What Everyone Should Know about Vitamin D and COVID-19

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Vitamin D insufficiency increases risk of illness

The COVID-19 pandemic has swept the globe, killing more than 4.5 million people as of this writing. The symptoms of a SARS-CoV-2 infection range from mild (similar to a common cold) to more severe manifestations such as bronchitis, pneumonia, acute respiratory distress syndrome, organ failure, and even death in those who are most susceptible.

The severity of COVID-19 infection is influenced by age, ethnicity, and “comorbidities” such as obesity, diabetes, hypertension (high blood pressure), and asthma.^[1],[2] People who are young and in good health are more likely to have mild symptoms.

Many of the groups at risk for COVID-19 are also at risk for vitamin D deficiency, including the elderly, overweight, chronically ill, and certain ethnic groups.^[3] In fact, vitamin D deficiency appears to be an important comorbidity in its own right, and one that is easily modifiable through supplementation.^{[4],[5],[6],[7],[8],[9],[10]} However, many people are unaware of the importance of vitamin D in COVID-19 risks and outcomes.

The evidence suggests that vitamin D may be one of the most important nutritional contributors to COVID-19 risks.

In 2020 we highlighted the importance of vitamin D in posts titled Vitamin D in the News and Study Identifies Widespread Inadequacies of Immune Health Nutrients. Since 2020, more than 800 peer-reviewed articles have been published on the subject.^[11] The evidence suggests that vitamin D may be one of the most important nutritional contributors to COVID-19 risks.

Sun exposure influences COVID-19 risks and outcomes

Vitamin D is made in the skin in response to solar UVB radiation (the type associated with suntans).^[12] It can also be obtained from the diet and (primarily) by supplementation.

Many variables affect sun exposure and vitamin D levels in the body: the latitude at which one resides, season (winter vs. summer), time of day, cloud cover, air pollution (which filters out UVB), skin melanin content (which absorbs UVB), age, and sunscreen use (which blocks UVB).^{[12],[13],[14],[15]}

In a study published in October 2021, scientists performed a comprehensive meta-analysis of all related published literature based on the association of vitamin D and COVID-19, which showed that a low vitamin D level is a critical risk factor for SARS-CoV-2 infection.^[16]

Additionally, there was a correlation between daily weather patterns and COVID-19 cases in 26 European countries. Low temperature, low UV index, and low cloud-free vitamin D UV dose (UVDVF) levels were correlated with COVID-19 prevalence. In other words, as the clouds rolled in, the cases began to rise.

These findings suggest that maintaining adequate vitamin D levels in the body, either through UV exposure or supplementation, may help reduce the risks of COVID-19.^[16] Most clinicians recommend supplementation, however, since excessive sun exposure increases the risk of skin cancer.

The significance of vitamin D deficiency for COVID-19

Vitamin D status is assessed by measuring circulating levels of its metabolite, 25-hydroxy-vitamin D (25(OH)D). A study of over 190,000 patients from all 50 states compared SARS-CoV-2 test results with matching 25(OH)D results from the preceding 12 months.^[17] There was a clear correlation between 25(OH)D levels and positive test results, as shown below. People with positive test results are very likely to have COVID-19.

The group classified as vitamin D deficient (<20 ng/ml) were 50% more likely to test positive than those with “adequate” vitamin D levels (30-34 ng/ml). The correlation persisted across latitudes, races/ethnicities, sexes, and age ranges. Moreover, higher vitamin D levels (>55 ng/ml) were associated with an even lower risk than the supposedly “adequate” group.

The vitamin D-deficient group had a 21% mortality rate, compared to 3.1% mortality in the group with adequate vitamin D levels – a difference of nearly sevenfold.

Vitamin D deficiency is also associated with a more severe disease course.^[18] One study of COVID-19 patients evaluated 25(OH)D levels in those who were asymptomatic and in those who were severely ill with COVID-19.^[19] Nearly all of the severely ill group (97%) were vitamin D deficient, compared with 33% of the individuals who were asymptomatic. The vitamin D-deficient group also had a 21% mortality rate, compared to 3.1% mortality in the group with adequate vitamin D levels – a difference of nearly sevenfold.

