50IU is a minuscule dose though, no? If people are recommended to supplement, they generally take a dose in the range of a few thousand.
The point is that from that N IU the 100 people will absorb anything from 0-N, it's very individual and varied.
The only way to be sure is to test your levels, which costs money every time. There really should be a simple and cheap home test kit for it. You'd sell millions every year just in the Nordics and Canada =)
I've been offered a (free) Vitamin D checkup exactly zero times. We should be getting them though, dunno why we don't.
Conversely, some studies have shown that 4k IU does contribute to hypercalcemia in a small number of cases (4 per 1000). So actually 4k is deemed "not completely safe" as a limit.
The point is, the amount you take needs to be adjusted by a clinician, as the safe range for you is unknowable otherwise.