Read somewhere that on long fasts, the body starts to remove Magnesium at an alarming rate to maintain Calcium and Phosphorous levels.
But some sites say these two are the 4th and 5th electrolytes - but this sub kinda skirts round the topic.
The body stores enough calcium so it’s not necessarily to supplement during the length of a fast, since most are under a month. Taking in too much electrolytes is just as dangerous, and it’s easier to overdo it with those.