But is the headache solely due to a lack of vitamins or can it often be caused by something else during the fast days? just to understand if it is something that I have to somehow accept or not
I think you are confusing “vitamins” with “electrolytes”. It’s unlikely, but perhaps possible, that you have a headache from lack of vitamins. It’s more likely an electrolyte issue, read and follow the FAQ and tweak it until the headache goes away. If you don’t have any luck, post back with a description of exactly what you are doing and what your symptoms are.