There is more to Vitamin D than sunshine.
Vitamin D is often called the “sunshine vitamin” because our skin can produce it when exposed to UVB radiation. But Vitamin D also comes from certain foods, fortified products and supplements — and it plays an important role in calcium absorption, bone health and several biological processes throughout the body. For skin health, the conversation is particularly interesting. Our skin is where Vitamin D production begins, but Vitamin D also interacts with cells and tissues throughout the body. So how much do we actually need, where do we get it from, and when does testing make sense?
What is Vitamin D?
Vitamin D is a fat-soluble vitamin that helps the body absorb calcium and supports normal bone health. There are two main forms:
Vitamin D2 — ergocalciferol
Found mainly in certain mushrooms and fortified foods, as well as some supplements.
Vitamin D3 — cholecalciferol
Produced naturally in the skin following UVB exposure and found in foods such as oily fish, fish liver oils, egg yolks and some animal-derived foods.
Both D2 and D3 can increase Vitamin D levels in the blood, although D3 generally raises blood levels slightly higher and for longer than D2.
Where can you get Vitamin D?
Vitamin D is relatively unusual because we can obtain it from both our diet and sunlight.
Food sources include:
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Oily fish such as salmon, mackerel, tuna and sardines
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Fish liver oils
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Egg yolks
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Beef liver
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Cheese
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UV-exposed mushrooms
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Fortified milk and plant-based milks
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Some fortified cereals and other foods
The amount found in foods can vary considerably, so checking food labels is useful when fortified products are part of your diet.
Vitamin D is fat-soluble — why does that matter?
Vitamin D is a fat-soluble vitamin, meaning dietary fat can help with its absorption. If you take Vitamin D as a supplement, taking it alongside a meal or snack containing some fat can improve absorption. This is one reason that the context in which we take a supplement can matter just as much as the supplement itself.
How much Vitamin D do we need?
Reference values vary between organisations and populations. The European Food Safety Authority sets an adequate intake of 15 µg (600 IU) per day for healthy adults, assuming limited exposure to sunlight. The US National Institutes of Health lists 600 IU per day for adults aged 19–70 and 800 IU for adults over 70. More is not necessarily better. For adults, the established tolerable upper intake level is 100 µg (4,000 IU) per day from all sources, unless a higher dose has been recommended and monitored by a healthcare professional. Excessive Vitamin D, particularly from supplements, can lead to abnormally high calcium levels and, in severe cases, kidney and cardiovascular complications.
What about Vitamin D testing?
This is where individual circumstances matter. A blood test can measure 25-hydroxyvitamin D [25(OH)D], the main marker used to assess Vitamin D status. However, testing isn't automatically necessary for everyone. The 2024 Endocrine Society guideline recommends against routine Vitamin D screening in generally healthy adults who don't have an established indication for testing. Testing may be considered in specific clinical situations depending on an individual's health, risk factors, symptoms, medications or medical history. At Woulfe, nutritional testing can be discussed as part of an individualised approach rather than assuming that one recommendation fits everyone.
And what about Vitamin K2?
Vitamin K is another fat-soluble vitamin involved in normal blood clotting and bone health. Vitamin K2 refers to a group of menaquinones, including MK-4 and MK-7. There is ongoing research into how Vitamin K interacts with calcium metabolism and bone health. However, current evidence does not establish routine K2 testing for everyone taking Vitamin D. If you are considering Vitamin D or Vitamin K supplementation, particularly at higher doses or alongside medication, it's worth discussing your individual circumstances with a qualified healthcare professional.
What does Vitamin D have to do with skin?
Vitamin D is produced in the skin and is involved in biological processes beyond bone health, including immune and cellular functions. Research has also investigated associations between Vitamin D status and a wide range of conditions. However, an important distinction is needed: An association between low Vitamin D status and a health condition does not automatically mean that taking Vitamin D supplements will prevent or treat that condition. The evidence for Vitamin D supplementation and disease prevention varies considerably between outcomes, which is why individual context matters.
The Woulfe perspective
At Woulfe, we believe healthy skin is about looking beyond the surface. Nutrition, lifestyle, skincare and internal health can all form part of the wider conversation around skin health. Rather than automatically reaching for a supplement, it can be more useful to ask:
What do I actually need?
Could my diet provide it?
Would testing be appropriate for me?
What does the evidence say?
These are the kinds of questions we explore as part of a more individualised approach to skin and wellness.
Want to learn more?
Join Edelle on 17 October for:
THE BASICS OF SKIN HEALTH
12–1pm + Q&A
The session will explore Vitamin D, essential fatty acids, the skin microbiome and more.
References & further reading
NIH Office of Dietary Supplements — Vitamin D
Vitamin D Fact Sheet
Endocrine Society — Vitamin D for the Prevention of Disease Guideline, 2024
Endocrine Society Clinical Practice Guideline
European Food Safety Authority — Dietary Reference Values for Vitamin D
EFSA Vitamin D Dietary Reference Values
NIH — Vitamin K Fact Sheet
Vitamin K Fact Sheet
National Cancer Institute — Vitamin D and Cancer
Vitamin D and Cancer
This article is intended for educational purposes and does not replace individual medical advice. If you are considering supplementation or testing, particularly if you take medication or have an existing health condition, speak with an appropriate healthcare professional.
