I have fasted with known B12 deficiency and suspected myriad of other deficiencies inc low protein. It is doable.. Doesn't mean it should be done. But the more minerals/ electrolytes/vitamins you lack, the much more bumpier the ride!
Physically and emotionally your fast will be trickier... Even anxiety and panic attacks even though you may have never had any before
It's much rougher. Refeeding is much rougher too, we don't realise but we need all those micronutrients to function inc. digestion! and without them we can become critical FAST! Especially the heart. During a fast if one is down on electrolyse the heart can experience arythmia... Which is dangerous.
Get a full blood analysis indicating all deficiencies, inc electrolye... Then remineralise and re-nutrition through the best healthiest foods to treat it.
After a month of eating well, then fast!