Boosting immunity and your vitamin D levels

Boosting immunity and your vitamin D levels

As the dark, long nights and chilly days begin to draw in, so does the inevitable cold and flu season. We all know the drill; runny noses, stuffy sinuses and those familiar aches, pains and feelings of fatigue. During this time of year, immunity becomes an important health focus for many of us. You may wonder, how exactly the immune system works, and is there anything we can do to boost its effectiveness to avoid catching the latest bug doing the rounds, and how might this relate to your vitamin D levels?

The immune system is a network of cells and organs and its primary role is to protect the body against pathogenic organisms such as viruses, bacteria and parasites. Immune response, and therefore effectiveness, is variable from person to person and dependent on several factors, including age, sleep quality, stress, genetics and of course, diet[1]. A whole range of vitamins and minerals, including zinc, selenium and vitamin C, are involved in keeping the immune system functioning at an optimal level, but one that is of particular importance – especially at this time of year – is vitamin D.

What is vitamin D?

Vitamin D is a unique nutrient because it functions as both a vitamin and a hormone within the body. Many people may know that vitamin D supports calcium and phosphorus absorption, helping to maintain healthy bones and teeth[2], but its work goes way beyond that. Vitamin D receptors can be found in almost every cell of the human body and therefore its role in maintaining health is widespread; from controlling inflammation to regulating mood, and even supporting our muscles to contract – there isn’t much this wonder nutrient can’t do! Most of our vitamin D requirements are met through its production under the skin on exposure to sunlight, specifically from UVB rays, which is why it is commonly known as the ‘sunshine vitamin’. However, a small amount of vitamin D can also be found in some foods, such as oily fish (salmon, mackerel and sardines), egg yolks, sun-exposed mushrooms and fortified food products[3].

How does vitamin D affect immunity?

Owing to its role as a hormone, vitamin D exhibits various actions within the body to support the immune system and keep us healthy. Firstly, vitamin D has been found to modulate immune function; it is now understood that vitamin D receptors are present on many of the immunological cells that form the immune system, including B cells, T cells and antigen-presenting cells, and is therefore required to allow them to function effectively. Vitamin D has been shown to regulate both the innate and adaptive immune responses[4], supporting the immune system to fight off both known and unknown pathogens and learn how to respond to them when exposed to them in the future. In fact, several studies have shown clear correlations between vitamin D deficiency and increased susceptibility to illnesses such as seasonal flu, upper respiratory tract infections and even COVID-19; sufficient vitamin D levels may also reduce the severity of these diseases[5].

Furthermore, vitamin D exhibits potent anti-inflammatory effects in the body, which may in turn encourage healthy immune function. While some inflammation is an essential mechanism in the immune response, chronic, irregulated inflammation can negatively alter its effectiveness. It has been demonstrated that vitamin D supplementation may reduce pro-inflammatory mediators and increase anti-inflammatory cytokines[6]. As such, vitamin D’s mediating effects on inflammation are crucial in promoting healthy immune function and may help prevent circulating viruses from taking hold.

However, vitamin D is not only useful for avoiding acute infectious disease but may also reduce the risk of developing autoimmune conditions, whereby the body’s immune system attacks its own organs and tissues. Deficiency of vitamin D has been linked to increased risk of autoimmune condition development including multiple sclerosis, type 1 diabetes and rheumatoid arthritis; in a large-scale double-blind clinical trial of over 25,000 participants, vitamin D supplementation was found to reduce autoimmune disease development by 22%[7]. Many adults in the UK have insufficient levels of vitamin D, especially during the winter months, due to lower levels of sunlight – which may explain why we are more susceptible to illness during this time. The National Diet and Nutrition survey shows that 1 in 6 adults have low serum vitamin D[8]. The best way to know whether your vitamin D levels are optimal is to test – our easy-to-use Vitamin D home testing kit allows you to accurately measure your Vitamin D with just the prick of a finger.

Low vitamin D levels

If your levels are low, it’s time to top up! As previously mentioned, the best source of vitamin D comes from that formed under the skin upon sun exposure. However, in the northern hemisphere hours of daylight are sparse during the winter months and not likely to be sufficient for optimal vitamin D production. As such, the UK government recommend supplementing with 10mcg per day. Eating vitamin D-rich foods, such as those listed above, can also help increase intake. Animal foods provide vitamin D3 (cholecalciferol) and plant-based foods provide D2 (ergocalciferol); while both forms are useful for increasing vitamin D levels, the general consensus is that vitamin D3 is more effective than D2 at raising blood levels of calcifediol[9]. Natural sources of vegan D3 are rare (you can learn more about them here, and most vegan-friendly food products are fortified with vitamin D2, so instead adding in a vegan D3 supplement may be advisable. Our Vegan Vitamin D3 capsules deliver 50mcg of cholecalciferol (vitamin D3) per capsule derived from lichen. Finally, it is important to note that vitamin D is a fat-soluble nutrient – as nature would have it, most food sources of vitamin D already contain some fat (such as oily fish) to support absorption. For other foods, like mushrooms, and supplements, it is recommended to consume these with a source of fat to ensure that vitamin D is readily absorbed by the gut.

To conclude, the role of the immune system is to protect the body against pathogenic organisms, prevent illness and support overall health and well-being. A range of nutrients are required for it to work optimally, including vitamin D. Vitamin D modulates cells within the immune system and reduces systemic inflammation to support healthy immune function. While sunlight is the best source, it is not always available, therefore topping up with food sources and supplements, particularly those containing vitamin D3, would be advisable.


[1] Calder, P.C. (2020) ‘Nutrition, immunity and covid-19’, BMJ Nutrition, Prevention & Health, 3(1), pp. 74–92. doi:10.1136/bmjnph-2020-000085.

[2] Harvard T.H.Chan (2023) Vitamin D, The Nutrition Source. Available at: (Accessed: 01 November 2023).

[3] NHS (2020) Vitamin D, NHS choices. Available at: (Accessed: 01 November 2023).

[4] Aranow C. Vitamin D and the immune system. J Investig Med. 2011 Aug;59(6):881-6. doi: 10.2310/JIM.0b013e31821b8755. PMID: 21527855; PMCID: PMC3166406.

[5] Shah, K. et al. (2022) ‘Does vitamin D supplementation reduce COVID-19 severity?: A systematic review’, QJM: An International Journal of Medicine, 115(10), pp. 665–672. doi:10.1093/qjmed/hcac040.

[6] Krajewska, M. et al. (2022) “Vitamin D effects on selected anti-inflammatory and pro-inflammatory markers of obesity-related chronic inflammation. Frontiers in Endocrinology, 13.

[7] Hahn, J. et al. (2022) ‘Vitamin D and marine omega 3 fatty acid supplementation and incident autoimmune disease: Vital randomized controlled trial’, BMJ [Preprint]. doi:10.1136/bmj-2021-066452.

[8] Public Health England (2020) National Diet and Nutrition Survey: Diet, nutrition and physical activity in 2020. Available at: (Accessed: 01 November 2023).

[9] Balachandar, R. et al. (2021) ‘Relative efficacy of vitamin D2 and Vitamin D3 in improving vitamin D status: Systematic review and meta-analysis’, Nutrients, 13(10), p. 3328. doi:10.3390/nu13103328.

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  • Great job on your blog post! The content was well-organized and presented in an accessible manner. I appreciated the practical tips and suggestions you provided, as well as the real-life examples that helped illustrate the concepts.


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Elisabeth Philipps

Karen Devine

Karen Devine

Shelley Harvey

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