Well, how do I explain... The mitochondria get used to using certain fuels, most people on an American diet having their bodies primed to use carbs 24/7, and the more you switch your body to sourcing a little or a lot of fuel from fat, the more you are able to run on an empty stomach, the more metabolically flexible you are, and the more steady your energy levels. It has a lot of side effects like losing fat, lowering inflammation all around the body, and even increasing the power your brain can draw, as the ketones (fuel from fat) that it draws from your blood when running more off of fat than usual are more oxygen efficient than the blood sugar.

You can increase this with intermittent fasting overnight (basically, delay breakfast or have dinner early or both), fasting multiple days with lots of water and electrolytes and possibly water soluble vitamins to avoid headaches and muscle cramps and vitamin deficiencies, eating anything from moderately low carb to extremely low carb. Exercise also helps, as does abstaining from eating sugar most of the time. Drinking vinegar at the beginning of a meal with carbs prevents glucose spikes, which helps too, and you can get this from a shot of Tabasco for flavor.

And for keto, don't believe the bullshit about needing to eat low protein on keto to avoid it turning into carbs, that's pseudoscience: your body allocates only as much protein to be converted into carbs as is necessary, it's smart, pretty much you should eat as much protein as you can get, on any diet, and especially keto. Too many gullible and unscientific people out there regurgitating that shit that's unfounded and proven false.