i'm guessing he meant 50k not 50. 50k is quite a large dose.
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.