Anna, how does daylight / the sun give us Vitamin D and why can’t our bodies retain it?

Question: Anna, how does daylight / the sun give us Vitamin D and why can't our bodies retain it?
- Keywords:
  body,
  daylight,
  sun
Asked by Khrystillianna to Anna on 14 Nov 2014.
- Anna Scaife answered on 14 Nov 2014:
  
  Hey Khrystillianna – the Sun doesn’t give us vitamin D directly, but the UV light from the sun allows our skin to make it through a chemical reaction. It’s called “photo-chemical” reaction because it requires both the radiation (“photo-“) from the Sun and the chemicals in our skin.
  
  In fact milk also has the same reaction and that’s how companies make vitamin D tablets – weird, huh?
  
  Our bodies take all the vitamin D we make straight to our liver where it gets processed into other things – that’s why we don’t store it.

Question: Anna, how does daylight / the sun give us Vitamin D and why can't our bodies retain it?