If you want to be totally sure then take your fasting level......drink the diet Coke......test at 1, 2 and maybe 3 hr intervals. That will give you a fairly accurate reading. However, if you fast, and the level is low......then you have no food other than a Diet Coke....you could well 'liver dump' and your Bg levels will rise anyway ! Hence that method is not exactly a good thing to do. :wink:
Personally I drink as much Diet Coke/Pepsi as I want and it has no effect whatsoever on my Bg levels. I have never tested as I mentioned above.....I think that is taking things to the extreme.
I have a meal and a diet Coke then test two hours later, if the levels are OK then the meal and the Coke were fine. I certainly don't test every single item of food or drink individually.
Ken