Posted this in response to a suggestion to try more fat:
I have actually been forcing more fat again. At one point I was consuming about a lb of USWellness's tallow in addition to my daily meal of fatty meat or pemmican. That soon became 'as much as I could stomach', which soon became under half a pound a day while fat fasting. USW tallow is unbelievably disgusting; honestly, I've consumed so many gross supplement powders and concoctions that I'm quite used to forcing myself to suck it up. Their tallow has reached a point with me where no amount of willpower will prevent some sort of gag reflex.
My point is that I switched to pure heavy cream for now, which is palatable enough. I've started eating between 1-1.5 lbs of 80% ground beef and one pint of cream every night, doubting my excess protein theory. Starting today I'm kicking that up to 2-3 pints of cream first then 'lean' ground beef.
The only thing I've found to help so far is absolutely ungodly amounts of water. It does seem like my body is still converting protein to fuel for whatever reason, and it is dehydrating me severely. I'm drinking 2+ gallons of water a day, and it seems like several of the adverse effects I was experiencing were actually due to dehydration.
The issue with this is that I'm drinking an absurd amount of water and visit the restroom 2+ times an hour. The possibility of water overdose and electrolyte disturbances are constantly on my mind, but I seem to be doing alright. The other issue is that, no matter how much I drink during the day, I always become extremely dehydrated during the night. I crawl out of bed exhausted and feel no relief until a few pints later.
Anyways, my assumption is that it's my body's gluconeogenesis that is causing this extreme dehydration- the kidneys demand large amounts of water to continuously filter out the nitrogen (thus the bubbles, which clear up once I've drank enough in a short enough time period.) At this point, my new hypothesis is that this is due to a lack of fat prompting my body to rely on gluconeogenesis for the majority of its fuel.
Considering all of the cases of failure to thrive on ZC diets I've read about, it seems a great majority are young, lean to begin with, and male (with a few instances of active athletes fizzling out and giving up). It doesn't make a whole lot of sense to me that ~2500 calories from fat alone + protein would not cover my energy needs when totally sedentary. Perhaps, though, I have some sort of fat 'debt' that needs to be satisfied before my body is comfortable burning fat for fuel. In light of the younger lean males failing at ZC is does seem a strong possibility that we've all greatly underestimated our bodies needs for fat.
A lack of fat almost seems too simple to be the case, but considering I've availed myself of many symptoms simply by drinking far more water, I think refocusing on the basics is wise enough.