What is vitamin D and what does it do?

What is vitamin D?

Vitamin D is a group of fat-soluble secosteroids (essentially a steroid where one of the bonds in the steroid rings is broken).

Vitamin D regulates the amount of calcium and phosphate in the body. In addition, this vitamin is also responsible for regulation of cellular proliferation, differentiation, apoptosis and innate and adaptive immunity.

There are two major types (there are more types which are out of scope for this post) of vitamin D – vitamin D3 (also known as cholecalciferol) and vitamin D2 (ergocalciferol).

vitamin d3 and d2 structures

Our major source of vitamin D comes from the synthesis of vitamin D3 in the lower layers of the skin epidermis (read this post to learn more about the skin’s structure) due to sun exposure (read how UV light affects skin ageing here).

However, vitamin D2 and D3 are both biologically inactive (keep reading to find out how they become active).

Conversion of vitamin D2 and D3 to a biologically active form

So, how do vitamins D2 and D3 become biologically active? Well, via a 2-step enzymatic hydroxylation process (see below).

conversion of vitamins d2 and d3 into their biologically active form

The conversion happens through a two-step enzymatic hydroxylation; initially vitamins D2 and D3 are converted to 25-hydroxyvitamin D in the liver (enzymes known as 25-hydroxylases facilitate this process).

Following this, the kidney utilises the 1a-hydroxylase enzyme to convert 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D2 or D3 (calcitriol). Calcitriol is the active form of vitamin D.

Eventually this leads to activation of vitamin D receptors in target tissues.

Sources of vitamin D

Often, inadequate exposure to the sun (ie. winter time) and a poor diet can lead to a vitamin D deficiency. Although it is not always as straightforward as consuming more vitamin D rich foods or sitting in the sun all day (which I don’t recommend).

There are a few other things that can affect vitamin D serum levels, namely – skin colour, body fat % (remember vit. D is fat-soluble), general health, lifestyle factors (alcohol/smoking), as well as genetic factors.

Keep reading to learn more about the sources of this vitamin.

Synthesis of vitamin D3 in the skin epidermis

As mentioned above, a major source of our vitamin D comes from the synthesis of vitamin D3 due to exposure to the sun.

Vitamin D3 is synthesised via the UV irradiation of 7-dehydrocholesterol to previtamin D3 in the skin of animals at UVB wavelengths of 290–320 nm. A further thermal isomerisation step to form vitamin D3 then occurs.

synthesis of vitamin d3 in the skin

P.S. you can still achieve adequate vitamin D synthesis whilst wearing sunscreen (incidental exposure to UV rays is all it takes), so make sure to use it!


Vitamin D2 and D3 can be ingested through our diet. There are several great sources of vitamin D. These can be split into foods with naturally occurring vitamin D2 or D3 – such as salmon, eggs or mushrooms etc – or foods fortified with vitamin D. Fortified foods include margarine and cereals (generally fortified with vitamin D2), amongst others.

So feel free to dig into a hearty brunch consisting of plenty of salmon and egg (especially in the winter)!

Vitamin D supplements

Vitamin D isn’t necessarily the most abundant vitamin – the foods you can consume to achieve your daily intake are generally quite limiting. So during the winter months it is very easy to become deficient. If you are aware that you are deficient in vitamin D (perhaps due to a blood test), or you struggle to consume foods containing vitamin D, then supplementation might be ideal.

Both vitamin D2 and D3 supplements are available. As to which one’s better, the jury is still out.

What we do know, is that the sun provides us with vitamin D3, whereas dietary sources can provide us with vitamin D2 or D3. Many regard vitamin D2 and D3 as equivalent and interchangeable. Many vitamin D supplements for prescription come in the form of vitamin D2. Whereas multivitamins often contain either D2 or D3.

However, there has been a shift to reformulate, so that the supplements come in D3 form – several studies have shown diminished binding of vitamin D2 metabolites to vitamin D binding protein in plasma. They have also shown a shorter shelf life for vitamin D2.

P.S. Whichever supplement you choose, don’t forget to consume it with fats (maybe with a meal) if possible, to facilitate absorption. And remember, it can be just as bad, if not worse, to over supplement. Please consult your doctor to see if vitamin D supplements could be right for you.


Dietary Reference Intakes for Vitamin D and Calcium, Ross, A. C., Taylor, C. L., Yaktine, A. L., & Del Valle, H. B. (Eds.). (2011). Dietary Reference Intakes for Calcium and Vitamin D. National Academies Press (US).

Tripkovic, L., Lambert, H., Hart, K., Smith, C. P., Bucca, G., Penson, S., Chope, G., Hyppönen, E., Berry, J., Vieth, R., & Lanham-New, S. (2012). Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. The American journal of clinical nutrition95(6), 1357–1364. https://doi.org/10.3945/ajcn.111.031070

Mostafa, W. Z., & Hegazy, R. A. (2015). Vitamin D and the skin: Focus on a complex relationship: A review. Journal of advanced research6(6), 793–804. https://doi.org/10.1016/j.jare.2014.01.011

Lisa A Houghton, Reinhold Vieth, The case against ergocalciferol (vitamin D2) as a vitamin supplement, The American Journal of Clinical Nutrition, Volume 84, Issue 4, October 2006, Pages 694–697,

Logan, V., Gray, A., Peddie, M., Harper, M., & Houghton, L. (2013). Long-term vitamin D3 supplementation is more effective than vitamin D2 in maintaining serum 25-hydroxyvitamin D status over the winter months. British Journal of Nutrition, 109(6), 1082-1088. doi:10.1017/S0007114512002851

Laura Tripkovic, Helen Lambert, Kathryn Hart, Colin P Smith, Giselda Bucca, Simon Penson, Gemma Chope, Elina Hyppönen, Jacqueline Berry, Reinhold Vieth, Susan Lanham-New, Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis, The American Journal of Clinical Nutrition, Volume 95, Issue 6, June 2012, Pages 1357–1364, https://doi.org/10.3945/ajcn.111.031070

Fuse, S., Tanabe, N., Yoshida, M., Yoshida, H., Doi, T., & Takahashi, T. (2010). Continuous-flow synthesis of vitamin D 3. Chemical communications46(46), 8722-8724.