I haven't heard this before so thank you for that. To answer your question though, I haven't done a long fast before. My longest fast was 7 days and I find humor in the bitter irony that the day I feel the worst is precisely two times my longest fast.
I favor "throwing caution to the winds" in this situation because I am quite dedicated to the 31 days I have chosen--I am partial to the time length. If it is truly dangerous in that it is life threatening to disregard this rule-of-thumb in gross proportions, then I will, reluctantly, break this fast. But I really rather not if it isn't a necessity.