Vitamin D from UV grown mushrooms
Want vitamin D? If you happen to be in a sunny place that allows certain UV rays to hit your skin, your body can make it. You can also take Vitamin D2-containing supplements (like cyanocobalamin vitamin B12 it is often vegan, but useless to the body), Vitamin D3 (animal sourced, so not acceptable to vegans), or eat an animal with vitamin D in it.
As a near vegan, I still take vitamin D3. Vegans often talk about respecting animals. D3 vitamin manufacture does not kill any animals at all. It is made from lanolin in the wool of sheep. The process of removing wool is beneficial to sheep, as parasites like maggots will be attracted to sheep wool near the anus if it is not shorn, and infect the sheep.
Hence, I don’t have any problems with using wool products or vitamin D3. If some vegans want to respect the lives of those maggots and parasites more than the sheep, that is their problem.
However, for pure vegans, there is now a source that is entirely animal-free. Now it seems that mushrooms grown under UV light also have vitamin D in them, and will retain the vitamin D for eight days after. Such mushrooms apparently are on the market in California already. Unfortunately, it seems that it is ergosterol, which is a precursor to vitamin D2, not D3.