Vitamin D has become more widely discussed in recent years. This is largely because of its known role in supporting mental well-being, immunity, and skeletal health. It’s gained even more attention since the introduction of COVID-19, which has highlighted the widespread deficiency of this very important nutrient. Vitamin D deficiency is now recognized as a pandemic itself.1
What is vitamin D?
Vitamin D is a fat-soluble vitamin, alongside vitamins A, E, and K. This means that, unlike water-soluble vitamins, vitamin D doesn’t dissolve in water and chemically acts more like an oil. It’s also better absorbed by your body when consumed with dietary fats.
There are two types of vitamin D. The first is vitamin D2, or ergocalciferol, which is made by plants. The other is vitamin D3, or cholecalciferol, which is produced by animals. There are some differences in how well these are utilized by the body, discussed later in this article.
The major roles of vitamin D as a whole are in maintaining bone strength, regulating calcium metabolism, and increasing its absorption, supporting gut integrity, and helping your immune system stay strong.2,3 In fact, vitamin D acts as a type of hormone that plays a role in immunity. Getting enough of it may also be beneficial for people looking to lose weight or who have high blood pressure.4,5
Not getting enough vitamin D has been linked to a number of health problems, including an increased risk for chronic diseases, like heart disease and certain cancers.6 Commonly, deficiency presents as bone loss and increased susceptibility to infection or illness.7,8 There is also evidence of vitamin D deficiency often being present in the case of mental health issues like depression.9
Some vitamin D is inherently produced by your skin when exposed to direct UVB sunlight. This exposure triggers your skin to convert cholesterol into the active form of vitamin D3.10 This occurs through a process of metabolizing vitamin D into 25-hydroxyvitamin D3 in your liver, and ultimately into 1-alpha, 25-dihydroxyvitamin D3 in your kidneys, before it can be effectively utilized by your body.11
While spending time outside in the sunlight is a convenient way to boost vitamin D stores, there are a number of factors involved in how much is produced. For instance, skin pigmentation, age, sex, and where you live geographically can either increase or inhibit how much vitamin D your skin is able to make.12
Vitamin D and the immune system
Perhaps the role of vitamin D that’s most relevant to current events is in immunity. In particular, having enough vitamin D in your body seems to be helpful in preventing and lessening severity in respiratory infections, like COVID-19, for some people.
For instance, a 2017 meta-analysis published in BMJ, which included 25 randomized controlled trials involving over 11,000 participants ages 0-100 years, examined the relationship between vitamin D status and susceptibility to acute respiratory infections.13
The authors found that supplementation of vitamin D was successful in reducing risk for acute respiratory infections among all of the participants. The benefit appeared to be even more amplified among those who started out deficient in vitamin D.
Other research shows that vitamin D likely plays a role in linking your innate and adaptive immunity. This means that not only does it help optimize your body’s initial immune response to potential threats but may also be important for T-cell function involved in your learned, prolonged immune response. On the other hand, vitamin D deficiency may hinder both of these functions.14
A 2020 review published in the journal Nutrients concluded that having vitamin D deficiency is also linked to a higher risk for developing immune-related disorders. Some of these included psoriasis, type 1 diabetes, sepsis, rheumatoid arthritis, tuberculosis, multiple sclerosis, respiratory infection, and even COVID-19.15
Why vitamin D matters for COVID-19
While there’s still a lot to learn about the roles of specific nutrients in COVID-19, a correlation between vitamin D status and severity of the infection has emerged over the past year. In particular, the association between vitamin D deficiency and risk for acute respiratory illness has been further demonstrated during this pandemic.
An August 2020 study found that 81% of the 42 study participants who required admission to the intensive care unit with severe COVID-19 infections also had vitamin D deficiency.16 While this is a small study population, the statistics suggest a nutritional connection worth further investigation.
