One of the key things in being able to fast in a healthy matter is entering the state of ketosis – that is, switching your metabolism from burning carbohydrate to burning fat. *Considering most people have been burning carbohydrate their whole lives* this can make fasting a challenge.
Upton Sinclair recommends doing quite long fasts – 12 days, but you may want to work your way up to longer fasts. Unless you are relatively keto adapted (or have experience with fasting of course), it’s important that you take precautions while doing a fast. Fasting isn’t inherently dangerous, *but diving into fasting when your body is in no way geared for it can be.* I recommend doing more research before doing longer fasts. “The Complete Guide to Fasting” (http://amzn.to/2tQjxXa) is a good book to start with.