Over the past 3 years I've spent a fair amount of time aiming to be roughly keto.  From 2 months of eat less than 10g carbs per day, to 2 8 week UD2.0 cycles to several 8-15 day runs at PSMF.  When I first started, I got all of brain fog/keto flu symptoms people talk about, but in the last year or so, it seems to me like I can now change over to virtually no carbs with no side effects.  For example, Christmas was a bit of a mess for me and December through to march I was probably averaging 300g carbs per day.  Over the past 8 days, I've dropped my carb intake it virtually nil (less than 15g counting all plant matter, so mostly fibre) while continuing to lift every other day and run every other way.  Macros have been roughly 210p/10c/100f each day.  I've got absolutely no brain fog or fatigue - I just feel normal (less hungry).  I also IF on a roughly 16/8 schedule if that's relevant. 

Has this ever been brought up before or qualified? 
I've considered the possibility that I just don't transition to ketosis at all, but that would mean that my body was converting a huge proportion of the protein I eat into glucose which doesn't really seem likely to me. 

I've found the same thing.  Having tried low carb before, when I first tried keto I didn't encounter any brain fog/flu symptoms.  I'm 8 months in, and whenever I go out of keto I don't feel any symptoms. 

As to how quickly it loses it, I'm not sure but would also like to know. 

I've found the same thing.  Having tried low carb before, when I first tried keto I didn't encounter any brain fog/flu symptoms.  I'm 8 months in, and whenever I go out of keto I don't feel any symptoms. 

As to how quickly it loses it, I'm not sure but would also like to know. 

I've never seen an iota of data on it.  Anecdotally, people who have done keto and adapted seem to re-adapt with little difficulty the next time.  This *suggests* some type of long-term or permanent adaptation but that's just my speculation.