If you happen to die during a 10 day water fast, chances are excellent that it wasn't constipation that killed you. The same goes for a 30 day water fast. But if your bowels don't move during that time one thing is a certainty: you will be miserable.
That's why regular saline solution cleansing enemas are recommended during a water or juice fast, preferaby once or twice daily (first thing upon arising and/or again at bedtime).