More than a dozen meta-analyses have examined the association between vitamin D deficiency and COVID-19 risks by collating the results from studies around the world. The majority of the reviews have concluded that vitamin D deficiency increases the risk of a severe outcome.^{[4],[16],[18],[20],[21],[22],[23],[24],[25],[26],[27]}

Vitamin D supplementation reduces COVID-19 risks

Patients who took vitamin D supplements were 64% less likely to require the intensive care unit as compared to patients who did not take supplements.

In addition to observational studies, direct studies of vitamin D supplementation have begun. They indicate that vitamin D supplementation is associated with less severe COVID-19 infection and better outcomes.^{[28],[29],[30]}

A meta-analysis of studies involving 532 hospitalized patients with COVID-19 (189 taking vitamin D supplements, and 343 receiving usual care/placebo) showed that COVID-positive patients who took vitamin D supplements were 64% less likely to require the intensive care unit (ICU) as compared to patients who did not take supplements.^[31]

In a population study of more than 85,000 individuals, the mortality risk was assessed in subjects taking supplements who had adequate vitamin D levels (serum 25(OH)D > 30 ng/ml) as compared to vitamin D-deficient subjects who did not take the supplement (serum 25(OH)D < 20 ng/ml).^[32] The risk of COVID-19 mortality was 50% less in patients with adequate vitamin D supplementation compared with those who were deficient.

A review published in October 2021 notes: “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, which is present in more than 80% of adults with COVID-19.”^[10]

Why is vitamin D still controversial?

Despite the evidence, this topic remains controversial for multiple reasons:

• Some studies were poorly designed and did not show a correlation between vitamin D and infections.^[33]
• Low vitamin D may be a consequence of COVID-19 infection as well as a predisposing factor.^[34],[35]
• Larger clinical trials are needed to confirm the population studies and to determine optimal vitamin D levels.^[36]

In addition to the scientific issues, controversy arises from the concern that consumers may ignore other public health recommendations if they think that vitamin D supplementation alone could reduce the risk of infections.^[37]

Regarding this point, Dr. Helena Gibson-Moore of the British Nutrition Foundation says: “Despite some media headlines, it is important that vitamin D (or any other nutrient, food or supplement) is never positioned as a ‘magic bullet’ against COVID‐19… Even if robust evidence emerges for vitamin D (or another nutrient), it should be considered in the context of the other established approaches to combat the disease, such as vaccines, social distancing, wearing of masks, keeping windows open and hygiene measures.”³⁷

While keeping these precautions in mind, many scientists think that vitamin D deserves greater emphasis now. A position statement from the Spanish Society of Geriatrics and Gerontology argues that “…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.”^[38]

Vitamin D: a prudent approach

Evidence to date suggests the possibility that the COVID-19 pandemic sustains itself in large part through infection of those with low vitamin D.

To raise awareness of vitamin D, a group of 220 experts have joined forces under the moniker of Vitamin D for All.^[39],[40] Their Open Letter states: “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.”  They recommend that adults increase vitamin D intakes from all sources to achieve serum 25(OH)D levels above 30 ng/ml.^[39]

Vitamin D for All suggests that adults whose 25(OH)D levels have not yet been tested consume 2,000–4,000 IU of vitamin D daily. People with an increased risk of deficiency due to overweight, ethnicity, advanced age, or comorbidities may need higher intakes.^[39] These recommendations are in line with established vitamin D guidelines issued by the Endocrine Society in 2011.^[41] Further clinical studies will help clarify the optimal 25(OH)D levels with respect to COVID-19.

Since vitamin D deficiency increases the risk of osteoporosis, frailty, diabetes, heart disease, other respiratory infections, and some cancers, supplementation to achieve adequate intakes has many potential health benefits.^{[42],[43],[44],[45],[46]}

Last but not least, Vitamin D for All acknowledges that numerous factors can predispose individuals to COVID-19 infections and their complications. They note, however, that “inadequate vitamin D may be the most easily and quickly modifiable risk factor with abundant evidence for a beneficial effect.”^[39] From a nutritional perspective, ensuring adequate vitamin D intake may be one of the best things we can do for our health, both during this pandemic and beyond.

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