Did they test "high dose taken frequently" at all? Bearing in mind that the guideline vitamin-D daily dose target is set at just about the lowest possible value that doesn't cause obvious deficiencies.
Something like 40,000-100,000IU/day for several months.
That's 8-20 pills per day of the most potent D3 commonly available.
> In the studies, daily low doses were 400 to 4000 IU per day, and higher-doses administered at longer intervals were 60,000 to 120,000 IU once per month or less.