Vitamin D - Fact Sheet

In this Article (Index)

• Vitamin D at a glance
• The Discovery of Vitamin D
• Vitamin D2 and vitamin D3
• The transformations of vitamin D
• Vitamin D: its role in the body
• Normal levels of vitamin D in the body
• Recommended Daily Intake
• Sources of Vitamin D
• Dietary sources
• Fortified Foods
• Cod Liver Oil
• Sunlight Exposure and Vitamin D
• Vitamin D Health Claims
• Health Effects of Vitamin D
• Bone Health and Rickets
• No protective effects on Cancer, Diabetes, or Cardiovascular disease
• The Risks of taking High Doses of Vitamin D
• Psoriasis and Vitamin D
• Vitamin D and autoimmune disorders
• Vitamin D and osteoarthritis
• Vitamin D and Migraine
• Vitamin D and Depression
• Vitamin D: risk of deficiency in Plant-Based Diets

The Discovery of Vitamin D

Rickets, now known to be caused by a vitamin D deficiency was studied by European doctors in the 1700s and 1800s. By the late 1700s, Percival in the UK proposed the use of cod liver oil to treat the condition, believing it was due to a nutritional factor. In Poland, in the early 1800s, Sniadecki noticed that rickets incidence was lower in the countryside than in cities and proposed that the lack of pure air or sunlight caused the disease. For over a century the debate between environment or diet continued.

Animal experiments with pure nutrients (carbohydrates, protein, fats) proved that the lack of some special nutrients found in food in minute amounts, caused diseases like beriberi, scurvy, and rickets, and that by supplying these "vital" compounds, the diseases could be cured. In 1906 English biochemist Frederick Gowland named these compounds "accessory food factors". The idea was taken up by Casimir Funk, a Polish biochemist working in London who in 1911 isolated the factor whose deficit provoked beriberi he named it "vitamine", combining the words "Vital" and "amine" because it was a chemical compound known as amine, with an amine group (NH₂). In 1913, American scientist Elmer McCollum detected that there were different types of "vitamines", and called them "fat-soluble factor A" and "water-soluble factor B." McCollum was investigating the effects of vitamin A found in cod liver oil on rickets in dogs; he eliminated the vitamin A yet the oil still cured the dogs so he correctly concluded that yet another new vitamin responsible for this effect. As vitamins A, B, and C had been discovered, he named it vitamin D.
As new discoveries showed that not al vitamins had amine groups, the name was changed and the final "e" dropped. The vitamines became vitamins.

At that time, Huldshinsky, in Vienna, and Chick and his team in England found that exposing children with rickets to sunlight or artificial UV light cured them. So the environmental factor was real!

Vitamin D2 and vitamin D3

Only later was the role of sunlight in vitamin D formation, and the different forms of the vitamin understood.

There is a variant of vitamin D, called vitamin D2. It was discovered independently in 1924 by two groups (Hess and Weinstock, and Steenbock and Black) who found that UV light could make certain inert foods antirachitic (could cure rickets). Later vitamin D2 was detected in light-exposed ergosterol (provitamin D).

Vitamin D2 is found in plants and fungi exposed to ultraviolet B light.

Vitamin D2 was chemically isolated in 1932 by Askew.

In 1935, a compound called 7-dehydrocholesterol was isolated by Windaus, who with Bock identified vitamin D3 in 1937.
Scientists at that time correctly surmised that vitamin D3 was synthesized by the body when 7-dehydrocholesterol in the skin was irradiated by sunlight, but this was not proven until 1978.

7-Dehydrocholesterol (7-DHC) is a type of compound called sterol, that the body uses to synthesize cholesterol, and when the sun penetrates the skin, 7-DHC is converted photochemicallty into vitamin D3.

The transformations of vitamin D

As mentioned, it has two main forms, vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) which have slightly different chemical structures. These are obtained from dietary sources or supplementation.

Vitamin D in either of its forms is absorbed in the small intestine, the presence of fat in the gut improves absorption. (1)

Transformation into a bioactive form

The vitamin D obtained from the diet, sun exposure, or supplements is inert and is modified by the body which turns it into an active compound that can be used. The first transformation takes place in the liver, where vitamin D is hydroxylated and becomes 25-hydroxyvitamin D or 25(OH)D, also known as calcidiol. Then it is hydroxylated again in the kidney becoming an active compound, 1,25-dihydroxy vitamin D, or 1,25(OH)2D, also called calcitriol. (1)

Vitamin D: its role in the body

Its main role in the body is to promote the absorption of calcium in the gut and maintaining the concentration of calcium and phosphorus in the blood to ensure bone growth, and muscle health, by mineralizing the bones and maintaining calcium levels in the muscles (helps regulate their contraction).

