> activate some digistive processes, and make the liver work. So you are not exactly fasting.

There's a difference between working a bit and a lot.

Go up in the night, go some steps to the toilet. -- Or go out running 10k. Then back to sleep. Repeat reach night.

Coffee or a big meal.

You're still fasting because it doesn't affect your blood sugar. That's why you can drink black coffee, tea, or water when fasting for surgery.

I heard this, but I can't find any study measuring the differences seriously.

Fasting has various effect, not all of them are linked to the sugar consumption.

What I've read in this area all comes back to "whether or not black coffee breaks the fast depends on the goal of your fasting", i.e., weight loss vs something more specific.

[Likewise for other things people don't always think about like vitamin / supplements.]

There is more info in [1][2] but tl;dr:

- Fasting for metabolic health/weight loss: likely does not break a fast

- Fasting for gut rest: does break a fast

- Fasting for longevity: likely does not break a fast

[1]: https://www.zerofasting.com/does-coffee-break-your-fast/

[2]: https://www.dietdoctor.com/can-use-cream-coffee-fasting