The link below details an interesting story of an athlete/trainer running smack into thyroid disfunction, which PN attributes to excessively fastidious intermittent fasting and too much "stress" (for the record, and in the interest of language precision, I like to think of "stress" as a good thing; how we respond to stressors makes them "good" or "bad").
The big lesson is - anything good can be taken too far. I love the liberty of waking in the morning and not needed food to feel good. I think IF is a great way to live - for me. But if anyone takes a basic idea like IF, and gets crazy with it, they can get into the deep end of a pool they are not prepared to swim in. Which is why, on nights like last night, I eat cake, ice cream, or other food treats with a clear conscience when I'm socializing and celebrating. About once a month, I even get a candy bar or something stupid like that to remind myself how little I like them (even though, from years of eating them when sugar dependent, I have a positive association to sweets - a HUGE one).