A review of 54 studies found that a regular exercise program strengthens the immune system, increases antibody response to vaccinations, and reduces risks for community-acquired infectious disease by 31 percent and death from infections by 37 percent in various populations (Sports Medicine, April 20, 2021). It does this by increasing IGA antibodies in all body cavities and CD4 T cells that help to kill invading germs. Even a single hard bout of exercise appears to enhance immune responses to vaccination in both younger and older individuals (Brain Behav Immun, Jul 2014;39:33-41). Many studies show that people who exercise regularly have higher antibody responses to vaccinations (J Gerontol A Biol Sci Med Sci, 2002 Sep; 57(9):M557-62). Aging progressively impairs your immunity, but the most active elderly patients have the highest protective immune responses (Ann N Y Acad Sci, 2002 Apr; 959():117-27).

Exercise Reduces Risk for COVID-19 Complications
Regular exercise can substantially reduce the risk of COVID-19-related hospitalization, intensive care unit (ICU) admission, or death, according to a study from the UK (Brit J of Sprts Med, Apr 14, 2021). A self-reported physical activity study of 48,440 adult patients found that those who did not exercise were at:
• 2.26 times the risk of hospitalization
• 1.73 times the risk of ICU admission
• 2.49 times the risk of death from COVID-19
Not exercising was the third most important risk factor for COVID-19 death, after being older than 60 or having had a previous organ transplant. Not exercising was way ahead of smoking, obesity, diabetes, high blood pressure, heart disease, and cancer as risk factors for death from COVID-19. The authors recommend, “Walk 30 minutes a day, five days a week at a moderate pace and that will give you a tremendous protective effect against COVID-19.”

How Exercise Can Improve your Immunity
This year, researchers discovered that walking and running transmit pressure on bone surfaces along arteriolar blood vessels into the bone marrow. This promotes the growth of new bone cells to strengthen and thicken bones and secrete a growth factor that increases the numbers of B and T lymphocyte cells that attack and kill invading germs (Nature, February 24, 2021;591:438-444).

Inactivity slows the formation of new bone and lymphocytes. Another review found that exercise reduces the loss of immune function associated with aging (J Sprt and Health Sci, May 2019;8(3):201-217). A study on mice showed how exercise helps to strengthen your immune system (Sci Rep, November 6, 2015;5:16364). When germs get into your body, you produce certain cells and proteins that attack the germs and try to kill them. When you exercise vigorously enough to damage muscle cells, your body uses these same cells and proteins to help heal your muscle cells. Exercise that is vigorous enough to damage muscles stimulates the same immune cells that heal muscles to help your immunity respond to and kill invading germs. If your immune system stays active all the time, the same cells and proteins that kill germs can attack your own cells. This is called inflammation.

When you are exercising properly by taking hard workouts to strengthen your muscles followed by easy workouts to allow your muscles to heal, you are also strengthening your immune system. When you damage muscles with hard exercise, your immune system uses the same white blood cells and proteins that attack and kill germs to start the healing process, so vigorous exercise turns on your immunity in the same way that an infection does. Then to keep your immune system from being too active and using the same chemicals to damage you, you produce anti-inflammatory chemicals to dampen down your immunity. A proper training program of vigorous exercise followed by easy workouts strengthens your immune system by turning it up and down.

Listen to Your Body
After intense exercise, you should expect to feel sore because of the muscle damage, which is good for you. If you do another intense workout when you are trying to heal, your body senses that you are causing further damage. Your body reduces the same white blood cells and proteins that you use to kill germs and heal damaged muscles.
• Try to alternate harder workouts with easy recovery ones on consecutive days.
• If you are training properly, expect to feel sore every morning when you wake up. If your muscles feel better after a 5-10 minute warm up, take your planned workout.
• If you don’t feel better during your warm up, go home because continuing to exercise will only increase your chances of injuring yourself.

How Much Is Too Much?
Exercise increases your ability to kill invading germs, unless you exercise too much (Front Immunol, Apr 16, 2018;9:648). If you keep on pushing your limits and exercising through muscle soreness, you turn on your immune system to heal the damaged muscles. Your immune system can be so highly activated that it eventually causes inflammation, which can damage organs throughout your body (J Amer Ger Soc, Aug 2004;52(7):1098-104).

Muscles are made up of hundreds of thousands of individual fibers. To make muscles stronger, you have to damage them. That means that you exercise more intensely on one day, damage your muscle fibers and feel sore on the next. Then when they heal, they become larger and stronger. You can tell that you are damaging a muscle by the burning you feel in the muscles during exercise, and the soreness you feel in that muscle on the next day. If you take an easy workout when your muscles feel sore, your muscle will become stronger. If you take an intense workout on sore muscles, you can tear them and become injured. The healing of muscles damaged by intense exercise is governed by your immune system:
• The exact same cells and cytokines that kill germs initiate the healing process.
• When you feel sore after intense exercise, your immune system goes into high gear to heal the damaged muscle tissue.
• If you do another intense workout when you still feel sore and are trying to heal, your body senses that you are causing further damage, so it reduces the amount of white blood cells and proteins that initiate muscle healing.
• Your body reduces your immune reactions because if your immune system stays active all the time, it can attack your healthy tissues in the same way that it attacks germs. This can lead to the many “auto-immune” diseases such as chronic fatigue syndrome or inflammatory bowel disease.

My Recommendations
A regular exercise program strengthens your immune system to help protect you from disease. However, too much exercise can cause inflammation that may impair your ability to kill germs. When your muscles feel sore, take very easy workouts or take the day off.