This relationship was also examined in an observational study published in November 2020. Researchers compared vitamin D status of people with active but asymptomatic COVID-19 infections to those admitted to the ICU with severe illness. While approximately one-third of the asymptomatic patients were deficient, 97% of the severely ill patients were deficient. The latter group also showed higher inflammatory markers.17
Furthermore, a higher death rate was seen among those patients with severe illness and vitamin D deficiency. The authors concluded that there should be a push for mass vitamin D supplementation among individuals at highest risk for severe COVID-19 infection.
This opinion is shared by many other scientists. A September 2020 statement from researchers on the subject was published in the Journal of Global Antimicrobial Resistance. It says that more studies are needed on the direct effect of vitamin D and COVID-19. But in the meantime, the enormous public health impact of the pandemic and the low risk for adverse effects of vitamin D supplementation should be enough to support mass use.18
The authors also note that vitamin D deficiency is an easily modifiable risk factor for acute respiratory infections, as seen through previous research, and that even a small reduction in COVID-19 from supplementation would make this intervention justifiable. Deficiency is widely observed among the elderly population, who are among the highest risk for severe COVID-19 outcomes.19
The specific mechanisms by which vitamin D may influence COVID-19 outcomes is yet to be determined. Some scientists believe that vitamin D is involved in certain pathways that increase respiratory severity of COVID-19 infections.
In a September 2020 comment published in the journal Clinical Medicine (London), scientists from Ghent University Hospital suggest that vitamin D binding protein (VDBP) may play a role in severe respiratory outcomes from this virus. The presence of VDBP has been found in the lung fluid of patients hospitalized with acute respiratory distress syndrome (ARDS). Based on similarities and hypothesized mechanisms, they recommend further investigation into this specific protein for COVID-19.20
How much vitamin D should you get?
It’s estimated that approximately 50% of people are vitamin D deficient.21 This is likely because most of us don’t spend enough time in direct sunlight consistently, and many people live in areas where UV exposure is insufficient.
Plus, most foods aren’t good sources of vitamin D. So, it can require intentionality to get enough vitamin D in your diet. Additionally, research has found that obesity can increase the risk for deficiency, independent of latitude and individual age.22
Because vitamin D levels can vary so significantly between individuals, it’s a good idea to have your levels checked by your doctor before starting supplementation. Knowing your baseline levels will help make sure you’re supplementing appropriately, including whether you need a prescription megadose to correct severe deficiency or if your levels are normal as is.
A vitamin D level that falls below 30 nmol/L is generally considered a deficiency.23 Optimal vitamin D levels are generally at least 50 nmol/L, and appropriate supplementation, sometimes in high doses, may be necessary to reach this.24 More is not always better, as a blood level above 375 nmol/L (150 ng/mL) is considered toxic. 25
Deficiency is widespread, so if you can’t get your levels checked right away, starting a moderate supplement may be prudent.
Due to its fat-soluble nature, supplemental vitamin D comes with a risk of toxicity. The upper daily limit for vitamin D is 2,000 IU. Taking too much vitamin D over a long period can lead to dangerously high levels of calcium in your body. This increases your risk for developing kidney stones, becoming dehydrated, and even damaging your kidneys.26
Vitamin D from food sources and sunlight are more manageable for your body. Overall, regularly getting a combination of food, supplements, and sunshine should be adequate for keeping your serum vitamin D levels within recommended levels.
Dietary sources of vitamin D
Most foods that make up the Western diet pattern are poor sources of vitamin D. In fact, most people are only able to meet around 5-10% of their vitamin D needs from food.
That being said, some of the best food sources of vitamin D include the following:
- Fatty fish. Some of the most concentrated fish are salmon, herring, and mackerel. Canned fish that contain small, edible bones, like tuna or sardines, are also a good source of vitamin D.
- Fortified milks and breakfast cereals. This includes both dairy and plant-based milks fortified to contain vitamin D. Check the ingredient label to determine whether it’s been added and how much it provides.
- UV-treated mushrooms. Certain mushrooms can be a decent source of vitamin D, if they’ve been grown using UV light. This process converts a plant compound called ergosterol into vitamin D2.
