As I mentioned fasting for 72hrs isn't meant for losing weight its meant to reset the body. In 72hrs you are not going consume your entire bodies resources unless you are already starving, if you are healthy its not a problem. If you are not equipped to do a 3-fast you will lose if there access to food (there is a difference between fasting and starving). During slow times of the year I burn 3000 calories a day on average and busy times about 4000 a day, most of the 3day fasts I've done was during working days when I'm busy because its a distraction from the hunger and to troll some of my weaker colleagues (conservatively 9000 calories during a fast). There is going to be a last meal before starting the fast, so really its only 2 days of no calories (6000) or just under 1.5 lb of fat burned. I'm aware the body will consume more than just fat its just a reference. I burn more calories than the average person.
On day 3 the strength/energy level drops about 20% and there is good amount of hunger. At 73hrs there could be a bit of a induced food coma (Its a bad idea to binge when coming out of fast and at the same time hard not too), After getting some sleep there is a spike in energy that last for a few weeks. Fasting to reset the body is built into human dna, it also starves out parasites (if infected). You don't have to fast if you don't want to. If you are relatively healthy with good weight you are equipped to do a 72hr fast (as Valentine1699 suggests). I have no way of proving this to you but just for fun I'll do 3 day day fast with a stupid (not recommend) dose vitamin C (12g). I usually take electrolytes and some vitamins (no protein powder) prevent dehydration and over flushing the good stuff out......and caffiene/black coffee.
You have inspired me. I'll should be around 12hrs after this post gets approved.