Had a friend in university, he did military service for 1 year and the only thing he had for breakfast during that time was a strong black coffee without any sugar nor milk.
He screwed up his digestive tract for good - there is apparently something in coffee that is quite aggressive on stomach and gets neutralized when mixed with any sugar/milk/food. He will never be able to eat vast array of common foods - things like marmelades and other basic food. Sorry I don't have any further info, has been few years ago.
He might have some unique predisposition for this, don't know, but better be careful and put at least some milk in it. It makes it much more filling anyway
For another anecdote, I have been drinking black coffee, and only black coffee, for the first four to six hours of the day. Sometimes water as well.
I have done this for about six years without damaging my digestion in any detectable way. I would bet money your friend has or had a Helicobacter pylori infection; treatment is antibiotics.
That definitely sounds like some predisposition.
The whole idea around intermittent fasting revolves around eating in a specific time window, outside of that you don't consume any calories at all.
Many people I know of do this, in order to lose some weight or gain muscles without adding too much fat, never heard of this problem.
Coffee can be quite acidic which with people with overactive stomachs can cause issues such as acid reflex, ulcer etc.
Instant coffee tends not to be as acidic same goes for cold brews and there are other ways to neutralize the acidity.
If you produce too much acid then intermediate fasting is not a good idea to begin with unless you also take supplements to control your stomach acidity.
He screwed up his digestive tract for good - there is apparently something in coffee that is quite aggressive on stomach and gets neutralized when mixed with any sugar/milk/food. He will never be able to eat vast array of common foods - things like marmelades and other basic food. Sorry I don't have any further info, has been few years ago.
He might have some unique predisposition for this, don't know, but better be careful and put at least some milk in it. It makes it much more filling anyway