The way I understood it, D3 is the easiest one to absorb. Both D2 and D3 do the same thing, but their efficiency is different.

https://vitamindwiki.com/ is a useful aggregate site for papers (peer-reviewed and others) on vitamin D. The latter is cholecalciferol (https://en.wikipedia.org/wiki/Cholecalciferol).

https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessiona... (excellent series for all vitamins)

Different manufacturers = varieties? It's the same stuff or should be.

As a vegan, I learned that there's D2 (comes from plants) and D3 (comes from animals), and they are the same as far as I know. Not sure about using them for treatments.

As a carnivore I’ve learned the same.

As an omnivore I didn’t know either of these things before now, so I thank my learned extreme-dieted colleagues for sharing.

> A friend of mine had cancer and they had him taking D3 immediately.

The last thing in the world a cancer patient needs is well nourished cancer cells, most grow and reproduce quickly.

There is some evidence that vitamin D deficiency is associated with prostate cancer: https://clincancerres.aacrjournals.org/content/20/9/2289.abs...

There are a number of other studies on this. From what little I've read, if the effect is real,it seems the it is relatively weak, but still worth doing.