How does sun on your skin make vitamin D?

06 September 2016

Question

Hi,
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

Answer

Kat Arney put this to Andrew Holding...

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

Kat - So, it's like a chemical reaction that's just happening?

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

Kat - So, there's these precursors that we're 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. It's 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 don't get enough UV hitting your skin?

Andrew - If you're an animal with fur, in fact, it's really interesting. They obviously can't get light to their skin especially if it's a dark coat. So, they actually put in oil to put on their coat and that's 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 - That's kind of disgusting.

Andrew - Well, cats do lick themselves.

Kat - I mean, we don't do that presumably.

Andrew - Do you lick cats?

Kat - I don't lick cats. I don't 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 we're 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 don't 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, we're 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 that's great.

Kat - I work too hard.

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