Fasting, on vegetable juices especially, can speed up the recovery process dramatically. But it can intensify very much the detox symptoms. Also, you have to fast properly, assisting the organs of elimination (liver, colon, kidneys, skin), otherwise you might get extremly ill. Breaking the fast is as well very very important, for many reasons but especially because fasting depletes some of the good flora along with the bad, because they need food/fiber to survive. It is kind of the same than with antifungals/antibacterials, even natural ones, like raw garlic or turmeric.
Personally when I fast, I use the enema bag a lot, activated charcoal to adsorb toxins, some fiber like psyllium/flaxseeds, and drink huge amounts of kefir (water kefir is less beneficial in my opinion, but milk kefir requires much more digesting time which can be counterproductive during a fast). I just can't understand why proponents of Water Fasting advise against doing anything else than drinking water, because water doesn't provide key nutrients for healing (vitamins, minerals, amino acids, enzymes, that vegetables provide), and doesn't assist the organs of elimination very much. It might do you some good, but nowhere near a complete vegetable juice fast protocol (in my experience).