I lost a lot of weight with fasting, sometimes up to 48 hours. The problem is my metabolism slowed down so much that when I went back to eating normally I gained it all back plus more.
Discussion
this is super common and why if it is used for weight loss it has to be balanced with a caloric matched (to your exercise) animal based diet.
it is much better to run a 250 calorie deficit daily for 6 months then to run a 1000 calorie deficit intermittently
slow and steady is the way.