Type 1 diabetic here. My body doesn't produce insulin, so if my glucose level goes up after eating or drinking something, that increase in glucose is a (very) approximate measure of insulin response in someone without type 1 diabetes. Insulin is released/produced/injected in response to glucose level increase (and other signals in someone who doesn't have type 1 diabetes).

In my experience, black coffee is absolutely fine outside of an eating window. I fast for at least 18 hours a day, only drinking black coffee and water when fasting. There was no measurable difference in glucose on occasions when I drank black coffee, vs occasions without (all other things as equal as possible).

I‘d say it depends whether drinking coffee triggers insulin release for you, or other growth factors like mTOR. According to a quick search, coffee actually seems to decrease mTOR - that would be ideal for fasting. Milk OTOH is ideal for promoting mTOR (babies need to grow fast), so I‘d avoid milk during a fasting window.

Common understanding of these matters says no it isn’t. So long as it is black coffe with now sugar. However, when I fast, the coffe I drink can have an adverse effect on my stomach.

The common wisdom is usually that black coffee is ok, as is anything without or with barely any calories.

To me, anything beyond water breaks the fast.

That doesn't mean you can't have coffee or a cup of tea, but I wouldn't put milk in it because that's literally food, with many calories, macro and micronutrients, etc.

I think depends on why you fast. I've used https://www.zerofasting.com/will-it-break-my-fast/ and https://www.zerofasting.com/will-it-break-my-fast-part-ii/

Coffee has a tiny amount of calories, but I've always seen people say it's allowed if you're doing TRE. If you set up your 8 hour window well though, it's probably not a huge deal to skip coffee outside the feeding window, if you're worried?

I skip breakfast and lunch but drink coffee/tea.

It's not about being perfect, do what works for you in the long run, a cup of coffee will always be better than a donut / milk & cereals / a full english breakfast when it comes to calories and insulin response

Just to add to the "what to drink while fasting" list, some people see an insulin spike when drinking soft drinks with artificial sweeteners. They don't break the fast for me (I've been one-meal-a-day for years now), but YMMV.

The paper is skimpy on methods. I didn't see if they were in a supplement or something. As such it only defined the size of the eating window and its time of day.

Conventional wisdom is <50 cal won't break the fast. Black coffee contains almost zero calories.

Generally coffee and tea are acceptable while fasting, provided you’re not adding anything to them.

My problem is: caffeine makes me more hungry, so I need to avoid coffee as well. Tea is OK.

Milk spikes glucose levels, so I guess it. depends on how much milk you use.