Not only is vitamin D important for bone and muscle health, but it plays a crucial role in the body's immune response. This has led scientists to question whether a daily dose of the sunshine supp could help to fend off the coronavirus – but the association between vitamin D and COVID-19 is not yet understood.
While low levels of vitamin D have been identified in people who experience severe outcomes of COVID-19, the National Institute for Health and Care Excellence (NICE) states, there is no evidence to confirm causality. Researchers are also yet to determine whether vitamin D can reduce the risk of infection or mitigate the severity of the disease. So, what do we currently know about vitamin D and COVID-19?
We spoke to Professor Adrian Martineau, clinical professor of respiratory infection and immunity at Queen Mary University of London, Professor Naveed Sattar, professor of metabolic medicine at the University of Glasgow, Dr Deborah Shoemark, senior research associate (Biomolecular Modelling) at the University of Bristol, and clinical nutritionist Suzie Sawyer:
How does vitamin D affect immune health?
Vitamin D is essential for a healthy immune system, which describes the complex network of cells and proteins that work together to defend your body against infection and disease. It's made up of 'innate' and 'adaptive' responses, and vitamin D acts on both.
Innate immunity is your first line of defence against foreign microbes. It's fast-acting and non-specific – the main purpose is immediately prevent the spread of the microbe. Adaptive immunity builds over time, as your immune system 'remembers' the microbes it destroys. It protects and heals the body when the innate immune system fails.
Not only is vitamin D crucial for activating your immune system defences, but it also boosts the function of immune cells, such as T cells and macrophages, while dampening down inflammation. It might seem unsurprising that vitamin D deficiency is therefore associated with increased susceptibility to infection, disease and immune-related disorders. However, looking at vitamin D alone may not tell the whole story.
'Vitamin D levels in the blood are affected by so many factors,' says Professor Sattar. 'Low levels are common in people who are obese, who smoke, who are less physically active, who go outdoors less often, who have poorer diets... And all those things lead to many common diseases. So it may be that low vitamin D is reflective of other lifestyle-type factors.'
What's more, vitamin D is a negative acute phase reactant. This means your levels decrease in response to inflammation or infection. Many people have inflammation before they develop a disease, Professor Sattar continues. 'This could cause the vitamin D to become lower – so it's the disease causing the vitamin D to become low, rather than the low vitamin D causing the disease.'
That's not to say vitamin D isn't important. 'If levels are really low, some people get low calcium and bone disease,' Professor Sattar says. 'We know that. But the only way that you know that vitamin D is relevant to chronic disease is by doing randomised trials.' Large studies looking at the effects of vitamin D supplementation on heart failure, diabetes, and cancer found no protective effects, he says.
'Yet in all of those conditions, low vitamin D levels are associated with higher risk,' Professor Sattar adds. 'But remember, obesity is more common than low vitamin D as predictive of diabetes, heart disease and heart failure. So are low activity levels, smoking, and poor diet. And it's those factors that probably cause the diseases as well as causing the vitamin D to be low.'
One area where vitamin D levels may be beneficial is acute respiratory infections. In a meta-analysis of 40 studies by Queen Mary University in 2017, vitamin D supplementation reduced the risk of acute respiratory tract infection among all participants – and was especially effective in those who were very deficient. However, at the time of writing, there is little evidence for using vitamin D supplements to prevent or treat COVID-19.
Vitamin D and COVID-19
Low levels of vitamin D have been associated with severe COVID-19 complications. 'There are a number of studies linking low vitamin D levels to increased risk of getting SARS-CoV-2 infection, and also to increased risk of severity,' says Professor Martineau. 'In other words, if you've got low vitamin D, and you get SARS-CoV-2, you're more likely to end up in hospital.' However, this does not prove a cause-and-effect relationship.
'It is not possible to confirm causality because many of the risk factors for severe COVID-19 outcomes are the same as the risk factors for low vitamin D status,' the NICE guidelines state. 'Vitamin D is a negative acute phase reactant, meaning its serum concentration falls during a systemic inflammatory response, which may occur during severe COVID-19 illness. Therefore, it is difficult to know if low vitamin D status causes poorer outcomes or vice versa.'
The only way to determine the true effects of vitamin D supplementation on COVID-19 is through a randomised clinical trial. Professor Martineau and the wider team at Queen Mary University are currently conducting such a study to investigate whether taking vitamin D could protect people from COVID-19. More than 6,200 participants with low levels of vitamin D in their blood have been given six months' supply of either 800 or 3,200 IU of vitamin D to take daily.
Researchers are tracking the incidence of acute respiratory infections in the participants – including COVID-19 – to determine whether the supplementation has any effect on the risk and severity of infection. The first interim data will be collected in March, with the study due to end between May and June, though the team is looking to extend it further to examine the effects of vitamin D on vaccine response.
Risk factors for vitamin D deficiency
Certain groups are more at risk of vitamin D deficiency than others. Four of the most common risk factors include:
🔹 Your diet
While the most efficient source of vitamin D is produced by the exposure of bare skin to sunlight, 'dietary intake makes up for the shortfall, particularly during the winter months in the northern hemisphere,' says Dr Shoemark. 'Social deprivation means that people who regularly rely on food banks may have fewer fresh foodstuffs in their diet.'
🔹 Lack of sunlight
Vitamin D deficiency is more common in people who spend little time outdoors, says Dr Shoemark. 'This likely includes many of the infirm, who are also more likely to reside in care homes.' Unfortunately, your skin also becomes less effective at converting sunlight into vitamin D as you age.
🔹 Your skin colour
The natural skin pigment melanin reduces your skin's ability to make vitamin D from sunlight. 'Members of the BAME (Black, Asian and minority ethnic) community need longer exposure times to sunlight to produce the levels of vitamin D they need,' says Dr Shoemark.
🔹 Your weight
Vitamin D is fat-soluble, which means it is stored in fat. 'Obesity raises the risk of vitamin D deficiency because adipose tissue sequesters fat-soluble vitamin D, which can reduce its availability,' says Dr Shoemark.
The bottom line
Guidelines recommend everyone in the UK takes 10 micrograms (400 IU) of vitamin D a day between October and early March in order to protect bone and muscle health. This advice is based on blood levels of vitamin D dipping in the winter and rising in the summer. 'Until we've got evidence that more is better, I don't think it makes sense to say that people should take more than that,' says Professor Martineau.
If you take a vitamin D supplement, treat it as an additional preventative measure. 'The worry I have is people out there are starting to take vitamin D and potentially getting false reassurance – 'This is protecting me, and therefore I'll carry on misbehaving',' says Professor Sattar. 'We don't know that. You need the randomised trials to report before we can say yes or no.'
Until then, continue taking steps to protect yourself and others from COVID-19: wear a face mask, avoid close contact with anyone you don't live with, and wash your hands regularly. Maintain a healthy lifestyle, try to be as active as you can within the constraints of government guidelines, and watch your weight, 'because excess weight is definitely a causal risk factor,' says Professor Sattar.
'Those things have much better evidence to protect you, because they protect against chronic disease,' he says. 'And if you get COVID when you've got chronic disease, you're more likely to succumb. If you get COVID when you're overweight, you're more likely to get severe disease. They're real and evidence-based. The rest is for the fairies at the moment.'
Last updated: 26-01-2020
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