[aldoushuxley001]

D3 mostly comes from fungi as far as commercial sources for supplements are concerned, I'm pretty sure.

[posix_me_less]

D3 is made by irradiating lanolin (from sheep wool) with UV light [1]. Natural source of D3 is oily fish, beef liver. It's the Vitamin D2 that is made from fungi - UV light helps to create it in mushrooms [2].

[1] https://www.quora.com/What-is-the-industrial-process-involve...

[2] https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessiona...