Fasting can be very stressful to the adrenals & so is parasite cleansing. In addition juice fasting can actually cause hypothyroidism because a lot of the things that are being juiced - thus going into the body in intensity of strength - are goitrogens (damage the thyroid), for ex. spinach and other things. You can check it out on the net.
A glass of juice a day probably won't hurt you. But you need to keep up your strength. There are things that get tossed out of the juicer that are actually helpful to your body.