When all else fails, stck to water only in your fasting window. It is said that black coffee and herbal teas are ok for fasting, but it depends on how your body reacts to them. Some folks see insulin movement.
I used to drink red bull every morning. When I decided to lose weight I switched to coffee. Milk & creamer. After a few months I started IF and started having it black and it was fine. My secret is Hazelnut coffee. That’s all I’ll drink, may not work for everyone but works for me.
I just keep the calories under 50. Idk if this is accurate, but my mom has been doing IMF for a long time, and she said that seems to be the consensus. Not saying it’s healthy, but she and I typically have an energy drink in the morning because the caffeine helps with cravings lol