Vitamin D: The Sunshine Vitamin

In this article, I will explore the sunshine vitamin, Vitamin D and its importance for overall health.

What are Vitamins?

Vitamins are essential, naturally occurring micronutrients found in small quantities in food and beverages. The exception is Vitamin D. Although some forms are available via diet our main source of Vitamin D is via the sun and its UVB rays. Vitamins usually act as co-factors that support enzyme activity in the body.

There are two types of vitamins, fat-soluble and water-soluble, Vitamin D is fat-soluble.

Most vitamins have specific deficiency signs associated with them and a severe deficiency can lead to death. Age, pregnancy, breastfeeding, exercise training, stress, illness, environmental and genetic factors may vary the need for certain vitamins.

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Ergocalciferol (D2) & Cholecalciferol (D3)

There are different forms of Vitamin D, including ergocalciferol (Vitamin D2) and cholecalciferol (Vitamin D3). Vitamin D is a steroid (cholesterol derived) prohormone that can be converted to a hormone in the body and is made in the skin when it is exposed to solar UVB radiation.

The natural form of Vitamin D is made in the body (cholecalciferol), which is made when exposed to sunlight.

D2 (ergocalciferol), is made by phytoplankton, yeast and fungi from the precursor, ergosterol when exposed to UV light.

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Metabolism, storage and excretion

D2 & D3 require metabolism in the liver and kidneys to produce the main form of active hormone in the body. In the liver, both forms are converted to 25-hydroxycholecalciferol (25(OH)D) by the enzyme 25-hydroxylase.

The liver releases 25(OH)D into the bloodstream and is taken up by the kidneys for conversion by enzymes to its major hormonal form, active Vitamin D (also known as 1,25-dihydroxycholecalciferol). This helps regulate cell proliferation and gene expression in addition to regulating calcium balance and bone growth.

The blood has the highest concentration of Vitamin D from the liver and has a circulating half-life of 2-3 weeks, with levels replenished from fat stores. Other storage sites include blood, liver, kidneys and muscles. Vitamin D3 is mainly stored in adipose tissue, D2 in muscle.

It helps maintain calcium and phosphate levels in the blood to support nerve and muscle function and bone growth. Vitamin D also regulates immunity, cell replication and growth.


  • Bone mineralisation: Vitamin D is essentail for normal bone growth and mineralisation. Deficiency may cause rickets.
  • Muscle and nerve function: Vitamin D maintains serum levels of calcium for delivery to the muscles and nerves. Calcium regulates permeability of nerve membranes and the release of acetylcholine which lowers the action potential of nerves, having a calming effect on the nervous system.
  • Brain function: Vitamin D activates receptors on neurons in the brain that regulate behaviour, inhibit age related changes in the brain cell and has protective functions in the brain. It also regulates the expression of genes involved in forming neuron connections and memories, calcium metabolism, neurotransmitter synthesis and control of damaging free radicals.
  • Assists cell replication and differentiation, anti cancer activity.
  • Immunity: It controls proliferation and activity of dendritic cells, macrophages, lymphocytes,adaptive and innate immune responses. It may help auto immune reactions in which the body attacks normal body tissues. It may also help prevent organ transplant rejection.
  • Antioxidant and anti-inflammatory activity: Vitamin D helps increase intracellular levels of the precursor to glutathione (master antioxidant) and inhibit production of inflammatory cytokines.
  • Insulin release: Vitamin D may have a role in stimulating insulin release, stabilising blood sugars.
  • Blood pressure regulation: Vitamin D inhibits renin production in the kidneys reducing the activity of the renin-angiotensin system that drives up blood pressure by costricting small arteries and increasing sodium retention.

Dietary Sources

There is a limited amount of Vitamin D in the food supply. Therefore it is impossible to maintain healthy blood levels of 25(OH)D solely from food sources. UVB exposure (sunlight) and supplementation are the most reliable sources.

Vitamin D3 is only found in fat-containing animal foods such as fish liver oils, eggs yolks, fatty fish such as sardines, mackerel and herring, full-fat dairy products such as butter, milk and cheese.

Farmed fish may only contain about 25 per cent of the D3 in wild-caught fish because of artificial feeding.

Vitamin D2 may be found in naturally grown field mushrooms and sun-dried mushrooms. Most commercially bought mushrooms are grown in dark conditions and do not contain significant amounts of Vitamin D. If you leave sho bought mushrooms out in the sun for 20 minutes before eating, this can boost their content.

Influencing factors

Dietary fat is needed to absorb Vitamin D, people on low-fat diets or those with fat malabsorption may have inadequate levels. Digestive issues such as gall bladder and pancreatic disorders, inflammatory bowel disease and cystic fibrosis, reduce the absorption of Vitamin D.

Absorption may also be decreased by drugs that inhibit cholesterol uptake. Parathyroid, liver or kidney disorders may impair metabolism.

