Water fasting with known deficiencies

Hey, I've completed short fasts before. I have a question about fasting with known deficiencies. I have a health problem that would benefit from fasting, but I also have a known vitamin D deficiency. I would imagine that's not all I am deficient in, possibly some minerals as well, but that is all that is confirmed. My diet has been pretty normal so the deficiency was likely caused by my health condition. So I don't really know what to do here. I need to do a water fast, but I'm worried about deficiencies.