I have recently read the Fast 800 book. Basically it is intermittent fasting 2 days out of 7. However before doing that, you stay on 800 calories a day for 2 weeks. They switch to the 2 days of 800 calories and 5 days of .Mediterranean style eating. I personally don’t follow the Mediterranean eating. I have been eating healthy 1900 calories 5 days a week and then 2 days at 800.
The book is cheap on Amazon, I got it for £5.
https://www.amazon.co.uk/gp/product/1780723628/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1