Can a natural BodyBuilder take Vitamin D3?

"Recently I found out that Vitamin D is a steroid hormone". Completely false.

As far as I know "natural bodybuilder" refers to anyone who doesn't take steroids, growth hormone or anything illegal/unhealthy. Taking regular supplements like vitamins, minerals or protein powders isn't illegal but they also might not be necessary.

Animal-based vitamin D (D3) is naturally produced by the human skin when you get sunlight. You can also find it in some fatty fish like wild-caught salmon, baltic herring and cod. Minor sources are eggs and fortified milk. There's also plant-based vitamin D2 which you get from some mushrooms. Some sources tell that the plant-based variant isn't as good. Some sources recommend a vitamin D supplement at least during wintertime if in the area sunlight is scarce.

Zinc and selenium are important for testosterone production. I eat one or two brazil nuts each day to get some extra selenium. Zinc is commonly found in meat, fullgrain and seeds for example.

Yes. Vitamin D 3 is not considered a performance enhancing drug. 75% of people are deficient in vitamin d. All it does is make your bones more dense so you are less likely to get a fracture. Taking it makes your bones about 1 to 3% more dense per year of taking it.