Vitamin D is known for helping to build and maintain the health of our bones, teeth and muscle function. It does this by enabling the absorption of calcium and phosphorus from the gut into the bloodstream and helps prevent calcium loss from the kidneys. Despite there being over 30 years of research substantiating this, is vitamin D’s importance to our immune system and the role it plays in protecting against autoimmune conditions, hypertension, infectious diseases and cancer. (14)

Vitamin D deficiency has been recognized as a world pandemic. (14) The countries with the highest rates of deficiency are mainly in the Middle East, China, Mongolia and India. About 60% of the UK population is permanently deficient in Vitamin D. This rises to 90% deficient in the winter months.

Less than 50% of the world’s population has adequate vitamin D levels (defined as serum 25-hydroxyvitamin D greater than 50 nmol/L (table 1), especially during the winter months. (15)

Vitamin D is a group of fat-soluble steroid hormones, so not a vitamin in the conventional sense. As vitamin D functions as a steroid, it is also important for many processes within the body related to hormones. (1) Vitamin D helps to regulate adrenaline, noradrenaline and dopamine production in the brain. It also protects against serotonin depletion. Low vitamin D levels increase a person’s risk of depression. (2) It’s no wonder that Vitamin D is known as the sunshine vitamin.

When UVB radiation from the sun interacts with exposed skin (not covered in clothes or sunscreens) an incredible reaction takes place. UVB rays interact with a compound embedded in our skin called 7-dehydrocholesterol, converting this into cholecalciferol, also known as vitamin D3. But this is just the start. This D3 moves through the bloodstream to the liver where the liver converts it to 25-hydroxyvitamin D, also known as 25-hydroxycholecalciferol. This is actually what most doctor’s test for when measuring our vitamin D levels. This once again moves through the bloodstream to the kidneys where it is converted to its final form, 125-dihydroxycholecalciferol, aka 125-dihydroxy vitamin D. This final form is known by the name D. Calcitriol. (3)

A circulating level of 25-hydroxyvitamin D of >75 nmol/L, is required to maximize vitamin D’s beneficial effects for health (table 1).

Almost every cell and tissue in our bodies have vitamin D receptors (VDR) sites. Researchers at the University of Oxford used DNA sequencing technology, chromatin immunoprecipitation followed by massively parallel DNA sequencing (ChIP-seq), to create a map of vitamin D receptor points binding across the human genome. (4)

The vitamin D receptor (VDR) is a protein activated by vitamin D, which attaches itself to DNA and thus influences what proteins are made from our genetic code (5) the researchers identified 2,776 genomic positions occupied by the VDR. These binding sites were unusually concentrated close to genes associated with susceptibility to autoimmune conditions such as MS, Chron’s, Lupus, rheumatoid arthritis and cancers such as chronic lymphocytic leukaemia and colorectal cancer. (4)

The research also showed that vitamin D had a significant effect on 229 genes including IRF8, previously associated with MS, and PTPN2, associated with Crohn’s disease and type 1 diabetes. (5) It is thought Vitamin D’s purpose in regulating these genes is to control cell growth and cellular differentiation and may be responsible for decreasing the risk of the cells being transformed into a malignant state. (14)

To quote Dr Andreas Heger from the MRC Functional Genomics Unit at Oxford, one of the lead authors of the study, “Our study shows quite dramatically the wide-ranging influence that vitamin D exerts over our health.” (5)

Immunomodulation is the ability of the immune system to increase or lower its activity as needs are. Vitamin D is an immunomodulator, which means it helps to support a beneficial immune response. In the case of autoimmunity, where the immune system is overreacting, vitamin D can help lower this reaction. In the case of infection or even cancer, then a heightened immune response is required to deal with the threat.

Every day thousands of cell mutations occur during the process of cell replication. When this happens Vitamin D plugs into these mutated cells to deliver instructions to the cell to repair itself and return to being a healthy cell that replicates normally.

If vitamin D doesn’t give this instruction properly and the cell fails to repair itself, vitamin D instructs the cell to destroy itself, a process known as apoptosis. This programmed cell death helps cells to destroy themselves rather than mutating into cancer cells.

More than 80 years ago the correlation between living at higher latitudes and increased risk of developing and dying of colon, prostate, breast, and several other cancers, was reported. As living at a higher latitude makes it harder for the body to produce Vitamin D, the link was made between a deficiency and cancer. (14)

A study published in the American Journal of Public Health states, “The majority of studies found a protective relationship between sufficient vitamin D status and lower risk of cancer. The evidence suggests that efforts to improve vitamin D status, for example by vitamin D supplementation, could reduce cancer incidence and mortality at low cost, with few or no adverse effects”. (13)

Vitamin D has also been shown to be important for protection against viral or bacterial upper respiratory infections as it protects against infection, supports the immune function in the throat, airways and lungs, helps to regenerate the protective endothelial barriers and regulate the inflammatory response to prevent cytokine storm. (6)

