Fasts = calorie-restriction. Calorie & carb-restriction causes all sorts of health problems. If you don't come from an eating disorder background or underweight than doing a couple day fast shouldn't be a problem. Dr. Graham of 80-10-10 does supervised fasts, but seems to recommend as a plan-b option if the mono-fruit route then 80-10-10 diet doesn't correct all of a person's problems.
As long as you keep your fat down, natural sugars shouldn't cause a problem.