We need to talk about vitamin D & COVID-19
It could save your life.
We need to talk about vitamin D, and why it’s the vitamin that might just get us through the SARS-Cov-2 (COVID-19) pandemic.
We’ve known for quite some time now that people with low vitamin D levels are more likely to contract COVID-19, require admission to the intensive care unit (ICU), and die from the disease.,,,,,,
Since January 2020, over 940 peer-reviewed articles have been published on the connection between vitamin D and COVID-19. The majority of evidence on the topic confirms that vitamin D is one of the most important nutritional influences on COVID-19 risk.
Correlating blood vitamin D levels and COVID-19 test results
Even before SARS-CoV-2 came along, we’ve known that vitamin D status influences respiratory health outcomes. Low vitamin D intakes significantly increase the risk of respiratory infections like influenza and pneumonia, and low vitamin D levels are also linked with more severe cases of these infections.,,, Fortunately, vitamin D3 supplementation was found in a 2013 meta-analysis to lower the risk of acute respiratory infections by over 30%.
A study published in September 2020 was among the first to illuminate the connection between blood vitamin D levels and SARS-CoV-2 infection risk. Considering over 190,000 patients from all 50 states of the USA, the authors looked at the patients’ vitamin D blood levels from the preceding 12 months and their recent COVID-19 test results. (Vitamin D is assessed by a blood test that measures the circulating levels of the vitamin D metabolite 25-hydroxy-vitamin D, or 25(OH)D for short.)
The data showed a significant correlation between the patients’ 25(OH)D blood levels and their COVID-19 test results. Specifically, of the patients with 25(OH)D levels of less than 20 ng/ml, 12.5% tested positive for COVID-19. Patients with higher (but still sub-optimal) 25(OH)D levels of 30-34 ng/ml fared better: 8.1% of them tested positive. Those with adequate 25(OH)D levels of over 55 ng/ml did the best: only 5.9% tested positive for COVID-19.
The study confirms that people with low vitamin D levels are more likely to test positive for COVID-19. But how do these folks fare in the face of the illness?
Not so well, as it turns out.
Vitamin D status and disease severity
Studies show that vitamin D deficiency can not only increase the risk of testing positive for COVID-19, but also the chance of enduring a more severe course of disease – and even of dying from the illness.
One such study – a November 2020 prospective observational study – examined adults ages 30 to 60 years with COVID-19 infections. Of the 154 patients followed, 91 were asymptomatic, while 63 were so ill they required care in a hospital intensive care unit (ICU).
Blood levels of 25(OH)D were measured in all patients, along with some markers of inflammation (serum IL-6, TNFα, and ferritin). The test results revealed that while 32.96% (29 of 91) of the asymptomatic COVID-19 patients had vitamin D deficiency, a whopping 96.82% (61 of 63) of those in the ICU were deficient in vitamin D. The blood markers of inflammation were also observed to be higher in the patients with low vitamin D levels.
Perhaps unsurprisingly, the low vitamin D and high inflammation levels were found to be correlated with increased mortality (death) rates from COVID-19. The fatality rate of vitamin D-deficient patients was 21%, versus only 3.1% in those with adequate levels. That’s a nearly seven-fold difference.
The authors summarize their findings with these words: “Vitamin D level is markedly low in severe COVID-19 patients. Inflammatory response is high in vitamin D deficient COVID-19 patients. This all translates into increased mortality in vitamin D deficient COVID-19 patients. As per the flexible approach in the current COVID-19 pandemic authors recommend mass administration of vitamin D supplements to population at risk for COVID-19.”
Nor is this the first study to demonstrate a connection between vitamin D deficiency and COVID-19 risk: The vast majority of reviews on the topic have confirmed that vitamin D deficiency increases the risk of severe COVID-19-related illness and death.,,,,,,,
Can taking a vitamin D3 supplement help?
A May 2021 meta-analysis of studies considered data from 532 patients hospitalized with COVID-19. Of these patients, those who took vitamin D3 supplements were 64% less likely to need ICU care than those who didn’t take the vitamin.
Another study – a population study of roughly 4.6 million people – shed further light on the matter. In the study, the health outcomes of adults taking vitamin D3 supplements were compared to the health outcomes of those who didn’t take the vitamin.
