Skip to content

Vitamin D: not really a vitamin

Vitamin D was named when it was discovered following the discovery of vitamins A, B and C. However, further research has shown that it is in fact a hormone and not a vitamin; this is due to the body’s ability to produce it from exposure to sunlight. It then undergoes activation by the liver and the kidneys so it is able to perform its essential functions, such as maintaining bone health. Other vitamins such as vitamin A and C do not need to go through the same activation process as vitamin D.

Humans can source vitamin D from sunlight via exposure to UVB radiation on the skin or from dietary sources such as oily fish, red meat and egg yolks. However, it is only present in small quantities in foods and, due to the current lockdown, questions must be raised about whether the population is spending sufficient time outside to synthesise vitamin D.

Vitamin D deficiency has been described as a global public health problem(1). It is well known that in the UK 30 – 40% of the general population are classed as vitamin D deficient during the winter months(2).

What effect can vitamin D deficiency have?

As mentioned earlier, vitamin D has important roles in maintaining the health of our bones and also has an important role for our intestines and kidneys. A deficiency in vitamin D can lead to bone diseases such as rickets in children and osteomalacia (a disease characterised by the softening of the bones) in adults. More recently the protective effect of vitamin D on our immune systems has also been described. Furthermore, data shows that optimal vitamin D levels may contribute to protection from respiratory infections (3).

Can vitamin D protect against COVID-19?

COVID-19, the disease caused by coronavirus, is a viral respiratory disease. With the recent outbreak, some focus has shifted to whether or not optimal vitamin D levels may be protective against the virus. The mechanisms by which infection with COVID-19 results in such severe effects in some people are not fully understood, however, evidence has shown that some of the immune system processes related to poor clinical outcome may be reduced by optimal vitamin D levels. It is also notable that vitamin D deficiency is highly prevalent among ethnic minorities and elderly populations in care homes and these populations are also thought to be at increased risk of COVID-19. Currently, there is no published evidence of a protective effect of vitamin D against COVID-19. However, there are on-going trials which are aiming to make clear any protective or treatment capabilities.

What are the vitamin D guidelines?

Existing guidance recommends that children from the age of one and adults in the UK have a daily intake of 10 μg (microgram) vitamin D4. The average dietary intake of vitamin D in the UK is 2 – 5 μg per day(4). The NHS has published a coronavirus update advising people to consider taking a 10 μg vitamin D supplement every day(5). This is due to the fact that people may not be synthesising enough vitamin D from sunlight due to spending more time indoors. They also ask that people do not buy more vitamin D than they need.


At the moment, there is not sufficient evidence to suggest a protective effect of vitamin D against COVID-19; however, as trials begin to take place, some mechanisms may emerge. It is important that the general public are aware of the importance of adequate vitamin D intake for prevention of other diseases and that more people may be deficient due to the current lockdown measures in place.


  1. World Health Organisation. (2015).  Vitamin D nutrition with a focus on the prevention of rickets and vitamin D deficiency in pregnant women. [accessed 24 April 2020].
  2. Public Health England (PHE). (2016). National Diet and Nutrition Survey. Retrieved from
  3. Martineau AR, Jolliffe DA, Hooper RL et al. (2017) Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data, BMJ 356, i658
  4. Scientific Advisory Committee on Nutrition (SACN). (2016). Vitamin D and Health Report.
  5. National Health Service (NHS). (2020). Vitamin D. [accessed 24 April 2020].
Share this content