Sorry, but fasting, as well as caloric deficit, will tighten skin very minimally. I would bet that you are talking greater than minute wrinkles. The caloric deficit got you there with the loose skin.
Been there, done that.
There is a non evasive method called radio frequency. It causes cell regrow and increases collagen production. It is a little expensive and works for about 5 months. I use it yearly before competition and it is progressive if you maintain your diet.
Now, if you have a lot of loose skin, this will not work like you are looking for.
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