My Mom and I have done “clean fasting” for around a year now so we stick to only water and black coffee during the fasting window. She’s lost around 60 pounds and has met her goal and I’ve lost around 20 but I still want to lose another 20-40.
We try to avoid anything else with “taste” because we don’t want to spike our insulin during our fasting period. Only exception is toothpaste lol.
Realistically speaking though people still lose weight doing “dirty fasting” where they consume a small amount of calories during their fasting window (ex. coffee with a little creamer or some bone broth) as long as an overall calorie deficit is maintained.
Technically anything with calories breaks a fasting window, but most of the research I have read suggests that weight loss with fasting only comes from caloric restriction. In other words if you eat 1500 cal in one meal or 1500 cal in three meals the weight loss will be very similar. There are some other benefits to fasting like autophagy, but I’m not sure how much that would be affected by 5 calories.