Vitamin D plays an important role in regulating calcium and phosphate levels, and is therefore vital for maintaining healthy bones, muscles and teeth (1). Recently, vitamin D has been in the spotlight in connection with the sars-cov-2-infection (COVID-19). This has led to confusion among both the public and healthcare professionals. This article will explore the connection between vitamin D and respiratory disease; including COVID-19.
Vitamin D and the Immune System
Vitamin D receptors are present in many cells of the immune system, such as macrophages, lymphocytes and natural killer cells (1). In addition, vitamin D is known to have anti-inflammatory effects, impact cytokine production, improve macrophage responses and regulate T-cell responses (2, 3).
Some observational studies have found an association between vitamin D deficiency and a higher risk of autoimmune disease. However, the overall evidence-base is conflicting (1).
Vitamin D and Respiratory Infections
Emerging research suggests that low levels of vitamin D may be associated with a higher risk of respiratory infections (4, 5).
For example, a large cross-sectional trial from 2009 which included almost 19,000 participants found a higher risk of respiratory infections in those with lower vitamin D levels; particularly in those with asthma and chronic obstructive pulmonary disease (6).
More recently, a meta-analysis which included almost 21,000 participants found that vitamin D deficiency was associated with a 64% higher risk of community-acquired pneumonia (5).
Another large meta-analysis from 2017 found that vitamin D supplementation significantly reduced the risk of respiratory tract infection, particularly in those with baseline vitamin D levels below the recommended level (25 nmol/L) (7)
Vitamin D and COVID-19
A recent opinion piece by two Irish lecturers about vitamin D and COVID-19 was widely reported in the media (8). Based on the evidence related to vitamin D and respiratory disease risk and the high rates of vitamin D deficiency in Ireland, the authors recommended that “all older adults, hospital inpatients, nursing home residents and other vulnerable groups be urgently supplemented with 20 - 50 micrograms of vitamin D per day to enhance their resistance to COVID-19, and that this advice be quickly extended to the general adult population”. However, this report was not based on research specific to COVID-19.
Data is beginning to emerge specifically related to vitamin D status in patients with COVID-19. For example, a preprint version of study which involved 212 patients from three hospitals in Southern Asia found that vitamin D levels were lowest in critical COVID-19 cases, and highest in mild cases of COVID-19 (average serum levels were 42.75 nmol/L and 78 nmol/L respectively) (9). It is important to note that this data is very new, and has not been peer- reviewed yet.
Similarly, a recent review reported a lower risk of death from COVID-19 in countries located below a latitude of 35 degrees North (10). This roughly matches the latitude at which vitamin D can be produced year-round from the action of sunlight on the skin (37 degrees North) (1). Although this is an interesting report, it does not provide robust evidence for a link between vitamin D status and COVID-19 risk or outcomes, as many other factors could have impacted this result.
Overall, the research related to COVID-19 and vitamin D is still in the very early stages so more studies are needed to investigate this potential link.
Vitamin D Recommendations
Public Health England (PHE) advises that those over the age of one should consider taking a 10 microgram vitamin D supplement during autumn and winter (October to March) (11). Those who get little or no exposure to the sun on their skin and ethnic minority groups with dark skin are advised to consider taking this supplement all year round (11).
Due to the current limitations of the COVID-19 lockdown, many people in the UK will be getting less exposure to sunlight than usual. Therefore, the NHS website has posted a ‘coronavirus update’ which states: “Consider taking 10 micrograms of vitamin D a day to keep your bones and muscles healthy. This is because you may not be getting enough vitamin D from sunlight if you’re indoors most of the day. There have been some news reports about vitamin D reducing the risk of coronavirus. However, there is no evidence that this is the case. Do not buy more vitamin D than you need.”
Similarly, the British Dietetic Association website recommends: “if you are having to self-isolate or if you are unable to go outside, you should consider taking a daily supplement containing 10 micrograms [of vitamin D] to ensure a healthy vitamin D status (for adults and children over the age of one)”.