There’s nothing to do to prevent loose skin from weight loss. Genetics, age, how long you’ve been overweight and overall health play into how much your skin can bounce back. The best thing you can do to help your skin is drink plenty of water and stay healthy.
A lot of people say longer fasts (like 36-48 hours) are good for getting rid of extra skin but I don’t know if that’s true or not.
There does seem to be a general consensus on the internet that fasting in general is great for loose skin though.
I just lost 50 lbs and noticed a lot of loose skin but after a week of not fasting a lot of that has tightened up.