It's practically impossible that that is all body fat. 1kg is over 7000 calories. The body is more than 50% water, so weight is highly variable based on water

It's practically impossible that that is all body fat. 1kg is over 7000 calories. The body is more than 50% water, so weight is highly variable based on water retention, and fasting causes the body to excrete a lot of water. For one, no salt intake will cause you to lose water. Secondly, glycogen depletion also causes you to lose water. The metallic taste in the mouth is ketosis, proof of glycogen depletion.