Lack of vitamin D leads to brittle, fragile bones and rickets in children. Adequate levels of vitamin D protect against osteoporosis in older adults.

It also helps reduce inflammation, enhances immune function, regulates cell growth, and plays a role in how glucose is metabolized. (1)

Normal Levels of Vitamin D

A blood analysis measuring the concentration of 25(OH)D (mentioned above) is an indicator of healthy or inadequate levels of vitamin D. 25(OH)D, after being hydroxylated in the liver remains in the bloodstream for around 15 days. Its concentration is measured in nanomoles per liter (nmol/L) and nanograms per milliliter (ng/mL). Note: One nmol/L is equal to 0.4 ng/mL, and 1 ng/mL is equal to 2.5 nmol/L.
One microgram (μg also mcg) of vitamin D is equal to 40 international units (IU).

There isn't a uniform standard on the minimal serum level of 25(OH)D. There are several. The UK Scientific Advisory Committee on Nutrition considers that vitamin D sufficiency begins at 30 nmol/l. The American Institute Of Medicine and other countries (e) define this level as 50 nmol/l, while the Endocrine Society considered 75 nmol/l as a baseline level until recently, but now no longer endorses this or any specific 25(OH)D levels to define vitamin D sufficiency, insufficiency, and deficiency. (f).

Interestingly, the 25(OH)D concentrations found in tribes in equatorial Africa, with a lifestyle and sun exposure similar to that of early humans are, on average, 115 nmol/l. (g).

Recommended Daily Intake

For men and women aged 51 to 70 years, 600 IU (International Units), equivalent to 15 μg (micrograms).
For men and women over 70, 800 IU, or 20 μg.

Sources of Vitamin D

Vitamin D is a fat-soluble vitamin that is naturally present in a few foods, added to others, and available as a dietary supplement. It is also produced by the body when ultraviolet (UV) rays from sunlight strike the skin and trigger vitamin D synthesis. We will look into each of these sources of vitamin D, starting with food.

Dietary Sources of Vitamin D

Good quality oily fish such as sardines, mackerel, trout, tuna, swordfish, and salmon are a great source of Vitamin D. So are eggs, milk, and certain fungi.

Below are some high-Vitamin D food sources according to the US government's dietary guidelines.

Dietary 25(OH)D

Animal-based foods not only provide vitamin D3, but they also supply vitamin D in its 25(OH)D form. Studies show that this animal sourced 25(OH)D is 5 times more potent than vitamin D3 in increasing human blood levels of 25(OH)D.

The 25(OH)D content in beef, chicken, pork, turkey, and eggs is 2 to 18 times higher than the amount of vitamin D3 in the food. (1)

Fortified Foods

Since vitamin D is fat-soluble, it can be toxic if consumed in very high doses. The US and Canada regulate the addition of vitamin D to food. It is mandatory for milk and margarine, and some manufacturers add it to cereal, yogurt, and other foods.
In the US, fortified foods provide most of the vitamin D requirements of Americans. Milk contains around 3μg per cup (120 IU) usually as vitamin D3. Plant-based milks are typicaly fortified with a similar level to cow's milk. Check the Nutrition Facts label to verify the vitamin D content in your milk. (1)

Cod Liver Oil

Since the 1600s, local coastal populations in Europe used cod liver oil to treat rickets. Schütte, a German doctor published a report in 1824 mentioning case histories of rickets cured by cod liver oil, which he had used for 25 years.

Cod liver oil is sold as a dietary supplement, it is obtained from the liver of cod fish and contains roughly 20% omega-3 fatty acids like DHA and EPA. It is also a source of vitamins A and D. One tablespoon of this oil contains 4,080 μg of vitamin A, and 34 μg (1,360 IU) of vitamin D. These are above the Dietary Reference Intake of vitamin A (900 and 700 μg/day for men and women respectively), and vitamin Dt for vitamin D (15 μg per day).
Those consuming cod liver oil as a source of omega-3 should avoid an excess intake of vitamins A and D.

Sunlight Exposure and Vitamin D

Sunlight is critical in the transformation of vitamin D precursors into active forms of the vitamin.

Type B ultraviolet radiation (UVB) can penetrate bare skin and convert the 7-dehydrocholesterol in the skin cells into pre-vitamin D3.

However, the season, time of the day, cloud cover, smog, skin tint, clothes, and use of sunscreen can block the UVB rays and limit vitamin D synthesis.

Dark-skinned people and older people have limited ability to produce vitamin D when exposed to sunlight.

