A lot of the initial “look at all this weight I’ve lost” is from water weight loss at the beginning. IF is great for losing water retention and inflammation.
I’m of the opinion that the first couple of weeks of IF aren’t really weight loss, but your body losing a lot of the water it’s been carrying around. So if you start eating normally, that water weight comes right back. In reality, you probably didn’t gain 2 kg. You probably gained a portion of weight, but a significant portion of that 2 kg is your body holding back onto that water.
At the same time, a portion of the weight you lost was fat but a significant portion of it was also water.
Think of fat cells like balloons. Once your body creates them it doesn’t destroy them it just empties them making them much easier to refill.
I think the autophagy phase of long fasting can break these down but it’s difficult.