Here, you allow healthy fats during the fast in addition to water and/or non-caloric beverages. While you probably would not eat a stick of butter, you could have bulletproof coffee (black coffee with butter, coconut oil or MCT oil), for example. Alternatively, you could add the fat to your tea.

https://articles.mercola.com/sites/articles/archive/2016/10/16/complete-guide-fasting.aspx