Supplementing enough vitamin D3 to raise 25(OH)D blood levels to an adequate level yielded significantly better outcomes. Specifically, patients on calcifediol (vitamin D3) supplements who achieved blood 25(OH)D levels ≥30 ng/ml had lower risk of SARS-CoV2 infection, lower risk of severe COVID-19, and lower COVID-19 mortality rates than the vitamin D-deficient patients who did not take vitamin D3 supplements.
Other study findings corroborate this one. In fact, an October 2021 review on the topic explains:
“There is a great deal of evidence that hypovitaminosis D [low vitamin D] is an independent (and easily modifiable) risk factor for severe forms of COVID-19 and death. Vitamin D supplementation is a simple, safe and inexpensive measure, which is effective in correcting hypovitaminosis D… [Low vitamin D affects] more than 80% of adults with COVID-19.”
What if somebody is already in the hospital?
A 2020 pilot study of COVID-19 patients admitted to the hospital for their condition studied the effects of vitamin D3 supplementation in those already sick with COVID-19. Specifically, the researchers considered a preparation of vitamin D3 known as calcifediol (also known as calcidiol, 25-hydroxycholecalciferol, or 25-hydroxyvitamin D3), a form of vitamin D that’s produced in the liver.
While all of the patients received regular treatment as indicated by the hospital’s protocols, a portion of the COVID-19 patients also received treatment with high dose oral calcifediol. These patients did remarkably better than the patients who did not receive calcifediol: Only 1 out of 50 patients (2% of) in the calcifediol group required ICU admission, and not a single one died. Of the non-calcifediol group, 13 of 26 patients (50%) required admission to the ICU, and two died. The administration of calcifediol thus reduced the chance of ICU admission by 93%.
Why hasn’t the NIH recommended vitamin D3 supplementation yet?
While the majority of studies suggest that low vitamin D levels increase the risk of getting sick with and dying from COVID-19, and that vitamin D3 supplementation can reduce those risks, the National Institutes of Health (NIH) has not yet recommended that people supplement with vitamin D3 in the face of the SARS-CoV-2 pandemic.
While larger clinical trials will help confirm the findings of the data thus far, what’s the harm in taking vitamin D3 in the meantime? We’ve known for a long time that vitamin D3 helps with a variety of health conditions – it has been shown to reduce the risk of respiratory infections, increase bone density, fight depression, lower the risk of some cancers, and even act as a hormone in the body.,,,,
We’ve known for a long time that vitamin D can do a lot of good for our health.
An open letter from a group of 220 experts urges us to consider the potentially life-saving benefits of taking vitamin D3 supplements: “Evidence to date suggests the possibility that the COVID-19 pandemic sustains itself in large part through infection of those with low vitamin D, and that deaths are concentrated largely in those with deficiency.”
Furthermore, a 2021 statement from the Spanish Society of Geriatrics and Gerontology argues: “…in an ideal world, health decisions must be made based on overwhelming evidence, but a time of crisis such as the current one may require a slightly different set of rules.”
Indeed: we have been playing by a different set of rules since the pandemic began. We have been urged to exercise “an abundance of caution” and to err on the side of protection whenever possible.
So why don’t the Powers-That-Be suggest that we all get our vitamin D 25-hydroxy levels checked and pick up a bottle of D3 from the local pharmacy or health food store while we wait for more data? They’ve recommended that we (and our kids) all get an experimental vaccine that was rushed to the market – why not add in a vitamin that has been around for decades, a vitamin with a well-known and favorable safety profile, at that?
I do not know.
But it has been suggested that public health officials are concerned that the public won’t follow their other recommendations – like the orders to avoid indoor gatherings, wear masks, and get vaccine boosters – if people think that vitamin D3 alone could spare them from COVID-19.
Personally, I think people are smart enough to handle taking a vitamin and follow other measures to slow the spread of SARS-CoV-2. It’s time we stopped believing in magic bullets and used all of the resources available to help us survive this pandemic.
Guidelines for vitamin D testing and supplementation
Vitamin D levels can be tested with a simple blood test known as “vitamin D 25-hydroxy,” or “25(OH)D” for short.
In my practice, I typically like to see my patients’ 25(OH)D levels well above 30 ng/ml – ideally between 55 and 80 ng/ml. (You should of course always check with your provider to determine what’s right for you.)