It shouldn’t be surprising that many peer-reviewed studies are affirming the connection between vitamin D deficiency and susceptibility to covid-19. Deficiency in vitamin D increases the chances of becoming infected and developing severe disease. According to the BMJ, the covid-19 ICU risk is 20 times higher in those with vitamin D deficiency. (10)

As vitamin D helps to regulate the immune response, those with an adequate level of vitamin D are less likely to enter into a cytokine storm. (8)

SARS-COV-2 is the virus that causes the disease covid-19. If SARS-COV-2 has been successful at replicating itself and spreading to surrounding cells, mild to moderate symptoms will appear, such as sore throat, cough, fever, muscle aches, which are all part of a normal immune response. A hyper-inflammatory response, also known as a cytokine storm, is an excessive immune response. These inflammatory cytokines along with the continued spread and replication of the virus, trigger more immune cell involvement and tissue damage. This leads to acute respiratory distress (ARD), multiple organ failure and even death. (9)

The immunomodulatory actions of vitamin D play a very important role in preventing this inappropriate hyper-inflammatory immune response.

Who is at risk of vitamin D deficiency?

Insufficient exposure to sunlight 

Even before the pandemic lockdowns, many people were spending the majority of their time indoors with working long hours, children not playing outdoors, exercising in indoor gyms. When people do venture outside they tend to cover up out of fear of premature skin ageing or skin cancer which prevents UVB from interacting with the skin. Skin cancer risk results from excessive exposure to sunlight that causes sunburn, not moderate sun exposure. (16)

Some of the sunniest areas of the world have high vitamin D deficiency levels due to covering up of skin for cultural or religious reasons.

Living at high latitude

The UK’s location in the Northern Hemisphere means that during the winter months the sun is lower in the sky and the days are shorter. It’s almost impossible to make vitamin D during the winter. This is also the time of year known as ‘cold and flu season, but as the pathologist, Dr Ryan Coles says, “There is no such thing as cold and flu season, there is only low vitamin D season”.

Darker skin

Dark skin has more melanin which protects it from UV light. People with dark skin need to spend longer in the sun to make vitamin D compared with paler skins. (6) It is thought this is why lighter skin and hair evolved as humans moved further away from the equator.

According to the American Journal of Nutrition, African Americans with very dark skin have an SPF of 15, and, thus, their ability to make vitamin D in their skin is reduced by as much as 99%. (14) People of African descent who live in the Northern Hemisphere have high levels of vitamin D deficiency compared to those living near the equator with a higher influx of UVB radiation, are not deficient. (14)


Pollution can reduce the amount of UVB radiation reaching the Earth’s surface so fewer UVB rays are penetrating the skin for vitamin D synthesis.


Vitamin D synthesis in the skin in response to UVB exposure falls by at least 50% in elderly people compared with young adults mobility or being institutionalized in care homes and not getting outside at all. (6)

A 2014 study published in the European Journal of Endocrinology of 11 nursing homes in Sweden reported that 80% had 25(OH)D levels below 50 nmol/l, and vitamin D deficiency was associated with increased mortality. (17)

Deficiency in breastfeeding mothers 

If a mother is low in vitamin D she can’t pass vitamin D on to her infant in her breast milk.

Lack of dietary fat 

A diet low in fat is not good for vitamin D which requires fat to be absorbed in the gut.

Poor kidney and liver function

It is also important that the kidneys, liver, gut and skin are healthy too. One of the most essential things for healthy kidney function is simply hydration. Ensuring intake of adequate amounts of water and also not overburdening the kidneys and liver with having to deal with a lot of ultra-processed foods. Our skin health reflects how we nourish our bodies and minds.


Vitamin D is fat-soluble so can get sequestered by fat tissue becoming unavailable for the body to use.

How to increase vitamin D levels

If humans were living in a natural setting, we would be getting 10% of our vitamin D from food and 90% from sunlight.

The best way to obtain vitamin D is from sunlight to interact with the skin in a safe way that doesn’t involve getting burnt. There is a free app called dminder that tells you the best time to be outside to make vitamin D.

Some food sources do contain vitamin D, but not enough to meet all our requirements.

Cod liver oil is by far the best food source. It has 446 IU per teaspoon. (11) Wild-caught salmon has about 988IU of vitamin D per 100g serving whilst farmed Atlantic salmon only contains 526 IU per 100g (11). This is probably due to vitamin D being plentiful in the food chain but not plentiful in the pellets fed to farmed fish.

Other oily fish such as halibut, herring and mackerel have around 100-300IU per 100g. (11)

Egg yolk contains 37 IU of vitamin D. Interestingly, pasture-raised chickens that spend their days outside in the sun produce eggs with vitamin D levels 3-4 times this. (11)

Mushrooms are rich in vitamin D2, not D3 which is only from animal sources. D3 is two times more efficient and effective for the body than D2. (11)

Wild mushrooms can contain 2,000IU of vitamin D2 per 100g, but farmed mushrooms are grown indoors so contain very little vitamin D.(11)

Quite a few highly processed food manufacturers have started adding vitamin D to breakfast cereals. However, this is vitamin D2. A large quantity of cereal would also need to be consumed to get an effective amount. (11)


During the winter in the UK, other Northern Hemisphere countries or people who aren’t getting adequate sunlight exposure will need to take a supplement. Taking vitamin D as a spray under the tongue is best as it’s more bioavailable and absorbed faster.

