Will a Celsius break your fast? Probably not enough to matter unless you’re a competitive faster or a lab rat. The 0–10 calories are a drop in the energy bucket, and the benefits of caffeine—focus, fat burning, delusions of grandeur—often outweigh the infinitesimal insulin blip. Just remember: the best fast is the one you can actually stick to. If that means enjoying a sparkly, cold Celsius while you avoid the siren song of a bagel, then crack that can open.
And if anyone gives you a hard time, just tell them you’re “performing a controlled metabolic experiment with citric acid and bravado.” Then take a sip, smile, and let the faux-sugar gremlins do their work. Your cells may be confused, but your day will be won.