Note that there is such a thing as too much vitamin D – it can lead to kidney stones, constipation, and even psychological changes, so I strongly discourage people from striving for high 25(OH) levels or taking high doses of the vitamin for prolonged periods of time without periodic blood tests.
That being said, 1,000 to 2,000 international units (IU) of vitamin D3 per day is typically a reasonable dose for most adults; 400 to 800 IU per day is likely safe for most babies and young children. (Note: vitamin D2 is not useful in these contexts; D3 is the appropriate form of the vitamin.)
When we consider that an analysis of data from the National Health and Nutrition Examination Surveys (NHANES) showed that 95% percent of adults who don’t take vitamins or supplements had vitamin D intakes below the estimated average requirement, it seems like a no-brainer to consider vitamin D3 supplementation for most individuals at this crucial time.
What if my doctor won’t order the test?
If your healthcare provider won’t order the Vitamin D 25-hydroxy [25(OH)D] test, don’t despair: you have options.
First off, you can look for another healthcare provider who will order the test.
Unfortunately, many health insurance plans in the USA are finnicky about paying for vitamin D tests. I suggest asking your ordering provider to put the ICD-10 code K90.9 (intestinal malabsorption, unspecified) on your order form: I’ve had good luck in getting that code covered.
If you need to go rogue, you absolutely can. Direct-to-consumer lab companies like AnyLabTestNow can be helpful. You may also be able to order your own testing and pay out of pocket through Ulta Labs depending on your location. (As of this writing, the cost through Ulta is $39 USD, plus an $8 blood draw fee).
Other companies also offer a home test where you poke your finger to collect a sample, which you then mail to a processing lab.
Can’t I just get vitamin D from the sun?
Our bodies make vitamin D when UVB radiation from the sun makes contact with our bare skin (that means skin without sunscreen). Many variables can affect the efficiency of that production, however – factors like latitude, season, time of day, cloud cover, and air pollution. Skin melanin content (race) and age also influence the body’s vitamin D production.,, Fortunately, vitamin D can also be taken in supplement form, as vitamin D3.
An October 2021 meta-analysis of studies relating vitamin D and COVID-19 found that a low vitamin D level is a critical risk factor for COVID-19 infection.
The analysis looked at the daily weather patterns and COVID-19 rates in 26 European countries, finding that low temperature, low UV index, and high number of cloudy days were all correlated with SARS-CoV-2 infection prevalence. In other words: the colder the weather, the less sun, and the more clouds, the more pandemic illness.
Fortunately, the study findings suggest that vitamin D supplementation may protect against COVID-19.
Because the skin’s absorption of UVB is so variable, and since too much sun exposure can damage the skin and lead to skin cancer risk, even people living in sunny climates may want to get their blood levels checked and take vitamin D3 in supplement form.
Aging can drop the skin and body’s ability to synthesize vitamin D by more than half, and the digestive changes that typically accompany aging can further decrease absorption of the vitamin (and other nutrients) from food. Supplementation may therefore be especially important in those above the age of 60, as SARS-CoV-2 hits seniors particularly hard.
People of color are also at increased risk of low vitamin D status, as melanin blocks the skin’s absorption of UVB, thus hindering the body’s production of vitamin D. This could be one piece of the COVID-19 disparity puzzle.,
In summary, studies have shown that people with low vitamin D levels are more likely to test positive for COVID-19, more likely to require admission to the intensive care unit (ICU) for treatment, and more likely to die from the disease.
Vitamin D3 supplementation is a cheap, easy, and likely effective way to reduce the risk of COVID-19-related infection, severe illness, and death. Using the vitamin D 25-hydroxy [25(OH)D] blood test can help guide dosage of vitamin D3 supplements.
Vitamin D3 supplementation is safe for most individuals and can easily be combined with other preventative measures like proper hygiene, wearing a mask in poorly-ventilated settings, avoiding indoor gatherings, getting vaccinated, exercising, avoiding alcohol, quitting smoking, maintaining a healthy weight, eating a nutritious diet, taking other immune-supporting vitamins/supplements, and getting good quality sleep every night.
Vitamin D just might take our COVID-19 protection to the next level.
Special thanks to my medical writer/editor colleague, Marina MacDonald, MS, PhD, for her previous writings on the topic of vitamin D and COVID, and for opening my eyes to the impressive body of literature on the subject.
