Your kidneys, located in the small of your lower back, can get clogged with toxins during ANY cleansing protocol (like fasting!).
Water flushes them, yes. But if any of your toxins are from heavy metals and the like, the kidneys can be overburdened quite quickly...especially if you become deficient in the very necessary nutrient called: sodium. Your kidneys will NOT release those heavier toxins IF the body becomes deficient in SALT...the kidneys NEED sodium to SQUEEZE out those nasty toxins. And remember, if you do this, make sure you get adequate amounts of water. You'll need it to flush those kidneys, especially on days when your back hurts. It just may be your KIDNEYS that are hurting and crying out for a little "back up" - cuz they're BACKED UP!
Mixing simple Seasalt (1/4 tsp per quart of water) turns your water into an electrolyte, also.