As UVB rays cannot penetrate glass, indoor exposure is useless, direct outdoor exposure is necessary. (1)

Exposing 50% of your skin area (without sunscreen) for 12 minutes at midday, in mid-latitude USA produces an amount of Vitamin D equivalent to supplementing 3,000 UI per day (7).

But the cold winter conditions keep you indoors and covered up. This makes it hard to expose the skin's surface to the sun, so it halts its natural production.

Sun Exposure and Cancer Risk

Avoid using tanning beds and exposure to UV radiation as it causes skin cancer. See our tips on how to protect yourself from the sun and avoid Sunburn.
Winter exposure to sunlight shouldn't increase the risk of cancer and will allow the body to synthesize vitamin D. (1)

Vitamin D Health Claims

The U.S. Food and Drug Administration, FDA) approved the following health claim regarding vitamin D:

• Adequate calcium and vitamin D, as part of a well-balanced diet, along with physical activity, may reduce the risk of osteoporosis.

The European Food Safety Authority, EFSA) recognized the following health claims about vitamin D, as backed by scientific evidence:

• normal function of the immune system.
• normal inflammatory response.
• normal muscle function.
• reduced risk of falling in people over age 60.

Health Effects of Vitamin D

• Bone Health and Rickets
• No protective effects on Cancer, Diabetes, or Cardiovascular disease
• Psoriasis and Vitamin D
• Autoimmune disorders
• Osteoarthritis
• Migraine
• Depression

Bone Health and Rickets

As mentioned in the previous sections, vitamin D supplementation prevents and cures rickets in children. (13)

Roughly 7% of the world population has a severe vitamin D deficit, mainly in North Africa, the Middle East, and some Asian countries. Older people and those with low serum levels of vitamin C can benefit as it improves bone health.

Taking vitamin D supplements for uses other than promoting bone and muscle health is not backed by science. Most Americans have an adequate intake of vitamin D from their diet and sun exposure, and a higher intake may pose risks.
A 2012 review (14) reported that vitamin D improves musculoskeletal health and reduces the rate of fractures and falls in adults aged 65 and older in daily doses of 800-5000 IU. However, it concludes that "for patients who are not at risk for developing vitamin D deficiency, sensible sun exposure is an inexpensive and enjoyable way to maintain [the body's] vitamin D stores".

These findings are supported by a 2014 study (12) that analyzed data from 53 trials and 91,791 participants and found that supplementation with vitamin D alone, does not reduce the risk of bone fracture in people who have adequate vitamin D levels. However, supplementation with vitamin D and calcium is linked to a reduction in the incidence of fractures. Concluding that "there is high quality evidence that vitamin D plus calcium reduces the risk of any type of fracture" with a risk reduction of 5%.

No protective effects on Cancer, Diabetes, or Cardiovascular disease

Supplementation trials on adults with adequate vitamin D levels (serum 25(OH)D concentration above 50 nmol/l) didn't reveal any effect on cancer, cardiovascular disease, or type-2 diabetes, they didn't improve bone density or lower the risk of falls. (13)

For those with vitamin D deficiency, supplementation could "modestly delays age-related bone loss and progression to T2DM, and improves lung function... In conclusion, supplementation of vitamin D-replete individuals does not generate overall health benefits." (13); T2DM is type-2 diabetes.

The Risks of taking High Doses of Vitamin D

As over 3% of adults consume 4000 IU/day of vitamin D or more, which is well above the RDA of 600-800 IU/day, a randomized trial (8) (the Calgary Vitamin D study) followed 373 healthy adults for 3 years to investigate the safety and effects of high doses of vitamin D (4,000 and 10,000 IU daily) compared to a control group with 400 IU/day.

Mild hypercalcemia was reported in some (3%-9%) of the subjects in the high intake groups. This is a condition where there are high calcium (Ca⁺⁺) levels in the blood, it can cause abdominal pain, confusion, depression, and abnormal heart rhythm.
Hypercalciuria occurred in all groups (400: 17%, 4,000: 22%, 10,000: 31%); it is elevated calcium in the urine. That can cause kidney disease and form kidney stones.

Very high doses above 25,000 IU/day can cause hypercalcemia, hypercalciuria, kidney disease, and calcification of soft tissue. It should be noted that The Institute of Medicine (IOM) set the tolerable upper intake limit at 4,000 IU/day (8)

High Doses of Vitamin D Have a Negative Effect on Bone Health

A second outcome of the Calgary Vitamin D study was that doses of 4,000 and 10,000 IU/day resulted in lower Bone Mineral Density (BMD) compared to 400 IU/day dosification. "These findings do not support a benefit of high-dose vitamin D supplementation for bone health." (9)

Psoriasis and Vitamin D

Psoriasis is a common chronic autoimmune skin disorder that affects around 2% of the world population. Since vitamin D has immunomodulatory (promotes immunity) and anti-inflammatory effects, there have been many studies investigating the association between low levels of vitamin D and psoriasis. The results have been contradictory and further research is needed to settle the issue. Below we mention some recent findings and their conclusions.

