We should be in ketosis during all weight loss stages. At maintenance, we should find a place at the foggy border between the two states where we neither gain nor lose and set our carb intake amount to keep us there permanently.

As far as I know, there are two methods to metabolize fats:

(1) The default method in which fatty acids go through series of processes and end with a fuel which is used for energy. After all energy needs are met, energy can be stored as body fat.

(2) The second method in which fatty acids breaks into ketone bodies and a different kind of fatty acids before it completes its journey. Ketones supply body cells with energy. After all energy needs are met, ketones can only exit the body. They cannot be stored as body fat.

To my believe, the body uses the first method alone when carb intake exceeds (100-120) carbs depending on the individual.

When carb intake is reduced below this amount, the two methods take place at the same time and method 2 share grows as the carb intake moves down twards zero.

So carb reduction acts like moving a mode switch slowly from one mode to another. The first mode is for a vegetarian human and the second mode is for a meat eating human.

The vegetarian human can metabolize fats, but probably not the perfect way. The meat eating human metabolizes fats better.

Ketones do not appear in the urine or saliva until they exceed the amount the body can use. This is why you don't notice them unless you eat 35 carbs or less.