[amenhotep]

The neat thing about vitamin D synthesis in the skin is that it stops once there's enough of it - it is literally impossible to overdose on vitamin D through sun exposure. So, there's a ceiling that makes "must be higher in sunny regions" far from obviously true!

[yesbut]

What happens to unused vitamin D consumed from supplementation?

[amenhotep]

That gets stored in the fat. Overdose through ingestion is a thing, our bodies aren't so smart about that; it's skin synthesis specifically that limits itself.

[yesbut]

How much causes overdose?