Water-only fasting was popular in sanatoriums and weight loss spas in the late 1800's/early 1900's... practice was stopped after lack of electrolytes caused several patient deaths from heart arrhythmias.

That's definitely true, electrolytes make it much easier.

Can you recommend any good sources to find out more about this need for electrolytes while fasting? I have been unable to find articles or books that seemed credible to my uneducated eye.