[aMorePerfectUnion]

—> Can you keep it at that level by sun exposure?

You live in Yuma, AZ and spend lots of time outside without sunscreen, yes.

If you live in the north half of he USA likely no
- particularly in winter.

[nickcarraway]

For what it's worth, my doctor said you should be able to build up enough in the summer to last over the winter. And you don'y have to be outside all day/

There are apps, D Minder is one, that allow you to track your Vitamin D by your location, the angle of the sun, etc.

Older people may not absorb as much. And the darkness of your skin will impact that too.