My subjective opinions on the matter: Ketosis only kicks in when carbs are sufficiently low, ketosis is required because protein as an energy source cannot pass

My subjective opinions on the matter: Ketosis only kicks in when carbs are sufficiently low, ketosis is required because protein as an energy source cannot pass the blood brain barrier and so needs to be converted to ketones before it can be utilised. This to me indicates the direction of ketosis being the backup system instead of the norm but I am not trained on the subject and definitely welcome being educated further. Note that this doesn't effect my opinion of fat as a great energy source. I personaly eat a relatively low carb (not keto), high-ish fat, high-ish protein, moderate carb diet. I'm also very active and this seems to work for me so far.