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?