The safety of the fast is entirely dependent on the quality of ones food reserves, and where nutrients are conserved within the process of autolysis during the fast: the least essential being used first for sustenance and where the remainder are conserved as far as it is possible.
Even in a protracted fast of over 40 days and beyond electrolyte deficiencies such as salts of potassium and sodium are extremely rare even with the most deficient of pre-fast diets.
As a matter of interest no one but no one has ever developed a case of Vitamin C deficient scurvy on a water-only fast, but is easily accomplished by a restricted or reducing diet.