Yes, it's fat soluble, and you don't lose fat soluble vitamins in urine the same way you do with vitamin C, but that's only a small part of the puzzle.

Questions you should ask:

1. OK, so fat soluble vitamins aren't excreted in the urine. But are there other processes by which the vitamin is consumed? Because if so, you could quickly run short on the vitamin whether or not it's fat soluble. If deficiency of the vitamin is known to be widespread, that's probably a sign that there is some form of consumption process like this.

2. What is the mechanism of toxicity? This matters because if the mechanism of toxicity is different than the normal mechanism of action, the dose you need to reach toxicity could be orders of magnitude higher than the dose you need to reach physiologic activity.

Regarding point 1: Vitamin D is inactivated inside mitochondria.

Regarding point 2: It's hard to get vitamin D toxicity because active vitamin D (calcitriol) is created "on demand" by cellular processes by tapping a much larger reserve of inert substance (calcifediol). Vitamin D supplements increase the amount of inert substance, not the amount of active substance.

> The evidence is clear that vitamin D toxicity is one of the rarest medical conditions and is typically due to intentional or inadvertent intake of extremely high doses of vitamin D (usually in the range of >50,000-100,000 IU/d for months to years).

https://www.mayoclinicproceedings.org/article/S0025-6196%281...