Diabetes Distilled: Sunlight exposure, vitamin D supplementation and COVID-19

This short report summarises the known evidence on the links between vitamin D levels, immune system function and respiratory diseases. In doing so, the authors examine the putative role of vitamin D in preventing and recovering from COVID-19 infection.

While vitamin D deficiency has often been correlated with acute respiratory infections, and potential causal mechanisms behind this link have been hypothesised, evidence of a direct link is lacking and the indirect evidence is confounded by seasonal differences in human behaviour and viral incidence, to name just two factors. Nonetheless, vitamin D is essential for good health, and so the authors conclude with useful advice on maintaining adequate levels, particularly during the winter months and during potential periods of limited sun exposure, such as when self-isolating or shielding.

In the UK, adequate vitamin D levels can be obtained from action of sun exposure on the skin between the months of March and September only. For people with white skin, 10 minutes with face, arms and legs exposed to the sun in the middle of the day is sufficient, but this rises to 25 minutes in people of South Asian descent, and still more may be required for people with black skin. The emphasis during the COVID-19 pandemic should be to achieve this exposure without leaving the home (i.e. in the garden or on a balcony; however, sun exposure through glass will not provide sufficient UVB rays for vitamin D synthesis).

From October to February, or if isolating/shielding indoors, supplementation with vitamin D (400 IU/day, or 10 µg/day) is required, and a vitamin D-rich diet (e.g. oily fish, red meat, egg yolk and fortified cereals) is recommended in addition. However, vitamin D supplement doses over 4000 IU/day (100 µg/day) may be harmful and should be avoided in the general population. There is no strong evidence to suggest that higher doses will aid the prevention or treatment of COVID-19.

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