 Pereira M, et al. Vitamin D deficiency aggravates COVID-19: systematic review and meta-analysis. Crit Rev Food Sci Nutr. 2020 Nov 4;1-9.
 Mercola J, et al. Evidence regarding vitamin D and risk of COVID-19 and its severity. Nutrients. 2020 Oct 31;12(11):3361.
 Lanham-New SA, et al. Vitamin D and SARS-CoV-2 virus/COVID-19 disease. BMJ Nutr Prev Health. 2020 May 13;3(1):106-10.
 Panagiotou G, et al. Low serum 25-hydroxyvitamin D (25[OH]D) levels in patients hospitalized with COVID-19 are associated with greater disease severity. Clin Endocrinol (Oxf). 2020 Oct;93(4):508-11.
 Rhein H. Vitamin D—let common sense prevail—on the balance of probabilities. Aging Clin Exp Res. 2021 Sep;33(9):2633.
 Sutherland JP, et al. Differences and determinants of vitamin D deficiency among UK biobank participants: a cross-ethnic and socioeconomic study. Clin Nutr. 2021 May;40(5):3436-47.
 Annweiler C, Souberbielle JC. Vitamin D supplementation and COVID-19: expert consensus and guidelines. Geriatr Psychol Neuropsychiatr Vieil. 2021 Oct 5. doi: 10.1684/pnv.2021.0955.
 PubMed. National Library of Medicine. Search for “Vitamin D Covid,” with filters set between dates of 01 Jan 2020 and 31 Dec 2021. Accessed December 20, 2021 Available from: https://pubmed.ncbi.nlm.nih.gov/?term=vitamin+D+covid&filter=dates.2020%2F1%2F1-2021%2F12%2F31
 Wacker M, Holick MF. Sunlight and vitamin D: a global perspective for health. Dermatoendocrinol. 2013 Jan 1;5(1):51-108.
 De Jongh RT, et al. Changes in vitamin D endocrinology during aging in adults. Mol Cell Endocrinol. 2017 Sep 15;453:144-50.
 Grant WB, et al. Seasonal variations of U.S. mortality rates: Roles of solar ultraviolet-B doses, vitamin D, gene expression, and infections. J Steroid Biochem Mol Biol. 2017 Oct;173:5-12.
 Webb AR, et al. Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. J Clin Endocrinol Metab. 1988 Aug;67(2):373-8.
 Mukherjee SB, et al. Seasonal UV exposure and vitamin D: association with the dynamics of COVID-19 transmission in Europe. FEBS Open Bio. 2021 Oct 5. doi: 10.1002/2211-5463.13309.
 Gunville CF, et al. The role of vitamin D in prevention and treatment of infection. Inflamm Allergy Drug Targets. 2013 Aug;12(4):239-45.
 Ginde AA, et al. Association between serum 25-hydroxyvitamin D level and upper respiratory tract infection in the Third National Health and Nutrition Examination Survey. Arch Intern Med. 2009 Feb 23;169(4):384-90.
 Pham H, et al. Acute respiratory tract infection and 25-hydroxyvitamin D concentration: a systematic review and meta-analysis. Int J Environ Res Public Health. 2019 Aug 21;16(17).
 Martineau AR, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017 Feb 15;356:i6583
 Bergman P, et al. Vitamin D and respiratory tract infections: a systematic review and meta-analysis of randomized controlled trials. PLoS One. 2013 Jun 19;8(6):e65835.
 Kaufman HW, et al. SARS-CoV-2 positivity rates associated with circulating 25-hydroxyvitamin D levels. PLoS One. 2020 Sep 17;15(9):e0239252.
 Petrelli F, et al. Therapeutic and prognostic role of vitamin D for COVID-19 infection: a systematic review and meta-analysis of 43 observational studies. J Steroid Biochem Mol Biol. 2021 Jul;211:105883.
 Jain A, et al. Analysis of vitamin D level among asymptomatic and critically ill COVID-19 patients and its correlation with inflammatory markers. Sci Rep. 2020 Nov 19;10(1):20191.
 Merzon E, et al. Low plasma 25(OH) vitamin D level is associated with increased risk of COVID‐19 infection: an Israeli population‐based study. FEBS J. 2020 Sep;287(17):3693-702.