Various sources of vitamin D is used in supplements. Lanolin is a wax secreted by the sebaceous glands of wool-bearing animals. Lichens are a unique combination of algae and fungi and an excellent vegan source of D3. Then of course there are supplements made from fish oils and mushrooms.

As Vitamin D is fat-soluble, fats are required for it to be absorbed. Be careful you don’t buy a supplement that contains soy or corn oil. Vitamin D should be taken with vitamin K2 as they work closely together to control calcium levels in the body. Vitamin D controls the absorption of calcium into the blood. Vitamin K2 controls where that calcium ends up. Over-supplementation of vitamin D3 without ample vitamin K2 leads to problems of excess calcium. If calcium isn’t laid into bone, it will find itself in other tissues, like arteries where it contributes to atherosclerosis and vessel stiffness.

Can you overdose on Vitamin D?

Much has been made about supplements and toxicity. The potential toxic effects of vitamin D, such as bone demineralization, hypercalcemia, hypercalciuria, or nephrocalcinosis with renal failure, are encountered rarely, generally only when the daily dose exceeds 10 000 IU of vitamin D over an extended period. (13) Toxicity in the past has been due to overdoses in the range of 50 000 to 150 000 IU per day on a long-term basis.

The research on Vitamin D concludes that it is best to supplement with vitamin D daily rather than taking large doses infrequently. Dosages of up to 10,000IU daily are considered safe (13). The dosage depends on the individual so always seek advice from a Naturopath.

Table 1

50-74nmol/L Adequate
25-50nol/L Insufficient
Less than 25nmol/L Deficient

NHS Guidelines

Serum Vitamin D Level


Siobhán Cosgrave is a registered Ayurveda Herbalist & Naturopath. Siobhan helps people uncover the underlying imbalances causing their symptoms using ancient wisdom and modern science. These underlying causes are rebalanced through nutritional changes, lifestyle modifications and bespoke herbal remedies. Thus enabling people to live full lives that are symptom-free, with abundant energy. Siobhán has a special interest in gut health, auto-immune conditions, and women’s hormonal health.


1 You and your Hormones: an education source from the Society for Endocrinology (2018) [Online] Available at:

2 C.Geng et al (2019) Vitamin D and depression: mechanisms, determination

and application [Online] Available at:

3 R. Nair & A Maseeh (2012) Vitamin D: The “sunshine” vitamin [Online] Available at:

4 S. Ramagopalan et al (2010) A ChIP-seq defined genome-wide map of vitamin D

receptor binding: Associations with disease

and evolution. Genome Research [Online] Available at: [accessed 23 September 2021].

5 Vitamin D found to influence over 200 genes, highlighting links to disease (2010) Science Daily [Online] Available at: [Accessed 20th September 2021]

6 G.Griffin et al (2020) Vitamin D and COVID-19: evidence and recommendations for supplementation. The Royal Society [Online] Available at: [Accessed 21st September 2021] The American Journal of Clinical Nutrition, Volume 87, Issue 4, April 2008, Pages 1080S–1086S,

8 R. Ghasemian et al(2021) The role of vitamin D in the age of COVID‐19: A systematic review and meta‐analysis NCBI [Online] Available at: [Accessed 2nd September 2021].

9 Grassroots Health Nutrient Research Institute (2020) Vitamin D Helps Fight COVID-19: Part 2 [Online] Available at: [Accessed on 20th September 2021]

10 The BMJ (2020) Is ethnicity linked to incidence or outcomes of covid-19? [Online] Available at [Accessed 20th September 2021].

11 7 Healthy Foods High in Vitamin D. [Online] Available at: [Accessed 20th September 2021].

12 J. Hathcock (2007) Risk assessment for vitamin D. The American Journal of Clinical Nutrition, Volume 85, Issue 1, January 2007, Pages 6–18 [Online] Available at: [Accessed 21 September 2021].

13 C.Garland et al(2006) The Role of Vitamin D in Cancer Prevention: AJPH [Online] Available at: [Accessed: 23rd September 2021].

14 M. Hollick, T. Chen (2008): Vitamin D deficiency: a worldwide problem with health consequences: American Journal of Nutrition [Online] Available at:[Accesssed 24th September 2021]

15 N. Vam Schoor, P/Lips (2017) Global Overview of Vitamin D Status. Endocrinology and Metabolism Clinics of North America Volume 46, Issue 4, December 2017, Pages 845-870 [Online] Available at: [Accessed 24 September 2021].

16 A.Spiro, J.Buttriss (2014) Vitamin D: An overview of vitamin D status and intake in Europe: Nutrition Bulletin, V39, issue4, p322-350 [Online] Available at: [Accessed 24th September 2021].

17 Samefors M, Östgren CJ, Mölstad SA et al. (2014) Vitamin D deficiency in elderly people in Swedish nursing homes is associated with increased mortality. European Journal of Endocrinology 170: 667–675.