Earlier quoted context omitted.
Here’s a piece of COMPLETELY anecdotal data, so do with it what you will. A friend with metabolic syndrome was struggling with weight despite intermittent fasting. She began to wear a continuous glucose monitor. She found that black coffee had the same effect as eating a donut, but if she added a boiled egg, the effect on her glucose was neutral. Now, I don’t know what effect all that has on other metabolic processes…
There’s no glucose in black coffee though. I’m not sure how that’s possible. Wouldn’t the glucose have to come from somewhere?
A continuous glucose monitor checks the level of glucose in your blood. Glucose can be stored in your body as glycogen (e.g. in the liver, or muscle tissue).
Your pancreas produces glucagon, which influences the process of converting it back into glucose, released into your blood.
That's generally accepted basic science, but from my own anecdotal friends/family experience, the pancreas also seems like a pretty buggy organ generally, prone to glitches both temporary (like perhaps in this coffee drinker) and chronic (type 1 diabetes), leading to undefined behavior, crashes, and even fatally bricking the system permanently.
So this anecdote isn't particularly surprising to me.
(Also, for those reasons, I would definitely wear a continuous glucose monitor myself if it wasn't so expensive and difficult to arrange when you don't actually have anything known to be wrong with your blood sugar levels. I predict that within a decade or two, they will be a fairly ubiquitous piece of human body instrumentation, like the sleep-tracking wristwatches and rings we have today.)