system
Given the following post, respond as if you're commenting on Reddit:
human
Does the body constantly produce vitamin D when out in the sun, or does it get to a point where it's got enough and shuts down the process?

I was speaking to someone about whether or not you can take too much vitamin D in supplement form. Then that led to the observation that if your skin makes vitamin D when exposed to the sun, wouldn't that mean you would OD on vitamin D from being in the sun too long? From what I've read you pretty much can't take too much vitamin D even in supplement form, but besides that I'm curious about how much your body will make for itself in a given time and whether or not it can turn the function on and off.
gpt
You can't OD on sun derived vitamin D. 

Sunlight, to be precise UVB rays in the range of 290-315nm, break a B ring bond in 7-dihydrocholesterol, also called pro-vitamin D3, the result is pre-vitamin D3. 

According to your body's needs some of this pre-vitamin D will be turned to vitamin D3, the rest will be turned to inactive by-products lumisterol and tachysterol. The latter can be turned back to pre-D if needed. 

In short, once pre-vit D is synthesized it turns either to D3 or inactive metabolites. This is the mechanism that protects from OD. 

Further, D3 is also inactive, but when your calcium levels drop, parathyroid hormone stimulates the kidneys to produce the active form of vitamin D, 1,25 OH2D3. The active form increases the absorption of calcium in the intestines.

Good to know: vitamin D3 can be stored in adipose tissue for up to two months. D3 has a half-life of 72 hours while that of 1,25 OH2D3 is 4 hours.