>>76485379
Again its about the insulin response, black coffee does have calories, but its low enough (I believe something like <10 per cup) and typically doesn't trigger an insulin response. You can test your piss for ketones if you want to see if you're actually still fasted or not after a black coffee, but milk coffee will certainly break a fast. If you've got ketones in your piss it means you're burning fat instead of glucose which is the point of fasting over just restricting calories, which is what you're doing now that you broke your fast. Test strips are cheap and a reliable way to check if you're actually in ketosis or not.

Usually its best just to drink water (or snake juice) during a fast if you want to actually be fasting.