Can vitamin D reduce the risk of COVID-19?

Image description: A mask and a vial of vitamin D

Amid the coronavirus pandemic the demand for vitamin supplements has skyrocketed as millions seek to ‘boost their immunity’ to combat infection. Although evidence underlying their efficacy in reducing case incidence and severity is sparse, a link between vitamin D deficiency and COVID-19 mortality has emerged in recent literature. This speculation prompts the all-important question: should we all be supplementing vitamin D?

In order to address this question, it is key to understand the functional importance of vitamin D (including its purported effects on the immune system). The term ‘vitamin D’ describes a group of fat-soluble prohormones, of which the most significant to humans are D3 (cholecalciferol) and D2 (ergocalciferol). While vitamin D can be obtained through the consumption of certain foods (oily fish and fortified cereals, for example), it is difficult to reach a sufficient intake through dietary means alone. A major natural source of vitamin D is UVB radiation from sunlight, which triggers the production of D3 in the epidermis of the skin. 

The subsequent conversion into the active hormone, calcitriol, allows vitamin D to exert its systemic effects. Despite usually being associated with bone health, vitamin D is implicated in almost every organ system. In particular, its association with the immune system stems from the use of cod liver oil – a potent source of vitamin D – to treat tuberculosis in the 19th century. The latest research indicates that vitamin D can activate the innate and suppress the adaptive immune systems – potentially reducing the risk and/or severity of infection through its anti-inflammatory effects. 

A 2017 meta-analysis by the BMJ found that vitamin D supplementation universally reduced the risk of acute respiratory tract infection, especially in those with severe deficiency. 25-hydroxyvitamin D (the main circulating metabolite of vitamin D) is reported to induce several antimicrobial mechanisms, as well as upregulating the production of antiviral and antibacterial proteins. But how is vitamin D implicated in COVID-19?

The demand for vitamin supplements has skyrocketed, as millions seek to ‘boost their immunity’ to combat infection

According to recent analyses, vitamin D deficiency may be linked with poorer outcomes in COVID-19 patients – a theory substantiated by relatively higher mortality rates in countries with higher levels of vitamin D deficiency. A systematic review conducted at Northwestern University used data from countries including China, Italy, the United Kingdom, and the United States to identify a connection between low serum vitamin D and ‘cytokine storm’.  This refers to a potentially lethal reaction generated by a severe hyperinflammatory state, often in response to infection. Ali Daneshkhah, a member of the Northwestern group, notes that “cytokine storm can severely damage lungs and lead to acute respiratory distress syndrome and death in patients.” The paper also discusses vitamin D and C-reactive protein (CRP) levels, assuming that elevated CRP serves as a marker for severe COVID-19 status. Researchers concluded that the risk of severe infection in those with vitamin D deficiency is 17.3%, compared to 14.6% in those with adequate vitamin D.

Despite abundant speculation, it is worth acknowledging this article is a ‘preprint’ and is therefore yet to be peer-reviewed. MedRxiv recommends that this research should not be used to guide clinical practice, especially as SARS-CoV-2 is a novel virus. It is hoped that future trials will clarify the capacity of vitamin D in improving patient outcomes. The limited data available suggests that vitamin D supplementation is most useful in those who are already deficient. However, this subgroup is hardly small considering an estimated 1 billion people worldwide are vitamin D insufficient or deficient (defined as a serum vitamin D of less than 31ng/ml and 20ngl/ml respectively).

According to the NHS, all adults require 10μg of vitamin D a day and are advised to take supplements if not spending adequate time outdoors. This advice is especially relevant to groups who are at an increased risk of insufficiency due to inadequate synthesis by the skin, namely: older adults, homebound individuals, and those with dark skin – who are also all at a greater risk of dying from COVID-19. In the current lockdown situation, vitamin D insufficiency has likely become even more widespread in the UK population. 

In light of this information, how can we ensure that we are getting enough vitamin D? The simplest method is regular sun exposure – while adhering to social distancing guidelines, of course. To maintain ideal vitamin D levels, one should aim for short periods of unprotected (i.e. uncovered skin and no sunscreen) sun exposure – although those with darker skin will require slightly longer periods, as increased levels of melanin impair vitamin D production. If this is not possible, the recommended dosage of vitamin D by the NHS is 10μg. Nevertheless, it is also vital to recognise the danger of vitamin D toxicity – although rare, adults should ensure that their vitamin D consumption does not exceed more than 100μg per day.

In the current lockdown situation, vitamin D insufficiency has likely become even more widespread in the UK population.

Regardless of its potential impact, the significance of vitamin D in this context pales in comparison to the need for social distancing and good hygiene. Moreover, it is not the only vitamin that is important for overall well being – and few would disagree that maintaining a healthy lifestyle and diet is crucial to aid our immune systems in the event of infection.

Photo by Dimitri Karastelev on Unsplash