According to the American College of Sports Medicine (ACSM), for the majority of fitness enthusiasts engaged in 30-60 minutes of exercise most days of the week, the number of sick days they take during flu season is reduced by at least 40 percent.
Yup! Regular physical activity actually strengthens your immune system.
The reason is thought to be that when you exercise, you improve blood flow and movement of your immune system’s lymphatic fluid, allowing your immune cells to circulate through the body more quickly to kill off bacteria and viruses more efficiently.
Like all good things, you can certainly overdo it… Research has shown that as few as 90 minutes of high-intensity endurance exercise can make you more susceptible to illness for up to three days after the session.
The reason for this is a release of certain hormones that can cause a temporary decrease in the proper function of the immune system. When you perform a hard exercise session, you significantly increase the release of cortisol and adrenaline, your body’s “fight or flight” stress hormones, which can suppress the immune system.
So, if you are already sick with a cold or respiratory infection, high-intensity exercise such as heavy weightlifting or very long exercise such as marathon training can further weaken the immune system.