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I have recently noticed that some supplements contain a high number of vitamin D e.g.at least 50mcg (2000IU).
That got me curious because references I have, written in early-mid 80s explicity state that vitamin D is toxic with as little as 5 times the recommended intake of 10mcg(400IU).
Searching online, I see that the thresholds has changed e.g. from web md

The Institute of Medicine's Food and Nutrition Board's old 1997 recommendations suggested that 2,000 IU per day of vitamin D is safe for adults and that 1,000 IU per day is safe for infants up to 12 months of age. Many observers expected a drastic increase in the IOM's 2010 update.

That didn't exactly happen. The IOM committee did increase its "upper level intake" -- that is, the boundary at which it feared vitamin D would become unsafe. That dose is 4,000 IU/day for adults, 3,000 IU/day for kids ages 4-8, 2,500 IU/day for kids ages 1-3, 1,500 IU/day for infants ages 6-12 months, and 1,000 IU/day for infants ages 0-6 months.

But some recent studies suggest that healthy adults can tolerate more than 10,000 IU of vitamin D per day. John Jacob Cannell, MD, executive director of The Vitamin D Council, notes that the skin makes 10,000 IU of vitamin D after 30 minutes of full-body sun exposure. He suggests that 10,000 IU of vitamin D is not toxic.

I don't understand, how is it that 2000IU was considered toxic in the mid 80s and it is safe nowadays with thresholds of upper of 4000IU as being more prone to start being problematic?

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    Doesn't the last paragraph of your quote pretty much answer your question?
    – Carey Gregory ♦
    Nov 21, 2021 at 17:25
  • @CareyGregory: Well, I suppose that the original recommendation was based on specific toxic cases that were documented. The last paragraph seems to me as an "indication" not as evidence. But may be I am misunderstanding the process for all of this
    – Jim
    Nov 21, 2021 at 17:29
  • @CareyGregory: But I do understand that something has changed otherwise I doubt companies would add toxic levels by ignorance etc
    – Jim
    Nov 21, 2021 at 17:32
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    The limit was raised based on "recent studies." That's how science works. I don't know what the previous limit was based on, but someone decided to study it more carefully and discovered it was wrong. To answer this question, the basis for the earlier limit needs to be found along with those "recent studies." Keep in mind you can answer your own questions.
    – Carey Gregory ♦
    Nov 21, 2021 at 17:41

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