How will I get b12 during a fast?
What is Vitamin B12?
If you pull a carrot out of the ground, there would be little bits of dirt and millions of microscopic organisms all over that carrot. That's B12. It is basically bacteria poop. It is present on the surface on root vegetables like carrots, beetroot, etc. If you wash and peel that carrot, enough B12 is still present in it.
So during a fast, you are actually getting even more B12 than usual because you are consuming an abundance of vegetable juices. Don't worry, during a fast, your body will actually get more nutrition than normal and you don't need to worry about b12 or any vitamin deficiency.
The question is not what to take for B12, but what to stop consuming which might be leeching it from your body. Items like coffee, tea, energy drinks and sugar all leech out B12 from the body.