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  • Writer's pictureP.K. Peterson

Exercise, Sunshine, and the Immune System

“Nobody can be in good health if he does not have all the time fresh air, sunshine and good water.”

Flying Hawk, Oglala Lakota warrior, historian, educator and philosopher

“Walking is man’s best medicine.”


As we are hitting our stride in the summer months, I am turning my focus of this Germ Gem blog to the importance of exercise for physical and, equally important, mental health. Especially during this time of increased distress due to the COVID-19 pandemic and the latest news stories about cases spiking in many regions of our country, it’s useful to remember what steps (and, yes, I mean that literally) we can take as individuals, every day, to help us stay on a path toward wellbeing.

Sedentary behavior: it’s bad for your health. We all know this already, right? But if that were indeed the case, we would not be facing our obesity epidemic in this country. Sedentary behavior (which excludes sleeping) is defined as sitting or lying down that is experienced at work, at school, at home, when travelling or during leisure time. It requires little energy expenditure and that’s the problem. Numerous studies have shown that sedentary behavior is a risk factor for many chronic diseases, including cardiovascular disease, high blood pressure, type 2 diabetes, muscle degeneration, obesity, colon cancer, and dementia. According to the World Health Organization, 60 to 85% of people in the world—from both developed and developing countries—lead sedentary lifestyles, making it one of the more serious public health problems of our time. According to a recent review in the American Journal of Public Health, the widely held view that sitting is as potent a risk factor for mortality as is smoking is overstated, but that doesn’t mean that we should be complacent about this serious problem.

What about the effects of exercise on COVID-19? As readers of this blog already know from past posts, some of the same diseases that result from a sedentary lifestyle (e.g., obesity) are also risk factors for COVID-19. All the more reason to get your body moving. Recent studies from the University of Bath, highlighted in Science News, suggest there is a powerful influence of regular, daily exercise on our immune system. The authors stressed the importance of continuing to work-out even in lockdown. But, be smart about it. In other words, use common sense: as the authors underscored in the article, while regular moderate intensity aerobic exercise, such as walking, running, or cycling is recommended, it is also important to maintain good personal hygiene when exercising, including thoroughly washing hands following exercise, as well as paying attention to adequate sleep and a healthy diet. A different article, this one in the May 2019 article in the Journal of Sport and Health Science, entitled “The compelling link between physical activity and the body’s defense system,” summarized the results of studies dating as far back as 1900. The authors, a group of exercise physiologists, concluded that the “data support a clear inverse relationship between moderate exercise training and illness risk.” For converts to exercise, the results of these studies will come as no surprise.

Benefits of sunlight on the immune system. The other helpful thing about exercise is that it gets us outside. According to an article in Environmental Health Perspectives (“Benefits of Sunlight: A Bright Spot for Human Health”), sunlight has two potential mechanisms whereby it might help us fight infection. The first is through a direct microbicidal (destructive to microbes) effect of the ultraviolet radiation (UVR) in sunlight. UVR can indeed inactivate many types of bacteria and viruses. (Warning: disinfecting your skin with any kind of UVR can lead to damage and increase your risk of skin cancer.)

The second mechanism whereby sunlight can affect our immune defenses is through its role in vitamin D metabolism. Vitamin D is a steroid vitamin that promotes the intestinal absorption and metabolism of calcium and phosphorus. Under normal conditions of sunlight exposure, no dietary supplementation is necessary because sunlight promotes adequate vitamin D synthesis in the skin.

Vitamin D also comes from our diet (eggs, fish, and dairy products) as well as being produced in the skin. Skin production of the active form of vitamin D, (1,25[OH]D), however, depends on exposure to sunlight. Active people living in sunny regions produce most of the vitamin D they need from their skin. In less sunny climes the skin production of vitamin D is markedly diminished in the winter months, especially among the elderly and the housebound. In that population, vitamin D supplements may be needed.

Vitamin D deficiency has also been suggested to explain some of the geographic variation in the incidence of COVID-19. In May, an article was published in Internal Medicine News entitled: “Vitamin D: A Low-Hanging Fruit in COVID-19?” Like in most articles that have titles ending with a “?”, however, the answer is “it’s complicated” and “it’s too early to know.”

Exercise caution. As is true in many areas of medicine, the COVID-19 pandemic has stirred up research interest in the fields of exercise immunology and in the health benefits of sunlight (vitamin D). Although many questions remain unanswered, all experts agree that SARS-CoV-2 is most infectious when people are in prolonged contact at close range, especially indoors, and even more so in superspreader events. But exercising outdoors in the sunshine—while at the same time practicing proper distancing and wearing a mask if you’re likely to encounter others—is good not only for your physical and mental health but also for your immune system. (For those of you in climates where you cannot safely exercise outdoors during the summer months because it’s too hot, use the same common sense noted above and get your body moving indoors.)

It seems to me that the famous 19th century British-American actress Lilly Langtry had it right. She captivated London Society and caught the eye and stole the heart of the Prince of Wales. I’m not sure of her physical exercise routine, but she certainly exercised good judgment when she remarked: “Anyone’s life truly lived consists of work, sunshine, exercise, soap, plenty of fresh air, and a happy contented spirit.”

139 views2 comments


Jul 08, 2020

may I remind my learned colleague that every person who's walked in the sunshine has sooner or later passed away:) my lawn chair awaits!


Jul 08, 2020

Thanks Phil - couldn’t agree more!


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Main Page images courtesy of Shuxian Hu, MD. Dr. Hu is a scientist in the Neuroimmunology Research Laboratory at the University of Minnesota.

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