However, most mushrooms found at the grocery store are traditionally grown in the dark. Note that if you find UV-treated mushrooms, their vitamin D concentration decreases with age and storage time so it’s best to consume before their best-by date. Some dried mushrooms have also been treated with UV.27-29
- Egg yolks. The amount of vitamin D in eggs depends on how much sunlight the chicken was exposed to and how much vitamin D their feed contained.30 The average egg yolk contains around 37 IU, or 5% of the DV for vitamin D. Free range chickens may produce eggs with 3-4 times the amount of vitamin D compared to conventionally raised chickens who spend their lives indoors. Some research also says chickens fed vitamin D-fortified feed can produce eggs that contain up to 6,000 IU.31
Still, unless you’re eating these foods on a daily basis, a more reliable and consistent source of vitamin D for most people is a dietary supplement.
Choosing a vitamin D supplement
When choosing a vitamin D supplement to add to your routine, it’s important to consider dosage, form, and quality.
Studies show that vitamin D3, or cholecalciferol, is more effective than D2 for increasing serum vitamin D levels. Plus, vitamin D2 supplements may be more likely to degrade over time due to a higher sensitivity to environmental factors like humidity and changes in temperature.32-34
One option for an everyday multivitamin that contains vitamin D is Daily Start. Three capsules per day will provide 250 IU of vitamin D, which may be ideal for people who get regular sun exposure and consume other vitamin D sources in their diet.
If you’re seeking a vegan supplement, note that while D2 is always vegan, it’s not well utilized. This leaves D3, which is typically derived from lanolin, the oil produced in sheep’s wool. However, some brands have started producing vegan D3, which are made using lichen.
Vitamin D for COVID-19 prevention and recovery
While vitamin D supplementation can be helpful year-round to help maintain optimal serum levels. This may be particularly important if you are in a population that is at higher risk of respiratory infections or deficiency.
For the purposes of everyday immunity, a supplement of at least 2,000 IU per day of vitamin D3 is generally considered prudent. Some scientists say that taking 5,000 IU per day for 3 months is more effective at optimizing vitamin D levels.35
If you know you’ve been directly exposed to Covid-19, speak to your doctor first. Some health professionals suggest that acutely supplementing up to 5,000 IU per day of vitamin D3 may be warranted to give your immune system a boost. However, this is above the tolerable upper limit for vitamin D, and therefore should only be used for a short period if not being taken under supervision of your physician.36,37
Interestingly, a small September 2020 case study published in the American Journal of Therapeutics evaluated the efficacy of vitamin D supplementation on outcomes of hospitalized COVID-19 patients. The authors discussed the effects of 4 vitamin D deficient patients being supplemented with either a moderate dose of 1,000 IU of vitamin D3 or a high dose of 50,000 IU of vitamin D2 daily for 5 days.38
The high dose patients experienced a quick recovery from COVID-19, including a shorter hospital stay, reduced oxygen requirements, and lower inflammatory markers. The researchers noted that therapeutic vitamin D supplementation could be helpful in preventing acute respiratory distress syndrome in patients admitted for treatment, especially when other resources are scarce.
This sentiment is shared by other researchers around the globe. For instance, in a July 2020 statement published in Mayo Clinic Proceedings, scientists in Slovenia recommend supplementing vitamin D to high risk populations. They also recommended supplementing s those already infected with COVID-19 to achieve optimal serum levels of the nutrient as soon as possible.39
In another randomized, placebo-controlled study out of India, 40 patients who were positive for mild or asymptomatic COVID-19 infections were given either a placebo or a very high dose of 60,000 IU of vitamin D3 per day for 7 days. The goal was to increase serum vitamin D levels to a minimum of 50 ng/mL, and intervention patients who did not reach this yet continued to receive the same daily high dose vitamin D3. Inflammatory markers were monitored.40
Most of the intervention patients achieved goal serum vitamin D levels by day 14 and experienced a reduction in the inflammatory biomarker fibrinogen. Overall, more initially deficient patients who achieved goal vitamin D levels experienced some degree of reduced inflammation compared to placebo. The authors concluded that correcting vitamin D deficiency may help improve COVID-19 outcomes for some patients.