A 2022 study using data from the UK Biobank study (10) followed 2,856 participants with psoriasis for almost 11 years.

The study found that those with low vitamin D levels (less than 25 nmol/L levels of 25(OH)D)had a 20% higher risk of developing psoriasis. Obese people, with a BMI above 30 kg/m²) had a 30% higher risk.

But, the risk of psoriasis decreased as blood levels of 25(OH)D increased from 25 to 50 nmol/L. Suggesting that a deficit in vitamin D is associated with psoriasis. It also noted that increasing vitamin D levels beyond 50 nmol/L didn't improve the outcome, and in fact, worsened it.

This effect was also modified by obesity; the protective effect of vitamin D was larger in this group.

Another study from 2022 (11) reported that "patients with psoriasis had significantly lower levels of serum vitamin D compared to patients without psoriasis, and that patients with psoriasis who were treated with oral vitamin D achieved greater clinical improvement compared to those who did not receive oral vitamin D therapy."

It also found that vitamin K2 and magnesium (Mg) taken together with vitamin D enhanced the effects and improved its absorption.

Apparently, serum levels of 25(OH)D greater than 30 ng/mL. are needed to ensure adequate immunity. These studies suggest that increasing serum levels in patients with psoriasis would improve their symptoms. It is probable that genetic factors found in persons with psoriasis affect the uptake, distribution, and biochemical modifications of vitamin D compared with people who don't suffer from this condition.

Vitamin D and autoimmune disorders

Low Vitamin D has been associated with other autoimmune disorders like vitiligo, atopic dermatitis, (a) lupus, (b) and multiple sclerosis (c)

Vitamin D and osteoarthritis

You should get tested for vitamin D deficiency. Studies have suggested that vitamin D deficiency may increase the risk of OA. The conclusion was that "vitamin D supplementation may alleviate joint pain in OA patients with low vitamin D status (<50 nmol/L)" (15).

Vitamin D and Migraine

Supplementation with Vitamin D3 reduced the Helicobacter species count in healthy individuals, and some studies show that low vitamin D serum levels could increase the risk of migraine. Vitamin D deficiency could trigger migraine. (3)

Vitamin D and Depression

The mechanism by which Vitamin D interacts with depression is yet unknown, but a study (5) reported that it may enhance serotonin production and therefore improve mood.

According to Melrose, (2015) and Young Sup Woo (2019) (3, 6), low levels of Vitamin D are linked with depression and people suffering from Winter Blues have deficient levels of this vitamin.

Winter and vitamin D levels

Low levels of Vitamin D are usually caused by unbalanced diets (eating food that lacks this vitamin) or lack of outdoor activity: sunlight is necessary for your body to make Vitamin D.

This means that during winter, Vitamin D must be supplemented. The current recommendations for Vitamin D intake are 200 - 600 IU per day. However, these are considered too low by many researchers, who suggest that people not getting enough sunlight exposure may require 800 - 1,000 IU per day to attain optimal blood levels of Vitamin D (30-40 ng/mL).

Bear in mind that Vitamin D is soluble in fat. Your body stores it in its fatty tissue. Overdosing can cause intoxication, and adverse reactions could occur with doses of 50,000 IU or more per day.

Vitamin D: risk of deficiency in Plant-Based Diets

Vitamin D is normally produced by the body when it is exposed to the sun, but there is a global deficiency of vitamin D influenced by lifestyle, geographic location (in more northern latitudes), the use of sunscreen, etc. So a dietary intake of vitamin D is necessary.
Most of the foods rich in vitamin D are animal-based (eggs, milk, beef, or fish) where it is present in the D3 form, which has a higher bioavailability than the D2 form found in plant-based foods. Vegans and strict vegetarians might be at risk of vitamin D deficiency. (4)

Closing Comments

Vitamin D has an important role in our bone and muscle health, but supplementation seems to be effective for those suffering from vitamin D deficiency like older people or those who don't get enough outdoor sunlight.

A balanced diet, eating fortified food, and exposing your skin to direct sunlight are the best, and cheapest ways to ensure an optimal level of vitamin D.

Those with autoimmune disorders, psoriasis, and vegans or vegetarians should consider supplementation.