The search for therapies for Covid-19 has led scientists to explore an array of drugs ranging from expensive anti-virals and anti-inflammatories to anti-malarials (hydroxychloroquine) and even anti-helminthics (use to treat parasitic worms). Despite this, the potential role of something much more simple – ‘vitamin D’ in either helping to prevent infection or reduce severity has got lots of attention in the last two months. Indeed, there has been over 50 peer reviewed articles published on the topic of ‘vitamin D and Covid-19’ and while evidence strongly supports a role, it still remains circumstantial.
But how might vitamin D affect Covid-19? In some ways, the designation of vitamin D as the fourth ‘vitamin’ at the turn of the last century was a bit of a misnomer. Traditionally, vitamins were substances obtained in the diet that were important for maintaining certains aspects of health. However, vitamin D is unique in that apart from our diet or taking supplements, about 80-90% is produced by our skin after exposure to sunlight.
Not only that, vitamin D is a hormone that appears to act all over the body. Its main role is in promoting adequate absorption of calcium from the gut and thereby maintaining normal bone health. It also appears to be important in muscle function and indeed supplementing some older adults who are deficient reduces falls risk. However, the discovery of the ‘vitamin D receptor’ in most organs suggests a role well beyond musculoskeletal health. Indeed, there has been an increasing body of evidence that strongly supports a positive affect of vitamin D on the immune system.
As we are reliant on sun exposure for a lot of our vitamin D, cocooning in older adults is very likely to have impacted on levels. Indeed, based on the TILDA study, nearly 15% of older Irish adults are already deficient rising to 20% in the winter. Taking this into account, it is estimated that 27% of the over 70s who were cocooning may be deficient. Importantly, while this is not good for their bones it might also have an affect on other health outcomes such as Covid-19.
Vitamin D appears to affect the immune system in a number of ways. For example, it has been shown to promote the production of a number of anti-bacterial proteins in the lining of the respiratory tract. It also has an effect on immune cells (T cells) that are involved in the production of several inflammatory cytokines.
In particular, vitamin D deficiency is associated with increased levels of the cytokines IL-6 and TNF which are elevated in Covid-19 and predict more severe disease. Importantly, vitamin D has also been shown to decrease the production of these very chemicals thereby having a potential role in counteracting the ‘cytokine storm’ due to Covid-19. In some in vitro and in vivo models, vitamin D has also been shown to slow the replication of viruses.
But what about real world clinical studies of vitamin D? In one analysis involving 21,000 people, low vitamin D status was associated with a 64% increased risk of pneumonia. Furthermore, in a pooled analysis of 25 studies where 10,933 subjects were supplemented with vitamin D, there was a 12% reduction in colds, flus and chest infections with greater benefit identified in those with the lowest levels.
Poorer vitamin D status in several European countries has also been associated with a higher incidence of Covid-19 and greater mortality. However, trying to tease apart other co-associated factors that might in part account for this is difficult. Notably, in England and Wales black people who have higher rates of vitamin D deficiency were four times more likely to die from Covid-19 though whether this was a factor is unclear. Nonetheless, public health bodies in the UK are now conducting a rapid evidence review of vitamin D in the context of Covid-19.
So, what food contains vitamin D? It is found in fortified milk and breakfast cereals but other sources in the diet are limited with small amounts in eggs and meat. Oily fish including salmon and also musthrooms are good sources. However, in most people dietary intake alone cannot make up for lack of sun exposure. On the other hand about 10-15 minutes of sun a few times a week from late March to September will allow you to make lots of vitamin D. Those who have more pigmented skin will however required exposure for a little longer.
But how much vitamin D should be taken to prevent deficency? Recommendations vary but during the winter it is sensible to take about 400 IU daily and up to 600-800 IU daily all year round in higher risk groups – especially those who are frail with limited sun exposure. In some cases, it may be appropriate to check a vitamin D level to guide dosing and response to treatment.
Dr Kevin McCarroll is a Consultant Physician in Geriatric Medicine, St James’s Hospital, Dublin.