Despite this research, the current stance of the National Institutes of Health and the World Health Organization is that there’s not enough data to support the use of vitamin D to prevent and help treat COVID-19.41 However, this appears to be based on studies that show vitamin D had a neutral or insignificant effect on health outcomes of patients.
Adequate vitamin D is beneficial for COVID-19
There is widespread evidence that deficiency seems to be associated with higher risk for severe COVID-19 outcomes, and that vitamin D deficiency is a global public health concern. Studies suggest that this particular deficiency likely contributes to a higher risk for immune-related diseases, including worsened outcomes for patients with COVID-19 infections.
Furthermore, controlled doses of vitamin D carry little risk for adverse effects, suggesting a lot of potential for its therapeutic use in the pandemic.
Whether you’re in a high-risk group for serious COVID-19 outcomes, or you’re looking for ways to boost immunity, your vitamin D status is a great factor to assess. Fortunately, vitamin D deficiency is preventable and easily treatable for most people, using a combination of sunlight, food, and supplements.
Get your vitamin D level checked at the doctor’s office so that you can supplement accordingly and correct a deficiency if needed. While more research is needed on vitamin D and COVID-19, maintaining ideal serum levels of this nutrient can only help.
- Holick MF, Chen TC. Vitamin D deficiency: a worldwide problem with health consequences. Am J Clin Nutr. 2008 Apr;87(4):1080S-6S. doi: 10.1093/ajcn/87.4.1080S.
- Ceglia L. Vitamin D and skeletal muscle tissue and function. Mol Aspects Med. 2008 Dec;29(6):407-14. doi: 10.1016/j.mam.2008.07.002.
- Battistini C, Ballan R, Herkenhoff ME, Saad SMI, Sun J. Vitamin D Modulates Intestinal Microbiota in Inflammatory Bowel Diseases. Int J Mol Sci. 2020 Dec;22(1):362. doi: 10.3390/ijms22010362.
- Mason C, Xiao L, Imayama I, Duggan C, Wang CY, Korde L, McTiernan A. Vitamin D3 supplementation during weight loss: a double-blind randomized controlled trial. Am J Clin Nutr. 2014 May;99(5):1015-25. doi: 10.3945/ajcn.113.073734.
- Vimaleswaran KS, Cavadino A, Berry DJ, Jorde R, Dieffenbach K, Chen L, Alves AC, et al. Association of vitamin D status with arterial blood pressure and hypertension risk: a mendelian randomisation study. The Lancet. 2014;2(9):719-729. doi:10.1016/S2213-8587(14)70113-5.
- Hewison M. An update on vitamin D and human immunity. Clin Endocrinol (Oxf). 2012 Mar;76(3):315-25. doi: 10.1111/j.1365-2265.2011.04261.x.
- Lips P, van Schoor NM. The effect of vitamin D on bone and osteoporosis. Best Pract Res Clin Endocrinol Metab. 2011 Aug;25(4):585-91. doi: 10.1016/j.beem.2011.05.002.
- Hewison M. Vitamin D and immune function: an overview. Proc Nutr Soc. 2012 Feb;71(1):50-61. doi: 10.1017/S0029665111001650.
- Anglin RE, Samaan Z, Walter SD, McDonald SD. Vitamin D deficiency and depression in adults: systematic review and meta-analysis. Br J Psychiatry. 2013 Feb;202:100-7. doi: 10.1192/bjp.bp.111.106666.
- Alshahrani F, Aljohani N. Vitamin D: deficiency, sufficiency and toxicity. Nutrients. 2013 Sep;5(9):3605-16. doi: 10.3390/nu5093605.
- DeLuca HF. Overview of general physiologic features and functions of vitamin D. Am J Clin Nutr. 2004 Dec;80(6 Suppl):1689S-96S. doi: 10.1093/ajcn/80.6.1689S.
