Science Questions

How does sun on your skin make vitamin D?

Mon, 5th Sep 2016

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paul ledger asked:


I listen to your programme Naked Scientist at 5am on Saturday mornings. My question is:-

How does sunlight shining on the skin make vitamin D?

When and by whom was this discovered?

Why aren't other vitamins made this way?

How much skin needs to be exposed and for how long to get your RDA of vitamin D?

Thank You Paul



Kat Arney put this to Andrew Holding...the sun

Andrew - So thatís a great question. So yes, we need UV to make vitamin D and unlike a lot of biological processes, itís actually done spontaneously. There's not an enzyme that does it.

Kat - So, itís like a chemical reaction thatís just happening?

Andrew - Yeah, so itís a specific type of chemical reaction called a pericyclic chemical reaction and thatís where the bonds rearrange spontaneously when the energy comes in the right source. Depending whether itís heat or UV will depend to how the shape of the molecule is after that reaction.

Kat - So, there's these precursors that weíre making and the UV light comes in, hits our skin, and does the chemistry.

Andrew - Yes. So we make a precursor up to a certain point. Itís cholesterol and then it does the conversion and then it gets taken a little bit more to make into vitamin D.

Kat - But that does depend on light actually hitting our skin. So what happens if you donít get enough UV hitting your skin?

Andrew - If you're an animal with fur, in fact, itís really interesting. They obviously can't get light to their skin especially if itís a dark coat. So, they actually put in oil to put on their coat and thatís where it gets UV and then they lick it back off.

Kat - So they're making the precursors, they're secreting them out themselves, the chemistry happens, and they eat it.

Andrew - Yeah.

Kat - Thatís kind of disgusting.

Andrew - Well, cats do lick themselves.

Kat - I mean, we donít do that presumably.

Andrew - Do you lick cats?

Kat - I donít lick cats. I donít even like cats very much at all. But what happens? Is there actually enough sunlight to make enough vitamin D?

Andrew - This is a big challenge actually today when weíre worried about getting too much sunlight. So, if you're inside your house and you decided to sunbathe behind your window, you're not getting enough UVB. So, UVB is the more burny one because the glass blocks it. So you have to go outside which isn't terrible but in the UK of course, we donít have much sun in winter. So then a whole load of other things come into play depending on your skin coloration. So if you're very pale and you get a lot more UV getting to the parts for what it needs to get to then you do produce still not great amount but probably enough. If you had a skin that absorbs lots of UV, you'll go and have to probably take supplements.

Kat - And you can get vitamin D from food as well. You're going to need a balance of sunlight and the right food, and then maybe supplements in the winter as well.

Andrew - So, I think the current advice now is we actually should all be just taking supplements because we cover up with stuff to stop skin cancer so sun cream that blocks most of it. So actually, weíre not doing ourselves a great favour on vitamin D front but we are doing ourselves a great favour on having great skin.

Kat - Well there you go, avoid the skin like a handbag by keeping nice and young, and staying out with the UV as much as you can I reckon.

Andrew - Yeah, but do get outside thatís great.

Kat - I work too hard.


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Put simply, ultraviolet rays in sunlight have sufficient energy to break bonds in molecules of 7-dehydrocholesterol within the skin. This unfolds one of the rings in the molecule producing a new substance called cholecalciferol, of vitamin D3. But this form - dubbed a pro-vitamin - isn't actually active in the role of boosting recovery of calcium (as well as phosphate, zinc and magnesium) from the GI tract and kidney.

To become active the cholecalciferol goes through two further modifications. First, in the liver, a hydroxy (OH) group is added to carbon number 25 in the molecule. This produces 25-hydroxy-cholecalciferol.

Next, a further hydroxyl group is added, this time to carbon number 1 to produce 1,25 dihydroxycholecalciferol. This is active vitamin D. The reason for the two separate anatomical sites of modification is that the kidney hydroxylation is the rate limiting step in the activation of the molecule. It's the gatekeeper, and is itself regulated by the amount of vitamin D circulating and calcium levels. This prevents the body from over-producing active vitamin D molecules, which could lead to hypercalcaemia.

You need about 15 minutes of summer Sun on your face to make a day's worth of vitamin D, but owing to the mechanism described above, any extra vitamin D obtained by more extensive solar worship will not lead to excessive vitamin D production.

Other hormones and vitamins are not made this way because the chemistry of these different substances is quite different and does not lend itself to the same metabolic mechanism. chris, Fri, 22nd Jul 2016

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