First, let’s talk about what’s actually in that can. Celsius is marketed as a “thermogenic” drink, which is just a fancy way of saying it’s supposed to burn calories. It’s loaded with caffeine, green tea extract, and a bunch of B-vitamins.
Here’s the kicker: it also contains zero sugar and only 10 calories. That’s almost nothing. By the strictest rules of fasting—the kind that says zero calories—those 10 calories technically break a fast.
But let’s be honest. Is a handful of almonds or a splash of heavy cream in your coffee worse? It’s all a matter of how strict you want to be and what your goals actually are.
The “Clean Fast” vs. The “Dirty Fast” Debate
This is where the friendly arguments start. If you are a purist—the kind of person who drinks black coffee and thinks even a mint is cheating—then yes, Celsius breaks your fast. Those 10 calories do require your liver to do a little work.
But if you are a human being who just wants to survive a morning workout without crying, welcome to the “dirty fast” club. Many experts argue that a fast isn’t truly broken unless you consume more than 50 calories. So, a 10-calorie Celsius? That’s basically a whisper to your metabolism.
The real question is about insulin response. Artificial sweeteners like sucralose, which are in Celsius, are a hot topic. Some studies say they cause a tiny insulin spike; others say it’s so small it doesn’t matter for weight loss or autophagy.
