How sunlight, the immune system, and Covid-19 Interact
For thousands of years, humans have recognized that the sun plays a role in the emergence and transmission of viruses.
Vitamin D is an essential nutrient, which means the human body needs it but can’t make it. While some foods contain vitamin D, people have traditionally gotten most of their vitamin D from the sun: When exposed to ultraviolet light, a chemical reaction takes place in the skin that results in the production of vitamin D.
For a just-published study in the journal Aging Clinical and Experimental Research, researchers examined the average vitamin D levels among residents of different European countries. They found a correlation between low vitamin D levels and higher rates of Covid-19 infections and — even more so — Covid-19 deaths.
“Previous studies have shown that vitamin D protected against acute respiratory tract infection overall, and older adults — the group most deficient in vitamin D — are also the ones most seriously affected by Covid-19,” says Petre Cristian Ilie, PhD, co-author of the study and a research director at the Queen Elizabeth Hospital in the U.K. “Our finding was that getting vitamin D levels into the normal range might help.”
Ilie says there are several mechanisms by which vitamin D could counteract Covid-19. First, vitamin D enhances the expression of an enzyme called angiotensin-converting enzyme 2, or ACE2. “Previous studies identified associations between higher levels of ACE2 and better coronavirus disease health outcomes,” Ilie says, adding that, in the lungs, ACE2 has demonstrated the ability to protect against acute lung injury.
What explains the vitamin D discrepancies among these countries? The authors of that 2019 study point out that diet, behavior, clothing choices, and skin color all affect vitamin D status. The darker a person’s skin, the more sun they require to make vitamin D. If relatively dark-skinned Europeans in countries such as Spain and Italy avoid the sun, slather on sunscreen, and wear clothing that covers much of their body, this could partly explain why their vitamin D levels may be lower than those of the Portuguese. That study also points out that the predominantly light-skinned residents of Nordic countries need relatively little sun to produce vitamin D. They also tend to eat diets rich in cod liver oil and other seafood sources of vitamin D, and many foods in those countries are fortified with vitamin D — all of which could explain that population’s enviable blood levels of the vitamin.
Also noteworthy, though highly speculative: African Americans tend to have much lower levels of vitamin D than white Americans. Some researchers have posited that, in addition to long-standing racial and socioeconomic factors, these vitamin D discrepancies could partly explain why black Americans are at greater risk than whites for heart disease, certain forms of cancer, and other diseases linked to vitamin D deficiencies. Black Americans have also suffered disproportionately from Covid-19.
“Right now, we know that 17% of African Americans have levels of vitamin D below 10 ng/ml, which virtually everyone agrees is seriously deficient,” says Walter Willett, MD, PhD, a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Medicine.
Willett has studied vitamin D and human health extensively. He says it’s possible, though far from proven, that vitamin D shortfalls may help partially explain the Covid-19 imbalances that are showing up between white and black Americans. He says it’s also possible that a vitamin D supplement may provide some protection against Covid-19 for those Americans who are deficient — which may be most of the population. While there isn’t broad expert consensus on what constitutes “normal” or “low” when it comes to vitamin D in the human body, a 2018 study concluded that up to 40% of U.S. adults may be deficient in the vitamin.
Ilie also says vitamin D has “multiple roles” in the immune system that may strengthen its ability to repel Covid-19. One example: Low levels of vitamin D seem to impair the development of macrophages — white blood cells that eat invading pathogens, including viruses. He says vitamin D also helps prevent inflammation from running amok. Furthermore, there’s evidence that the low vitamin D levels are associated with immune system–related dysfunction and disease.
Although his study did not look specifically at sun exposure, Ilie says sunlight is a natural source of vitamin D. His analysis partly relied on a 2019 study from the European Journal of Endocrinology finding, somewhat counterintuitively, that older adults living in Portugal tend to have much higher vitamin D levels than the same demographic in neighboring Spain, and that older adults in the Nordic countries tend to have higher levels than those living in Italy and other sunnier Southern European countries. While both Spain and Italy have been hit hard by Covid-19, Portugal and the Nordic countries have thus far experienced relatively light rates of death and infection.