Black coffee shouldn’t have calories, but it apparently does trigger metabolic responses in the brain, gut and liver.
Most people debating things like coffee, tea, lemon, even fish oil (which definitely has calories) have lost the forest for the trees, and they often can’t explain why they are fasting in the first place or their goals.
Unless you induce diarrhea, there’s food residues in the gut which keep adding to your energy bottomline for up to a few days. It’s unlikely that having a coffee or a supplement has a noticeable impact on your fasted state during intermittent fasting (when fasting 3+ days, things are different). To be sure, one could just measure serum insulin and ketone levels. Coffee will raise blood sugar, but it’s also lipolytic, so it’s both „bad“ and beneficial.
Nitpicking perhaps, but there's definitely measurable food energy in black coffee. A small mug of paper-filtered might be <1 kcal, but a double espresso might be 5+.
Most people debating things like coffee, tea, lemon, even fish oil (which definitely has calories) have lost the forest for the trees, and they often can’t explain why they are fasting in the first place or their goals.