Metabolism is about converting food into energy, and it does so through three main pathways:
the glucose pathway: the body processes carbohydrates and produces energy. This is the priority pathway for the brain and red blood cells—the former because of its importance, and the latter because they have no mitochondria.
the beta-oxidation pathway: it transforms fat into energy. When insulin drops, this pathway increases.
the ketosis pathway: it allows the energy from beta-oxidation to be distributed throughout the body in a context of low glucose.
These three pathways are designed so that we can withstand periods without eating or bouts of intense exercise in which glucose is reduced, which is why it takes a very long fast for the body to actually run out of energy.
When I have gone through periods of eating very little or fasting and experienced what is known as “keto flu,” what happens is that this shift from one pathway to another has a period of mismatch in which the brain has not yet been equipped with what it needs to consume ketones, and until it develops that capacity, the brain has less energy and the kidneys lose water during the adjustment. Once adapted, the body works perfectly well with few carbohydrates. The same thing happens with sugar cravings: a metabolic mismatch.
BTW, dude, keto flu sucks.