 Munshi R, et al. Vitamin D insufficiency as a potential culprit in critical COVID‐19 patients. J Med Virol. 2020 Jul 27.
 Daneshkhah A, et al. Evidence for possible association of vitamin D status with cytokine storm and unregulated inflammation in COVID-19 patients. Aging Clin Exp Res. 2020 Sep 2.
 Orrù B, et al. Inhibitory effects of Vitamin D on inflammation and IL-6 release. A further support for COVID-19 management? Eur Rev Med Pharmacol Sci. 2020 Aug;24(15):8187-93.
 Mardani R, et al. Association of vitamin D with the modulation of the disease severity in COVID-19. Virus Res. 2020 Nov;289:198148.
 Akbar MR, et al. Low Serum 25-hydroxyvitamin D (Vitamin D) level is associated with susceptibility to COVID-19, severity, and mortality: a systematic review and meta-analysis. Front Nutr. 2021 Mar 29;8:660420.
 Yisak H, et al. Effects of vitamin D on COVID-19 infection and prognosis: a systematic review. Risk Manag Healthc Policy. 2021 Jan 7;14:31-8.
 Mariani J, et al. Association between vitamin D deficiency and COVID-19 incidence, complications, and mortality in 46 countries: an ecological study. Health Secur. May-Jun 2021;19(3):302-8.
 Pugach IZ, Pugach S. Strong correlation between prevalence of severe vitamin D deficiency and population mortality rate from COVID-19 in Europe. Wien Klin Wochenschr. 2021 Apr;133(7-8):403-5.
 Liu N, et al. Low vitamin D status is associated with coronavirus disease 2019 outcomes: a systematic review and meta-analysis. Int J Infect Dis. 2021 Mar;104:58-64.
 Ghasemian R, et al. The role of vitamin D in the age of COVID-19: A systematic review and meta-analysis. Int J Clin Pract. 2021 Jul 29;e14675.
 Teshome A, et al. The impact of vitamin D level on COVID-19 infection: systematic review and meta-analysis. Front Public Health. 2021 Mar 5;9:624559.
 Szarpak L, et al. A systematic review and meta-analysis of effect of vitamin D levels on the incidence of COVID-19. Cardiol J. 2021;28(5):647-54.
 Radujkovic A, et al. Vitamin D deficiency and outcome of COVID-19 patients. Nutrients. 2020 Sep 10;12(9):2757.
 Annweiler G, et al. Vitamin D supplementation associated to better survival in hospitalized frail elderly COVID-19 Patients: the GERIA-COVID quasi-experimental study. Nutrients. 2020 Nov 2;12(11):3377.
 Castillo ME, et al. Effect of calcifediol treatment and best available therapy versus best available therapy on intensive care unit admission and mortality among patients hospitalized for COVID-19: a pilot randomized clinical study. J Steroid Biochem Mol Biol. 2020 Oct;203:105751.
 Shah K, et al. Vitamin D supplementation, COVID-19 and disease severity: a meta-analysis. QJM. 2021 May 19;114(3):175-81.
 Oristrell J, et al. Vitamin D supplementation and COVID-19 risk: a population-based, cohort study. J Endocrinol Invest. 2021 Jul 17;1-13.
 Annweiler C, Souberbielle JC. Vitamin D supplementation and COVID-19: expert consensus and guidelines. Geriatr Psychol Neuropsychiatr Vieil. 2021 Oct 5. doi: 10.1684/pnv.2021.0955.
 Castillo ME, et al. Effect of calcifediol treatment and best available therapy versus best available therapy on intensive care unit admission and mortality among patients hospitalized for COVID-19: A pilot randomized clinical study. J Steroid Biochem Mol Biol. 2020;203:105751.
 Vitamin D. National Institutes of Health [Internet]. April 21, 2021. Accessed December 20, 2021. Available at: https://www.covid19treatmentguidelines.nih.gov/therapies/supplements/vitamin-d/
 Annweiler C, et al. Learning from previous methodological pitfalls to propose well-designed trials on vitamin D in COVID-19. J Steroid Biochem Mol Biol. 2021 Jul;211:105901.
 Murai IH, Fernandes AL,Sales LP. Effect of a single high dose of vitamin D3 on hospital length of stay in patients with moderate to severe COVID-19: a randomized clinical trial. 2021; Published online ahead of print. Available at: https://pubmed.ncbi.nlm.nih.gov/33595634/.
