Once the food has gone through the stomach and small intestine, and glycogen stores are depleted, the body will start burning fat. Being āfat-adaptedā greatly aids the process, and coming from a ketogenic diet helps, too, but a few parts of the body have to have glucose, and the body will start producing it in the liver, pretty much no matter what, usually using fat.
Most people will lose about a half pound of fat (200g +) per day. Bigger people and people with more fat can lose more. Most times, starting a fast causes some fluid loss, and this can alter what the scale says a lot more than the fat loss. So can other electrolyte considerations, like how much salt one is getting, or hormonal issues. But if youāre not eating, youāll be losing weight in the long run, one way or another.