- “Time for more vitamin D.” Harvard Medical School. Published Sept 2008. Available from: https://www.health.harvard.edu/staying-healthy/time-for-more-vitamin-d
- Martineau AR, Jolliffe DA, Hooper RL, Greenberg L, Aloia JF, Bergman P, Dubnov-Raz G, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017 Feb;356:i6583. doi: 10.1136/bmj.i6583.
- Hewison M. Vitamin D and immune function: an overview. Proc Nutr Soc. 2012 Feb;71(1):50-61. doi: 10.1017/S0029665111001650.
- Charoenngam N, Holick MF. Immunologic Effects of Vitamin D on Human Health and Disease. Nutrients. 2020 Jul;12(7):2097. doi: 10.3390/nu12072097.
- Carpagnano GE, Di Lecce V, Quaranta VN, Zito A, Buonamico E, Capozza E, Palumbo A, Di Gioia G, Valerio VN, Resta O. Vitamin D deficiency as a predictor of poor prognosis in patients with acute respiratory failure due to COVID-19. J Endocrinol Invest. 2020 Aug 9:1–7. doi: 10.1007/s40618-020-01370-x.
- Jain A, Chaurasia R, Sengar NS, Singh M, Mahor S, Narain S. Analysis of vitamin D level among asymptomatic and critically ill COVID-19 patients and its correlation with inflammatory markers. Sci Rep. 2020 Nov;10(1):20191. doi: 10.1038/s41598-020-77093-z.
- Zemb P, Bergman P, Camargo CA Jr, Cavalier E, Cormier C, Courbebaisse M, Hollis B, et al. Vitamin D deficiency and the COVID-19 pandemic. J Glob Antimicrob Resist. 2020 Sep;22:133-134. doi: 10.1016/j.jgar.2020.05.006.
- Ilie PC, Stefanescu S, Smith L. The role of vitamin D in the prevention of coronavirus disease 2019 infection and mortality. Aging Clin Exp Res. 2020 Jul;32(7):1195-1198. doi: 10.1007/s40520-020-01570-8.
- Speeckaert MM, Speeckaert R, Delanghe JR. Vitamin D binding protein in COVID-19. Clin Med (Lond). 2020 Sep;20(5):e136-e137. doi: 10.7861/clinmed.Let.20.5.2.
- Nair R, Maseeh A. Vitamin D: The “sunshine” vitamin. J Pharmacol Pharmacother. 2012;3(2):118-126. doi:10.4103/0976-500X.95506.
- Pereira-Santos M, Costa PR, Assis AM, Santos CA, Santos DB. Obesity and vitamin D deficiency: a systematic review and meta-analysis. Obes Rev. 2015 Apr;16(4):341-9. doi: 10.1111/obr.12239.
- “Vitamin D: Health Professional Fact Sheet.” Office of Dietary Supplements. NIH.gov. Updated 9 Oct 2020. Available from: https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/#h4
- Ross AC, Manson JE, Abrams SA, Aloia JF, Brannon PM, Clinton SK, Durazo-Arvizu RA, et al. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know. J Clin Endocrinol Metab. 2011 Jan;96(1):53-8. doi: 10.1210/jc.2010-2704.
- Ozkan B, Hatun S, Bereket A. Vitamin D intoxication. Turk J Pediatr. 2012 Mar-Apr;54(2):93-8. https://pubmed.ncbi.nlm.nih.gov/22734293/
- Galior K, Grebe S, Singh R. Development of Vitamin D Toxicity from Overcorrection of Vitamin D Deficiency: A Review of Case Reports. Nutrients. 2018 Jul;10(8):953. doi: 10.3390/nu10080953.
- Cardwell G, Bornman JF, James AP, Black LJ. A Review of Mushrooms as a Potential Source of Dietary Vitamin D. Nutrients. 2018 Oct;10(10):1498. doi: 10.3390/nu10101498.
