Part of the issue with vitamin supplements is that the bioavailability can be unpredictable. The actual amount absorbed can vary between 10% and 100% depending on the time of day, supplement formulation, what foods if any are taken together, as well as the particular characteristics of the individual's intestines, which are difficult to assess. Because supplements are not regulated as pharmaceuticals in the United States, this variability can be severe; in the worst cases, supplements do not even contain the active principle.
So, I am not surprised that someone needs to take 5000 IU to get 600 IU worth of effect. Institutional medical authorities are (rationally) quite defensive when cautioning readers about supplement consumption; they must consider the worst case (100% bioavailability) when assessing the risk of overdose.
As an alternative to vitamin supplements, exposing common dietary mushrooms to ultraviolet light converts (by an uncatalysed photochemical reaction) the ergosterol therein to calciferol. How best to achieve this in a home setting is unclear.
So, I am not surprised that someone needs to take 5000 IU to get 600 IU worth of effect. Institutional medical authorities are (rationally) quite defensive when cautioning readers about supplement consumption; they must consider the worst case (100% bioavailability) when assessing the risk of overdose.
As an alternative to vitamin supplements, exposing common dietary mushrooms to ultraviolet light converts (by an uncatalysed photochemical reaction) the ergosterol therein to calciferol. How best to achieve this in a home setting is unclear.