Generally speaking, you don’t eat any food at all while spending your fasting hours. Any food and calorie intake will break your fasting process.
But you can drink beverages with very few calories, such as water, plain tea, black coffee.
Here are some examples of calorie-free drinks:
Plain tea (e.g. black, green, herbal)
Black coffee
Water
Sparkling water (OR flavored with zero-calorie)
Citrus OR berry flavored water (a few fruits slices)
Diet/no-sugar soda (Diet coke, no sugar Coke)
You may add 1 tablespoon of milk or unsweetened plant-based milk, and a small amount of artificial sweetener in your drinks to enhance the flavor if you can’t have them plain.