- Jäpelt RB, Jakobsen J. Vitamin D in plants: a review of occurrence, analysis, and biosynthesis. Front Plant Sci. 2013 May;4:136. doi: 10.3389/fpls.2013.00136.
- Sławińska A, Fornal E, Radzki W, Skrzypczak K, Zalewska-Korona M, Michalak-Majewska M, Parfieniuk E, Stachniuk A. Study on vitamin D₂ stability in dried mushrooms during drying and storage. Food Chem. 2016 May;199:203-9. doi: 10.1016/j.foodchem.2015.11.131.
- Kühn J, Schutkowski A, Kluge H, Hirche F, Stangl GI. Free-range farming: a natural alternative to produce vitamin D-enriched eggs. Nutrition. 2014 Apr;30(4):481-4. doi: 10.1016/j.nut.2013.10.002.
- Yao L, Wang T, Persia M, Horst RL, Higgins M. Effects of vitamin D(3) -enriched diet on egg yolk vitamin D(3) content and yolk quality. J Food Sci. 2013 Feb;78(2):C178-83. doi: 10.1111/1750-3841.12032.
- Glendenning P, Chew GT, Inderjeeth CA, Taranto M, Fraser WD. Calculated free and bioavailable vitamin D metabolite concentrations in vitamin D-deficient hip fracture patients after supplementation with cholecalciferol and ergocalciferol. Bone. 2013 Oct;56(2):271-5. doi: 10.1016/j.bone.2013.06.012.
- Tripkovic L, Lambert H, Hart K, Smith CP, Bucca G, Penson S, Chope G, Hyppönen E, Berry J, Vieth R, Lanham-New S. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012 Jun;95(6):1357-64. doi: 10.3945/ajcn.111.031070.
- Houghton LA, Vieth R. The case against ergocalciferol (vitamin D2) as a vitamin supplement. Am J Clin Nutr. 2006 Oct;84(4):694-7. doi: 10.1093/ajcn/84.4.694.
- Diamond T, Wong YK, Golombick T. Effect of oral cholecalciferol 2,000 versus 5,000 IU on serum vitamin D, PTH, bone and muscle strength in patients with vitamin D deficiency. Osteoporos Int. 2013 Mar;24(3):1101-5. doi: 10.1007/s00198-012-1944-7.
- Simonson W. Vitamin D dosing considerations in COVID-19 [published correction appears in Geriatr Nurs. 2021 Jan 9;:]. Geriatr Nurs. 2020;41(5):648-649. doi:10.1016/j.gerinurse.2020.08.011.
- Institute of Medicine (US) Committee to Review Dietary Reference Intakes for Vitamin D and Calcium; Ross AC, Taylor CL, Yaktine AL, et al., editors. Dietary Reference Intakes for Calcium and Vitamin D. Washington (DC): National Academies Press (US); 2011. 6, Tolerable Upper Intake Levels: Calcium and Vitamin D. Available from: https://www.ncbi.nlm.nih.gov/books/NBK56058/
- Ohaegbulam KC, Swalih M, Patel P, Smith MA, Perrin R. Vitamin D Supplementation in COVID-19 Patients: A Clinical Case Series. Am J Ther. 2020 Sep/Oct;27(5):e485-e490. doi: 10.1097/MJT.0000000000001222.
- Siuka D, Pfeifer M, Pinter B. Vitamin D Supplementation During the COVID-19 Pandemic. Mayo Clin Proc. 2020;95(8):1804-1805. doi:10.1016/j.mayocp.2020.05.036.
- Rastogi A, Bhansali A, Khare N, et al. Short term, high-dose vitamin D supplementation for COVID-19 disease: a randomised, placebo-controlled, study (SHADE study) Postgraduate Medical Journal. doi: 10.1136/postgradmedj-2020-139065.
- “Can vitamin D protect against the coronavirus disease 2019 (COVID-19)?” Mayo Clinic. Published 10 Feb 2021. Available from: https://www.mayoclinic.org/coronavirus-and-vitamin-d/expert-answers/faq-20493088