Black Coffee vs zero cal flavored water (LaCroix, etc.), is only Coffee safe while fasting?
So my wife has been IF for something like a year now, and seen great results. Over the past 2 and a half years I've been dieting through other methods (mainly just reduced calorie intake and eating less processed stuff) but she lost more, faster, and seemingly easier, in one year then I did in two and a half (a little over 50 lbs for me but that only put me about half way to a healthy weight for my height), so she finally convinced me to join her in intermittent fasting.
I've let her largely dictate everything for my eating situation, however, one of the rules was surprising to me, so I just thought I'd check in if this is backed by science or not, or just something that worked for her. She told me I can't have my usual flavored fizzy water (we have a carbonation machine and 0 cal flavor essences I use to make 0 cal drinks for myself) during the fasting time, only during my eating window, but I can have Black Coffee.
She's a Coffee drinker, so that worked out well for her, but I've never really cared for Coffee, I'm more of a tea drinker. Unfortunately, while she told me I could have unflavored zero sugar/milk tea, I've found I can't have tea on an empty stomach or it upsets said stomach something fierce. Although lacking the caffeine of tea or Coffee, I had thought maybe I could use fizzy flavored drinks to fill in the gaps during the fasting periods when I want a different taste then just pure water, but again, per her, those taste too much like food, or at least things I would associate with being sweet, and thus would make this a dirty fast if I drank them because even if in small amounts, they might spike insulin, or at the least, make me feel hungry, making fasting more difficult.
But is Coffee actually better? I feel like its got a very distinct flavor itself. While its not my favorite flavor, I could say the same about plenty of vegetables whose flavor, despite not being sweet, I would also associate with eating and which carry some amount of calories. I guess my concern is that Coffee is impacting me just as much, however minimally, as a flavored fizzy water would. And if that's the case, I'd rather just drink the flavored water. But at the same time, clearly this only water and coffee method worked for her, so I don't want to move outside the proven box and unintentionally screw up my own fast efforts.
TLDR: Are there studies comparing black coffee and other 0 calorie flavored drinks, like fizzy drinks, that show black coffee is actually better during fasting?