology 75, no. 5: 536–44
Greg Wells Ph.D. (www.drgregwells.com, @drgregwells) is an Assistant Professor at the University of Toronto in the Faculties of Medicine and Kinesiology. He was the sport science analyst for the Olympic Broadcast Consortium during the 2010 & 2012 Games, and is the author of Superbodies: Peak Performance Secrets from the World's Best Athletes. Jessica Caterini is a member of the Human Physiology Research Unit in the Faculty of Kinesiology at the University of Toronto.