Vitamin D is not an essential nutrient, because the human body can actually manufacture it when the skin is exposed to the sun’s ultraviolet rays. And yet, in more than 20 years of testing patients I have found that most people are deficient in vitamin D.

We simply do not get enough sun exposure to maintain optimal vitamin D levels. And older people make less vitamin D in their skin than younger people.

In addition, people with darker skin require more sun exposure to produce vitamin D than those with lighter skin. This is especially problematic for darker skinned people living in Northern climates.

Vitamin D is required for proper immune system functioning. Vitamin D receptors are found on immune cells such as B and T cells, which are needed to not only fight viral and bacterial infections but also to provide immunity against them.

Low vitamin D levels have been correlated with higher COVID-19 morbidity and mortality rates. And a review article found that vitamin D can reduce risk of infections through several mechanisms.

“Vitamin D has been shown to lower viral reproduction rates and reducing concentrations of pro-inflammatory cytokines that produce the inflammation that inures the lining of the lungs leading to pneumonia, as well as increasing concentrations of anti-inflammatory cytokines,” wrote the authors.

The RDA for vitamin D is 600 IU per day for adults. But at the first sign of a viral illness, I ask patients to take 50,000 IU of vitamin D3 per day for four days.