The original guidelines for vitamin d dosage was base simply on bone health. There's substantial evidence that vitamin d is critical for immune health. There has been a study since 2021 that showed that it 50 nanogram per milliliter in the blood you could have achieved near zero deaths from COVID. And the authors of that point out that you need 5,000 IUs per day to achieve that level. 50 nanograms per milliliter is nowhere near to being dangerous. Official US guidelines go up to 90 nanograms per milliliter as being acceptable. Furthermore the most recent studies have demonstrated that as much as 50,000 IUs per day of vitamin d have only beneficial health effects and no detrimental effects.