@Honeyblossom - My BMI is 18.5-19, so don't have much wiggle room for weightloss, but I have sometimes used fasting, albeit usually on long haul flights and such like, where I feel confident the food wouldn't make me happy, so I just choose to swerve it.
When I want to do that (the fast will usually be 36-48 hours- depending how you add in or take away time differences), I usually do a few days of carnivorous eating beforehand. I usually just do the carni - fattier meats, whether that is lamb, a fattier steak, belly pork or chicken thighs, right up until I get onto the aircraft, then that's it until I arrive, and have slept.
I don't think about what level of calories I'm taking in on the carni phase, but it'll be a few.
Many, many find the lack of gallbladder to be a red herring when low carbing. Personally, I have my gallbladder, but I do have a fat threshold. If I just eat too much - usually nut based fats, I experience steatorrhea. I won't describe it, but should it occur, I just dial back on the fats/nuts a bit until it subsides.
When I come out of the fast, my first meal will always be light. else my tummy can feel like I've eaten a boulder.
Obviously, we're all different, but for me it's doable provided I prepare.