This is not meant to be evidence one way or another, and might not be even related, but I've noticed on those IF days where I don't feel hunger past the 16 hours, and don't get to eat until dinner, that I feel a rapid heart rate and a slight sense of anxiousness and feel slightly out of breath.
I think what happens is that cortisol or adrenals gets squirted in order to get some fat converted into glucose ketones, and muscle protein into glycogen to feed the brain, red blood cells, and part of the side effect of this is the rapid heart rate.
I usually IF only on days I work out, so this is only 3x a week, and I only fall into this pattern of going past 16 hours of IF rarely, maybe less than 1x/month, and that's mostly by accident or lack of hunger.
It's interesting to notice this at a high level and ask "Why the anxiousness?" and then figure out, oh right, I skipped both lunch and breakfast, and it's dinner time, best to eat something immediately. I didn't notice fatigue, or brainfog, so it was unexpected effect.
Now as per leangains, I do take about 10g of Branched Chain Aminos in the mornings that I IF, but of course, at the point to where it gets past 16 hours, I've not taken more than just in the morning. Mostly what allows me to get to that point is having coffee (some of them decaf) with coconut oil.