First, I wouldn’t assume that the weight you lost is the same as the weight you regained. It could be fat, muscle or bone.
Second, fasting drives autophagy that cleanses the body of old weak or defective cells. So even if a fat cell is emptied and then refilled - or broken down and then rebuilt - the new cells are healthier and more rejuvenated.
Third, a lot of weight is actually water and food stuffs passing through your system… so again - weight doesn’t mean much.
How do you feel? How do your clothes fit?
When I fast, my metabolism does drop by 20% at least and as much as 30% on an extended fast. Here are my measured RMR results
The first is baseline at 2400 and the second is after a 19 day fast at 1700. The good news is that refeeding recovers RMR quickly and heavy protein and fat can bring it back to the very high levels.
Also, as you fast longer, your body will use fat more than lean mass… The more fat you have, the more you can burn without having to resort back to lean tissue.
In my experiment - I lost some lean mass but recovered almost all of it afterwards. I did stay active though.