Latitude and Seasons

Serum levels vary according to seasons, with the lowest levels at the end of winter, and highest levels at the end of summer. Higher altitude regions have greater UVB exposure than lower altitudes.

Anyone living north of 35 degrees latitude will not absorb enough UVB light during winter and late autumn months to metabolise Vitamin D. The body relies on stores built up over summer. Regular sun exposure can produce the equivalent of 70 mcg (2800 IU) of vitamin D daily.

Peak D3 production occurs in the middle of the day when the sun is at its strongest. One full day in the sun that causes a slight reddening of the skin can create the same amount of Vitamin D as an oral intake of 500mcg (20,000 IU).

Deficiencies are more common in those with darker skin pigmentation, elderly in nursing homes, shift workers, or those that do not spend any time outdoors. Skin exposure to UVB light is restricted by air pollution, indoor lifestyles, nightshift work, excessive clothing or regular suncream use.


It is estimated 10-15 minutes of midday sun exposure on the face, hands, arms, legs or back without sunscreen at least twice a week may be enough to produce adequate Vitamin D in summer.

To make Vitamin D effectively, sufficient unprotected bare skin must be exposed and UV radiation must be strong enough. Melanin blocks UV rays and D3 may be decreased in heavily tanned or pigmented skin. It is estimated those with darker skin pigmentation may need 10-50 times more exposure to UVB in order to make vitamin D compared to those with fair skin types.

Sunscreen factor 8 or above when properly applied, will reduce the ability to make Vitamin D by 95-99%.

Deficiency Effects

Vitamin D deficiency is a worldwide health issue, especially in those with dark skin living at higher latitudes. A study in Australia demonstrated that 33% of men and 40% of women had low levels of Vitamin D.

A recent large observational study suggested that ~40% of Europeans are vitamin D deficient, and 13% are severely deficient.

Lack of Vitamin D may cause:
  • Decreased serum (blood) calcium levels, triggering the release of the parathyroid hormone to mobilise it out of the bones and into the blood. Bones may become soft, weak, painful and fracture more easily.
  • When Vitamin D is combined with calcium deficiency bone abnormalities are more severe.
  • Elevated parathyroid hormone can also cause fatigue, depression, constipation, heartburn, peptic ulcers, nausea, vomitting, appetite loss, kidney stones, colon cancers and benign growths in the bones.
  • Low serum calcium levels in children with developing bones cause rickets, also affecting nerve and muscle weakness, causing pain and delayed motot development. Other effects can be irritability and lethargy. It may also cause congestive heart failure, dysfunction of the left ventricle, shock and death.
  • Low Vitamin D in adults causes osteomalacia and osteoporosis. This may lead to pain, increase risk of fractures, muscle aches and a waddling gait. Pain symptoms may be misdiagnosed as fibromyalgia or symptoms of depression.
  • Through its effects on calcium metabolism a deficiency can also cause nervous tension, irritability, anxiety and insomnia.
  • In pregnancy low levels may cause pre eclampsia, low birth weight, poor post natal growth, auto immune issues and bone fragility.
Indicators of low levels may include:
  • Bone weakness, pain, fractures, osteomalacia, osteoporosis
  • Muscle weakness, pain, loss of balance, chronic fatigue
  • Nervous tension, irritability, anxiety, insomnia, mood disorders
  • Headaches and pressure in the head
  • Psoriasis
  • Lowered immunity for infections, autoimmune disorders, diabetes, MS, asthma
  • Inflammatory and cardiovascular disorders, elevated blood pressure
  • Dementia, autism, schizophrenia
  • Cancer

Adequate levels

Vitamin D status is measured in the blood, which represents the amount in the liver.

To improve your immune function and lower your risk of viral infections, you’ll want to maintain a Vitamin D level between 60 ng/mL and 80 ng/mL (150 nmol/L and 200 nmol/L).

While conventional medicine insists a Vitamin D level of about 20 ng/mL is adequate, that really only applies to bone health. For optimal health, immune function and disease prevention, you want a blood level between 60 ng/mL and 80 ng/mL year-round. In Europe, the measurements you’re looking for are 150 nmol/L and 200 nmol/L.

The best way to know what supplementation you need is to test your levels. It is generally considered safe to take 4-5000 IU/ day. Higher doses and loading doses may be given under the advice and guidance of a doctor based upon testing.

Those with darker skin pigmentation or in areas higher than 35 degrees latitude, may need higher doses. As Vitamin D is a fat-soluble vitamin and may be toxic at high levels, it is best to test serum levels and recheck them again after 3-6 months of supplementation.

You can calculate dosage here.

Ways to improve levels

The specific dosage required to maintain an optimal Vitamin D level can vary widely from person to person depending on a variety of factors, including skin pigmentation, age and weight.

Gut health can also play an important role in how well you absorb the Vitamin D you take. When you have a healthy gut, beneficial bacteria produce butyrate by breaking down dietary fibre. Butyrate, in turn, helps increase Vitamin D, so the more butyrate you have, the more Vitamin D your body can absorb.

