My experience with Vitamin D is pretty weird, it also might chime with other members on this forum.
I was diagnosed with Mast Cell Activation Syndrome (MCAS) last Xmas and my doctor put me on a course of 1000iu daily Vitamin D (as well as 2 antihistamines).
Initially the Vitamin D started to do something positive - it seemed to be reducing my brain fog and has improved my tolerances to medications - but at the same time was worsening my CFS and achiness.
This doesn't make much sense to me - if the CFS and sensitivities are all being caused by MCAS, why would Vitamin D help some aspects of it while worsening others?
Part of me had hoped the Vitamin D might have been fixing an underlying issue (say mould or other infections) and that the "die off" was triggering my Mast Cells, but I feel this is unlikely.
My doctor (MCAS specialist) in our most recent consultation said the Vitamin D was intended to directly reduce my Mast Cell reactions (rather than anything underlying) and advised I keep taking it at 1000iu. But I literally cannot tolerate it now at all, it gives me really intense back pain and definitely worsens my symptoms, if feels like it's directly triggering my MCAS, so I've had to stop entirely.
I'm now at a loss as to what to make of it, and what if anything the Vitamin D helped while I was able to take it. And why I no longer can. If anyone has any insight here I would be massively grateful.