How does sun on your skin make vitamin D?

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Offline thedoc

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How does sun on your skin make vitamin D?
« on: 22/07/2016 15:23:01 »
paul ledger asked the Naked Scientists:
     I listen to your programme Naked Scientists 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

What do you think?
« Last Edit: 22/07/2016 15:23:01 by _system »


Offline chris

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Re: How does sun on your skin make vitamin D?
« Reply #1 on: 22/07/2016 23:17:16 »
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.
I never forget a face, but in your case I'll make an exception - Groucho Marx