Magnesium is required for the conversion of Vitamin D into its active form. According to a scientific review published in 2018, about half of Americans taking Vitamin D supplements may not get significant benefits as Vitamin D simply gets stored in its inactive form, and the reason for this is because they have insufficient magnesium levels.

Research by GrassrootsHealth shows you need 146% more Vitamin D to achieve a blood level of 40 ng/ml (100 nmol/L) if you do not take supplemental magnesium, compared to taking your Vitamin D with at least 400 mg of magnesium per day.

Vitamin K2 intake can also affect your required Vitamin D dosage. According to GrassrootsHealth, “combined intake of both supplemental magnesium and vitamin K2 has a greater effect on Vitamin D levels than either individually,” and “those taking both supplemental magnesium and Vitamin K2 have a higher Vitamin D level for any given Vitamin D intake amount than those taking either supplemental magnesium or vitamin K2 or neither.”

Vitamin D supplements should be taken during the morning so it does not affect melatonin levels.

Vitamin D and Covid-19

For the last few years, we have been at a grand solar minimum, meaning we are exposed to weaker UVB levels. We also spend much less time outdoors as a society than we used to. In addition, we have been terrified so much about the sun and skin cancer, many slather factor 50 on when in the sun.

It has been documented in the past that at times of solar minimums, pandemics occur. In the last 2 years, we have heard experts and Governments deny that adequate levels of Vitamin D have any impact on severity and mortality from Covid-19. However, the evidence is mounting.


It’s been previously established that low Vitamin D increases the risk of respiratory infections, including ARDS (acute respiratory distress syndrome), while Vitamin D supplementation helps to reduce the risk of such infections. One of the roles of Vitamin D in the immune system is to increase anti-microbial peptides in cells, and this includes the epithelial cells that line the respiratory tract.

In one randomized controlled trial of elderly people in long-term care, monthly high dose Vitamin D supplementation was shown to reduce the incidence of upper respiratory infections, in the centre’s residence.

This is great news since COVID-19 affects the immune system and respiratory system. The research specifically on SARS-CoV-2, the virus that causes COVID-19, is beginning to paint a similar picture.

And more evidence

If you search for Vitamin D and Covid-19 in google scholar 122,000 articles and research papers appear, a handful of which are summarised below.

One study looked at Vitamin D levels of COVID-19 patients in Spain, Italy and Switzerland and found low vitamin D levels correlate with more COVID-19 cases, more complications and higher mortality rates. The study also noted that Vitamin D levels are lower in the ageing population and these tend to be the people most vulnerable to COVID-19 severity and death.

In another article looking at the population in Switzerland, low Vitamin D levels were associated with positive SARS-CoV-2 tests whereas those with negative tests were more likely to have higher Vitamin D levels.

A recent review article showed that infection rates are lower in the Southern Hemisphere as the infection hit at the end of summer when Vitamin D levels tend to be higher versus in the Northern Hemisphere where the pandemic spread at the end of winter when Vitamin D levels are at their lowest. They also cite evidence that Vitamin D reduces the risk of influenza.

Research looked at the links between COVID-19, Parkinson’s disease and Vitamin D, concluding that Vitamin D has the potential to both slow the progression of Parkinson’s while offering protection from COVID-19.

Taken together, this evidence for Vitamin D reducing the symptoms and severity of respiratory infections and possibly COVID-19 makes it an important tool in our prevention toolkit.

It’s important to note many scientists admit that more research is needed to make a clear connection with Vitamin D’s role in COVID-19, but because Vitamin D is safe and has so many other health benefits, optimizing Vitamin D levels is a key part of overall preventative medicine. One definitely worth focusing on.

To conclude

It is clear that Vitamin D, the sunshine vitamin, is important for many functions in the body. Many of us remember being given Cod Liver Oil by our parents and grandparents, but this has faded over time. Adults and children alike spend more time indoors or cover themselves from head to toe in sunscreen, blocking the bodies ability to absorb and utilise Vitamin D.

In the current climate, it is also clear low levels of vitamin D lead to greater severity and mortality from Covid-19 and also other infectious diseases. Anyone who lives above 35 degrees latitude would benefit from supplementing with vitamin D in autumn and winter.

It is considered safe to take 4000-5000 IU per day, taken with Magnesium improves utilisation by the body and taken with K2 helps keep Calcium in the bones and serum levels within range. In summer, 10-15 minutes of bare body exposure may be enough to top up levels.

Although food sources provide us with some Vitamin D, being outside in the sun is where it is at. Having been at a solar minimum for a long time now, supplementation directed by testing should be implemented in those particularly vulnerable groups.


Links to studies/articles are linked in the text above.

Paxton, Fay (2015). Foundations of Naturopathic Nutrition. Allen & Unwin

Mindell, E (2011). New Vitamin Bible. Grand Central Life & Style

Holford, P (1997). The Optimum Nutrition Bible. Piatkus

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