I’m familiar with why people fast, nargoth16, and it’s not necessary to get the benefits you are wanting.
Eat at a slight caloric deficit. Consume no caffeine or alcohol or supplements (this does not include prescription medications!). Consume water and green tea. Eliminate soda from your diet. Consume organic fibrous green veggie carbs and organic fruit. Juice veggies if you like, but I’ve always felt removing the fiber from fruit removes some of the health benefits. Increasing fiber intake is part of cleansing.
Cut out any food group that is potentially allergenic. There’s a list if you do a search on the Internet – things like milk, eggs, soy, corn, wheat, shellfish, etc. Cut out all processed foods and starchy carbs. Do not do resistance training and cardio. Walking is okay, though.
Do consume grassfed meat and free range chicken/turkey.
If you limit your “modified” fast to a max of one week in duration, you’ll enjoy all of the benefits without losing any LBM.