In theory, exercise should help strengthen our immune systems. The National Institute of Health (NIH) explains that while science isn’t clear on how and why exercise might improve our immune function, research has suggested that physical activity flushes bacteria from lungs and airways, reducing the chance of cold, flu, and other respiratory illnesses.

Exercise might also cause changes in antibodies and white blood cells, which are the most important cells for fighting disease. We know that physical activity increases blood flow, and this might allow white blood cells to circulate more rapidly throughout the body, thus allowing these cells to detect and fight illnesses earlier. In addition, experts theorize that the rise in body temperature during and immediately after exercise might prevent bacteria from growing, similar to what happens when you get a fever. Lastly, exercise slows down the release of stress hormones, making the body less vulnerable to illness.

While the exact mechanisms of exercise’s effect on the immune system are not clearly known, the NIH recommends a moderate, regular exercise plan simple enough do on a daily basis, such as 20 to 30 minutes of daily walking, bike riding a few times per week, trips to the gym every other day, or regular games of golf.1

Several studies have been published that provide some insight into the most effective way to exercise for optimal immune performance. Some of these studies suggest that exercise can actually weaken the immune system if you overdo it. A study published in 2008 looked at two groups of mice: one group that rested comfortably in their cages for three days, and another that ran on little treadmills until they were exhausted. After three days, the researchers exposed the mice to the flu virus. More of the exhausted mice contracted the virus and had more severe symptoms than the sedentary mice.2

If you find that first study to be a little discouraging, another experiment on mice presents a slightly clearer picture of the relationship between exercise and immunity. In this study, three groups of mice were infected with the flu. Group 1 rested after receiving the virus, Group 2 ran for 20 to 30 minutes, and Group 3 ran for two and a half hours. The groups continued this regimen for three days, until they began to show symptoms. During the 21-day study period, 50 percent of the sedentary mice died and 70 percent of the mice in Group 3 died, but only 12 percent of the mice that ran for 20 to 30 minutes died. The researchers in charge of this study noticed a very unique immune response in the jogging mice, and this could help to explain why they were the most successful group. When an immune system encounters a virus like the flu, it results in an increase in T1- and T2-helper cells. These cells are responsible for causing a strategic level of inflammation within the body that presents a defense against these viruses. This process is most effective when the T1- and T2-helper cell response is timed in a particular way so that the T1 cells trigger this inflammatory response only to a certain level, and then the T2 cells tone down this response before it becomes too severe and damages the body. In the mice that jogged, this balance of T1- and T2- helper cells was more effectively calibrated than those in the sedentary or intense exercise groups.3

These studies suggest that to avoid getting sick, the key might be moderation in exercise. Instead of ramping up exercise during the cold and flu season, aim to maintain a regular habit of moderate exercise all year round, much like the NIH recommends. If you are an athlete or are training hard for a marathon during the fall or winter, focus on getting adequate nutrition, plenty of sleep, and your flu shot, because these studies suggest that you might be more susceptible to illness when training intensely.

SOURCES 1) Medline Plus site. Exercise and Immunity. February 7, 2018. https://medlineplus.gov/ency/article/007165.htm. Accessed February 13, 2018. 2) Murphy EA, Davis JM, Carmichael MD. Exercise stress increases susceptibility to influenza infection. Brain Behav Immun. 2008 Nov;22(8):1152–1155. 3) Reynolds, G. Phys Ed: Does Exercise Boost

Immunity? The New York Times (site). Oct 4, 2009. https://well.blogs.nytimes. com/2009/10/14/phys-ed-does-

exercise-boost-immunity/. Accessed February 10,

2018. NHR