Hi Loosie,
If you think this may be a magnesium deficiency than I would just full body soak in Epsom Salts which contains Magnesium sulfate and which when used through soaking, can soothe muscle pains/seizures and help improve rough patches in the skin: amongst many other benefits.
The body's magnesium level increases thru the skin when soaking with magnesium sulfate.
I would try this before you start your fast for at least a couple of weeks and then when you do fast, if the problem has not been resolved, then a magnesium deficiency was not the original cause.