Additionally, how do we get Vitamin D from a nuclear reaction? It doesn’t seem intuitive that a nuclear fusion reaction would suffice me with an important nutrient.
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The nuclear reaction doesn’t produce Vitamin D itself.
It’s that nuclear reaction keeps the Sun very hot. Very hot bodies radiate energy, which means they emit electromagnetic waves. Because we’re lucky enough that we have an atmosphere, most of the radiated energy is not lethal. For the most part we get (non lethal) ultraviolet, visible light and infrared.
All this gives plants their energy and triggers in humans the production of Vitamin D. In particular, Vitamin D needs UVB (Ultraviolet Type B), while photosynthesis relies on visible light from blue to red.
(*) When I say non lethal, I mean non-ionizing radiation. Excessive UV from sunlight can still cause skin cancer, but it doesn’t do that by forcing electrons to detach from atoms.
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