Here’s something you won’t hear about in the news or from most mainstream doctors—COVID-19 has been added to the long list of ailments that are closely connected to a vitamin D ­deficiency. Because of its important role in immune system ­functioning, having a vitamin D deficiency leaves you more vulnerable to a wide swath of diseases and ­disorders, including heart disease, type 1 diabetes, multiple sclerosis, rheumatoid arthritis, respiratory infections, inflammatory bowel disease, breast and colon cancer and, we’ve now learned, COVID-19. 

Studies done here and abroad since the start of the pandemic have found that the risk of getting the virus and of dying from it is far greater among people with suboptimal levels of vitamin D. Those at highest risk for COVID-19—the elderly, people who are obese and people of color—also are at greater risk of being vitamin D–deficient. Shoring up your vitamin D level is a simple, inexpensive and safe ­strategy to stay healthy. Leading ­vitamin D ­researcher ­Michael F. Holick, PhD, MD, explains… 

Vitamin D: Why Levels Matter

Initially vitamin D is processed in the liver, where it is ­converted to its major circulating form, 25-­hydroxyvitamin D. It is then activated in the kidneys to 1,25-­dihydroxyvitamin D, which is responsible for regulating bone health. 25-hydroxyvitamin D also is activated in many cells in the body including immune cells. This active form exerts a strong influence not only on regulating cellular growth to reduce risk for malignancies—it also a has modulating effect on the immune system, reducing risk for autoimmune diseases such as multiple sclerosis. And it ­decreases inflammatory activity, thereby reducing risk for cardiovascular disease. In addition, the locally activated vitamin D helps the immune cells fight bacterial and viral infections. There are vitamin D receptors (proteins that bind to activated vitamin D) in cells throughout the body, such as in the prostate, breasts, skin, intestinal tract, blood vessels and brain as well as immune cells. The activated vitamin D has the ­ability to exert a positive influence on many aspects of your health.

According to the Endocrine ­Society, vitamin D status is broadly described in one of three ways…

Deficiency: A blood level of ­25-hydroxyvitamin D below 20 nanograms per milliliter (ng/mL) causes bone disease and increases risk for many chronic illnesses.

Insufficiency: A blood level between 21 ng/ml and 29 ng/mL—the minimum for bone health but lacking what is needed for other important benefits.

Sufficiency: A blood level of 30 ng/mL to 100 ng/mL.

However, with new ­information regarding COVID-19 as well as ­other health threats, it may be better to aim for an optimal blood level between 40 ng/mL and 60 ng/mL, which is considered the ­preferred range.

Vitamin D and COVID-19: What We Know

A number of studies have looked at the connection between COVID-19 and vitamin D. In terms of prevention, at Boston University School of Medicine, we, in collaboration with Quest Diagnostics, did a retrospective ­observational analysis of blood samples from 192,000 people from all 50 states representing all ethnicities, sexes and ages, including the ­elderly. Findings showed a steady decline in infectivity as blood levels of 25-hydroxyvitamin D increased. We observed a 54% higher COVID-19 positivity rate among patients whose blood levels were less than 20 ng/mL compared with patients who had blood levels of at least 55 ng/mL. 

One study found that those with sufficient vitamin D were 51.5% less likely to die from COVID-19. We also know that vitamin D can help to reduce the excessive immune responsiveness known as the ­“cytokine storm” that some COVID-19 patients experience. 

How Much D Do You Need?

Guidelines set by the National Academy of Medicine for vitamin D suggest 600 international units (IU) daily up to age 70 and then 800 IU. But these amounts are based only on what’s needed to avoid a deficiency and maintain good bone health. This amount is not enough to provide adequate defense against COVID-19 and other health issues.

Research at Boston University showed that to achieve the optimal level (25-hydroxyvitamin D of 40 ng/mL to 60 ng/mL in people at a healthy weight), it takes 4,000 IU to 6,000 IU a day for most adults and teens and 1,000 IU to 2,000 IU for children, depending on age. Surprisingly, even infants and children need vitamin D supplementation. 

Finally, it is very difficult to reach unsafe levels of serum 25-­hydroxy-vitamin D. We did a study in which all participants were given the equivalent of 3,000 IU daily and followed for six years. Even those who started out as sufficient were still within a safe zone of 50 ng/mL to 80 ng/mL, far from levels that would be of concern for vitamin D toxicity (above 150 ng/mL). We even have tested giving a supplement of up to 10,000 IU per day to healthy adults for six months with no adverse effects.

Vitamin D Sources: Finding the Right Mix

About 80% of people with COVID-19 are vitamin D–­deficient—and the numbers in the general population could be just as high—meaning that everyone can benefit from getting more vitamin D. It’s extremely hard to get significant amounts of vitamin D from food. You can get some from truly wild salmon (about 600 IU in a four-ounce serving), sundried mushrooms (amounts vary by mushroom type and drying process) and cod liver oil (1,360 IU per tablespoon). While some dairy products and juices are D-fortified, one cup of milk or vitamin D–­fortified orange juice supplies around 100 IU to 120 IU. 

Even most people who live in sunny areas are deficient in ­vitamin D because they wear sunscreen or avoid sun exposure at the best time for vitamin D production (between 10 am and 3 pm). All this means that a high-­quality over-the-counter supplement is the best way to get your daily vitamin D requirement. Supplements are available as vitamin D-3 and, for vegetarians/­vegans, vitamin D-2. I recommend that children get 1,000 IU daily…teenagers and adults, at least 2,000 IU daily…and to achieve maximum benefit, I have most of my patients on 3,000 IU to 5,000 IU daily. I personally take 6,000 IU daily.