system
Given the following post, respond as if you're commenting on Reddit:
human
Does taking a vitamin D supplement stop natural vitamin D production?

 If I would for example take 1000 IU of oral vitamin D every day, does my natural vitamin D production in the sun stop due to the supplement taking?
gpt
Probably not. Consider that vitamin D3 comes from 7-dehydrocholesterol. The sun turns 7-dehydrocholesterol into vitamin D3, but 7-dehydrocholesterol is also converted into cholesterol. All cells must produce cholesterol, as it is an integral part of the cell membrane. Any feedback regulation that would inhibit 7-dehydrocholesterol would also inhibit cholesterol formation in the cell.

However, vitamin D3 is not the active form of vitamin D. There are two more hydroxylation steps, one in the liver and one predominantly in the kidneys. The active form activates an enzyme that deactivates the active form molecule, essentially creating a negative feedback loop.