On intermittent fasting. Can stevia in my coffee break my fast?

Well if you don't consider coffee to be breaking your fast, I can't see how adding stevia makes any difference.

No it doesn't.

If a substance doesn't create a change in any metabolic processes like black coffee, water, unsweetened tea or herbal tea's which doesn't involves enough calories like in milk, cream or sugar that may elicit an insulin response, then your good to go .

IF doesn't necessarily facilitate weight loss unless you actually consumed less calories than you normally would from shortening the window for food consumption and most people either don't know or don't care why there might be other benefits. Here are a few.

A few factors involved with IF and probably the main goal and focus is to increase insulin sensitivity which helps regulate blood sugar levels.

IF also reduces inflammation by recruiting fewer monocytes that are a type of white blood cell that can cause inflammation and with IF the monocytes that remain are also less inflammatory and that increase in insulin sensitivity previously mentioned also reduces inflammation.

IF also triggers autophagy, a process where cells remove damaged components and recycles them and helps reduce cellular stress and also inflammation. Oxidative stress is also reduced which can increase the production of antioxidants that help reduce oxidative stress, which guess what, reduces inflammation.

IF promotes the production of brain derived neurotrophic factor (BDNF), which supports brain health and cognitive function and it also supports the growth of new neurons and synapses, and fasting can enhance this process. Also through BDNF which is basically activated by ketones during the night it also increases human growth hormones (HGH) which increases protein synthesis and helps breakdown fat through this whole process of IF which is why it helps increase protein synthesis, basically it helps protect the muscle we have.

Of course the smaller the window the more engaged all these processes become and a minimum of a 12:12 window is the basic starting point and many people are in this window without even knowing, but the effects are quite minimal and suspect a few are almost non existent. The scientists that work with IF feel closer to the 18:6 window the better.

For the general population most of this really doesn't matter and IF's popularity is because it helps people lose weight and if it's working for you then that should tell you that eating right after you get up until you go to bed might have something to do with regulating weight loss.