Talk to your doctor and consult the Holy Spirit before you participate in this option but a complete fast is just that. For 21 days, you would only drink liquids (i.e. water, light juices, etc) for the entire day or designated portions of the day. We've seen this type of fast in the Book of Esther. Queen Esther called the entire nation of Israel to fast for their deliverance (Esther 4:16). It is amazing how long a human body can stand without food, but not without water. However, if God is calling you to this kind of fast, always follow His leading first.