system
Given the following post, respond as if you're commenting on Reddit:
human
Does my skin make vitamin d at a constant rate in the sun or does it level off after a while and how does it do that?

If your body supposedly gets the vitamin d it needs in around 15 minutes (I know it varies but that's the general guideline) what happens when I'm in the sun for hours? Does my skin know to slow down or does it keep cranking it out? What does my body do with the excess?
gpt
Yup your are correct in that the body gets sufficient vitamin D after ~10 mins of exposure to natural sunlight. Exposure to UVA light will activate enzymatic pathways involving the liver, kidney, and parathyroid that will produce 25-(OH)D2, the active form of vitamin D. After it has cranked out enough 25-(OH)D2 the enzymes involved in the production are inactivated via negative feedback loops. Therefore 
you can't actually get vitamin D toxicity from excess sun exposure.
 
That being said, taking exogenous vitamin D (in the form of vitamin D drops or pills) can lead to vitamin D toxicity because it is already in the 25-(OH)D2 form, and doesn't need to be processed by enzymes. Human vitamin D levels are usually on the low-side, so toxicity isn't a big issue.