Agreed, and there still is a lot of fear-mongering that one sees about “you have to eat some carbohydrates…”
My total layperson’s take on blood sugar is that yes, most of our body runs just fine on ketones, but it won’t be a perfect, linear relationship between what is considered normal blood sugar for a “standard” diet and what works for somebody burning fat.
“Normal” is usually said to be around 85mg/dl or 4.7mmol/l.
If, for example, a fat-burning person would be running on 75% ketones with the remaining 25% being glucose, then I think because of certain inefficiencies in the body (or other factors I’m not aware of) that while lower blood sugar, per se, is fine because less is needed, a blood sugar level 25% of “normal” might still be too low, and result in the person feeling common “low blood sugar” syptoms.
Putting together all the anecdotal accounts I’ve read, plus some reports and studies on long-term fasting people, I’d give a range as follows:
45mg/dl or 2.5mmol/l ~ some people start feeling faint or otherwise “not right” close to this level.
40 or 2.2 ~ a higher percentage of people will be feeling it.
35 or 2.0 ~ an even greater percentage will be feeling it. Still, lots of people - usually well fat-adapted and often experienced longer-term fasters - feel fine.
2.7 or 1.5 ~ this area is where most people will be feeling it, and - my opinion - this is where concern should really begin, even if a given individual isn’t reporting feeling bad.