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Reported cases of vitamin D deficiency have been increasing in incidence not just in the UK, but on a global scale, and even in countries where sunlight exposure is high. The risk of deficiency increases during the cooler winter months and both the short and long term public health impact of widespread low vitamin D is of significant concern. And especially since we now know that vitamin D acts not only on the skeletal system but on almost every other body system too, including the immune system.

Vitamin D’s crucial involvement in immune health has been of particular interest in 2020, as the world has been learning how to deal with Covid-19. In this article we take a closer look at how vitamin D supports immune health and what you can do to ensure your levels remain optimal as we approach the cooler winter months and risk of deficiency starts to increase.

So how does vitamin D actually support immune health?

The most important point to understand about vitamin D and immune health is that vitamin D is involved in both innate and adaptive immune function. But what exactly does that mean?

The immune system can be divided into 2 vital parts that help to defend us against foreign invaders. These are innate and adaptive immune function.

1. The innate immune response can be likened to first responders at the scene of a crime. Innate immune responses are non-specific and include physiological responses such as inflammation, fever, the release of antimicrobial chemicals and the arrival of phagocytic white blood cells that engulf and digest foreign entities. Vitamin D is essential for the innate immune system to function optimally.

2. The adaptive immune response is more like the specialist team working behind the scenes and recruited a bit further down the line. And this specialist team are particularly helpful in that they remember previously encountered criminals! Adaptive immune responses are therefore more specific, take 4 – 5 days to appear and generate immunologic memory which improves with every encounter. Adaptive immune responses include lymphocytes, memory cells and antibodies. Vitamin D is essential for the adaptive immune system to function optimally.

Without sufficient vitamin D, many aspects of innate and adaptive immune function may be compromised and this is reflected in clinical research:

• A 2009 study published in the Journal of Translational Medicine found that vitamin D insufficiency is common in hospitalised patients. This study demonstrated an association between critical illness and lower vitamin D levels, and a positive association between vitamin D status and an antimicrobial compound named cathelicidin.1 A 2018 study also found vitamin D status to be positively associated with plasma cathelicidin levels.2

• A large 2017 study published in the British Medical Journal found vitamin D to be effective for preventing colds and flu.3

• In a study published in September 2020, researchers analysed data of 235 patients hospitalised with COVID-19. They found a significant association between vitamin D sufficiency and reduction in clinical severity, inpatient mortality, serum levels of C-reactive protein (CRP) and an increase in lymphocyte percentage. The authors concluded, “the significant reduction in serum CRP, an inflammatory marker, along with increased lymphocytes percentage suggest that vitamin D sufficiency also may help modulate the immune response possibly by reducing risk for cytokine storm in response to this viral infection”.4

How can I ensure optimal vitamin D?
Unlike most essential nutrients, the main source of vitamin D is not food, but sunshine; our bare skin produces vitamin D when it comes into contact with the sun’s rays, hence why deficiency rates tend to increase during the cooler months when we spend more time covered up and indoors. In fact, in April 2020 Public Health England responded to growing concerns around high levels of vitamin D deficiency during lockdown when people were spending more time indoors by changing their recommendations on supplementation. During this time, they said that everyone should consider supplementing with vitamin D daily, not just at risk groups.

Test don’t guess
Whilst it’s a good idea to supplement with a daily maintenance dose to keep your vitamin D levels topped up, the best way to evaluate your supplement requirements is with a test. Your GP can check your vitamin D status, or alternatively there are many private companies offering vitamin D tests with a simple at-home finger-prick testing kit for around £30. If you do test, it is important that you work with a healthcare professional to evaluate your optimal daily dose. Ideally everyone should test twice yearly; at the end of summer and at the end of winter, to keep a close eye on any changes.

Supporting immune health with vitamin D
Vitamin D is important for both innate and adaptive immune health and research has demonstrated benefits of keeping levels within an optimal range. Everyone should supplement with a daily maintenance dose during the cooler winter months; it is recommended however that you check your current vitamin D level to ascertain the optimal daily dose for you. Vitamin D is certainly not a cure all or panacea but should be considered an important part of the bigger picture of immune health.

Find out more on the many different diet and lifestyle ways you can support your immune health here.

1. Jeng L, Yamschikov AV et al. Alterations in vitamin D status and anti-microbial peptide levels in patients in the intensive care unit with sepsis. J Transl Med. 2009; 7: 28
2. Georgieva, V et al. Association between vitamin D, antimicrobial peptides and urinary tract infection in infants and young children. Acta Paediatr. 2018
3. Martineau AR, Joliffe DA et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ 2017; 356: 16583
4. Maghbooli Z, Sahraian MA et al. Vitamin D sufficiency, a serum 25-hydroxyvitamin D at least 30 ng/ml reduced risk for adverse clinical outcomes in patients with COVID-19 infection. PLOS One 25 September 2020

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