 Smolders J, et al. Letter to the Editor: vitamin D deficiency in COVID-19: mixing up cause and consequence. Metabolism. 2021 Feb;115:154434.
 Chakhtoura M, Fuleihan GE. Reply to vitamin D deficiency in COVID-19: mixing up cause and consequence. Metabolism. 2021 Feb;115:154434.
 Trovas G, Tournis S. Vitamin D and COVID-19. Hormones (Athens). 2021 Mar;20(1):207-8.
 Holick MF. The vitamin D deficiency pandemic: approaches for diagnosis, treatment and prevention. Rev Endocr Metab Disord. 2017 Jun;18(2):153-65.
 Weaver CM, et al. Calcium plus vitamin D supplementation and risk of fractures: an updated meta-analysis from the National Osteoporosis Foundation. Osteoporos Int. 2016 Jan;27(1):367-76.
 Ensrud KE, et al. Circulating 25-hydroxyvitamin D levels and frailty status in older women. J Clin Endocrinol Metab. 2010 Dec;95(12):5266-73.
 Mirhosseini N, et al. Vitamin D supplementation, serum 25(OH)D concentrations and cardiovascular disease risk factors: a systematic review and meta-analysis. Front Cardiovasc Med. 2018 Jul 12;5:87.
 Umar M, et al. Role of vitamin D beyond the skeletal function: a review of the molecular and clinical studies. Int J Mol Sci. 2018 May 30;19(6).
 PR Newswire. 110-plus international experts release open letter: COVID-19 preys on those with vitamin D deficiency [Internet]. New York (NY): PR Newswire; 2021 [cited 2021 Oct 10]. Available from: https://www.prnewswire.com/news-releases/110-plus-international-experts-release-open-letter-covid-19-preys-on-those-with-vitamin-d-deficiency-301197338.html
 Tarazona‐Santabalbina FJ, et al. Vitamin D supplementation for the prevention and treatment of COVID‐19: a position statement from the Spanish Society of Geriatrics and Gerontology. Revista Española de Geriatría y Gerontología. 2021;56(3):177-82.
 Gibson-Moore H. Vitamin D: what’s new a year on from the COVID-19 outbreak? Nutr Bull. 2021 Jun;46(2):195-205.
 Vitamin D for All. Over 200 scientists & doctors call for increased vitamin D use to combat COVID-19 [Internet]. Vitamin D for All; 2021 [cited 2021 Oct 10]. Available from: https://vitamindforall.org/letter.html
 Reider CA, et al. Inadequacy of immune health nutrients: intakes in US adults, the 2005-2016 NHANES. Nutrients. 2020 Jun 10;12(6):E1735.
 MacLaughlin J, Holick MF. Aging decreases the capacity of human skin to produce vitamin D3. J Clin Invest. 1985 Oct;76(4):1536-8.
 Wyskida M, et al. Prevalence and factors promoting the occurrence of vitamin D deficiency in the elderly. Postepy Hig Med Dosw (Online). 2017 Mar 13;71(0):198-204.
 Murphy AB, et al. Predictors of serum vitamin D levels in African American and European American men in Chicago. Am J Mens Health. 2012 Sep;6(5):420-6.
 Cyrus E, et al. The impact of COVID-19 on African American communities in the United States. medRxiv. 2020 May 19 [preprint]
 White C, Nafilyan V. Coronavirus (COVID-19) related deaths by ethnic group, England and Wales: 2 March 2020 to 10 April 2020 [Internet]. Newport (UK): Office for National Statistics; 2020 [cited 2020 Jun 16]. Available from: https://www.ons.gov.uk/peoplepopulationandcommunity/birthsdeathsandmarriages/deaths/articles/coronavirusrelateddeathsbyethnicgroupenglandandwales/2march2020to10april2020
 Liu X, et al. Vitamin D deficiency and insufficiency among US adults: prevalence, predictors and clinical implications. Br J Nutr. 2018 Apr;119(8):928-36.
 Pourshahidi LK. Vitamin D and obesity: current perspectives and future directions. Proc Nutr Soc. 2015 May;74(2):115-24.
 Popkin BM, et al. Individuals with obesity and COVID-19: a global perspective on the epidemiology and biological relationships. Obes Rev. 2020 Aug 26.