Celsius where I live has sucralose and 1kcal. Haven’t tested my insulin before and after drinking 100 ml (serving size). I didn’t want to risk it. Technically calories break fasts, but there’s a lot of debate about how many calories it takes so some stick with the rule of 0. Insulin response is what breaks the fast and sweet tasting things even like 0 kcal teas can cause your body to start producing insulin, which breaks the fast. It also depends on why your fasting, if it’s for weight loss, autophagy, etc. The fasting method podcast talks a lot about this and calls it a crutch- what you need to get you through that fast but to eventually phase them out and train your fasting muscles.