There is a lot of talk about vitamin D and its potential link to a variety of chronic illnesses. This raises the possibility that vitamin D deficiency could possibly cause a variety of health problems and that taking vitamin D to bring our level to normal could possibly prevent those same health problems. The difficulty is that we just don't have the scientific evidence to tell us if that is the case.

How Did Interest in Vitamin D Begin?

It is estimated that 1 billion people in the world now have vit D deficiency which is attributed to modern lifestyle that has restricted sunlight exposure. (1) While we have long known that vit D deficiency can cause rickets and exacerbate osteoporosis due to its impact on calcium absorption, there is now observational data that suggests an association between normal vit D levels in the range of 40-60 ng/ml and a decreased mortality, as well as reduced likelihood of a number of chronic illnesses including cancer, coronary heart disease, type 2 diabetes, depression, and certain neurological diseases. (2)

How do we get vitamin D?

Vitamin D is produced by skin exposure to sunlight in as little as 5 minutes but varies based on a persons skin tone as well as geographical location. Vitamin D found in food comes in two forms, vitamin D2 found in plants, especially mushrooms, and vitamin D3 found in animal sources, such as cold water fish.

What is Vitamin D?

Vitamin D is a family of compounds in the body that occur in varying forms. The major form existing in the blood gets converted into an active form by the liver and kidneys. This active form acts as a hormone in the body helping to regulate levels of calcium, magnesium, and phosphorus, making it essential for bone health. Vitamin D has also been shown to influence the growth and differentiation of many other tissues and to help regulate the immune system. These other functions have implicated vitamin D in other disorders, such as autoimmunity and cancer.

Vitamin D supplements can be of either form, but the D3 form is preferred since it is far more effective in raising low levels than vit D2. Although short 5-10 minute periods of unprotected sunlight exposure twice weekly are probably sufficient to get enough vit D, the American Academy of Dermatological recommends avoidance of sun exposure to reduce skin cancer risk.

The best source of naturally occurring vit D is cold water fish: salmon, tuna, mackerel, herring, and sardines.

There are a variety of foods that are fortified with vitamin D2 such as milk, orange juice, and soy milk, but this fortification is probably far less effective than eating fish or taking cod liver oil. Finally supplementation with vitamin D3 capsules taken with a fatty meal is another common way to increase one's vitamin D levels in the blood.

How do I know if I need vitamin D supplementation?

A blood test will determine if someone is deficient in vitamin D. Those most at risk are the elderly, those with darker skin, or anyone who has limited exposure to sunlight.

The Endocrine Society defines vit D deficiency as a blood level < 20 ng/ml while insufficiency is 21-29 ng/ml. While they do not advocate for testing those not at risk for deficiency, their guidelines for vit D have not been revised since 2011, so are perhaps a bit dated. (3)

The U.S. Institute of Medicine has concluded that a 25-hydroxyvitamin D level of 20 ng/mL or above is adequate for bone health, and that levels greater than 30 ng/mL do not offer significantly better effects, while levels above 50 are associated with risks from hypercalcemia (high calcium levels in the blood) and increased risk of cancer and all cause mortality. (4) This means that we do not have data to show that taking vitamin D supplementation or cod liver oil to raise low vitamin D levels into the normal range will offer benefit in preventing the chronic illnesses currently felt to be associated with low levels. Therefore, choosing to test one's level or taking supplemental vitamin D or cod liver oil should be an individual decision best made through discussion with your primary care physician, factoring in your personal risk factors.

What about vitamin D and Covid-19?

Because the vitamin D receptor is expressed on immune cells such as B cells, T cells, and antigen-presenting cells, and these cells can synthesize the active vitamin D metabolite, vitamin D has the potential to modulate immune responses. (5) In the US vitamin D deficiency is more common among Hispanics and African Americans, both groups which seem to be at a higher risk of Covid-19. Likewise, vitamin D deficiency is more common in the elderly and those with obesity or hypertension, and all of these groups appear to have a higher complication rate from Covid-19. We know that taking vitamin can increase T cells and their activity (part of our immune system) and a meta-analysis of randomized controlled trials suggests that vitamin D supplementation can reduce the odds of getting respiratory tract infections, but whether this will apply to Covid-19 we simply do not know at this time. There are observational studies currently being conducted to determine the role of vitamin D in both preventing as well as treating Covid-19. Information on these clinical trials can be found at ClinicalTrials.gov

References:

Charoenngam N, Shirvani A, Holick MF. Vitamin D for skeletal and non-skeletal health: What we should know. J Clin Orthop Trauma. 2019 Nov-Dec;10(6):1082-1093 https://pubmed.ncbi.nlm.nih.gov/31708633/
Vitamin D and chronic diseases: the current state of the art https://doi.org/10.1007/s00204-016-1804-x
https://www.endocrine.org/clinical-practice-guidelines/vitamin-d-deficiency
https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/#disc
Aranow C. Vitamin D and the immune system. J Investig Med. 2011;59(6):881-886. Available at: https://www.ncbi.nlm.nih.gov/pubmed/21527855.
Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583. Available at: https://www.ncbi.nlm.nih.gov/pubmed/28202713.
https://clinicaltrials.gov/ct2/results?cond=COVID&term=vitamin+d&cntry=&state=&city=&dist=

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