A lot of people counter argue that the body/brain run more efficiently primarily on glucose and glycogen rather than ketones. While it is true if you introduce sugar (glucose) into the blood, cells will generally begin to produce energy/atp primarily from glucose over ketones. However, it is not because the glucose is more efficient, in fact ketones are smaller making them the a more efficient fuel source of the cells. For example, bc they are smaller they cross the blood-brain barrier more easily and are more efficiently used by brain cells to be converted into energy/atp (in the mitochondria).
I think there is a reason we call it “getting knocked out of ketosis” because that is our natural state. That is forget high fat/low carb and abstain from all calories and drink only water to stay hydrated and the body will go into ketosis, which is what we are talking about when we discuss “fasting” as a metabolic state (vs say a culturally/religiously defined “fast”).