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I believe that autophagy and ketosis are dials, not switches, and that a few calories from cream won’t throw you out of them.
That said, I’d ask yourself why you need the cream and if you can live without it—less so for physiological reasons and more so for psychological ones. Part of fasting, for me, is getting comfortable with being uncomfortable.
Dr. Fung published an article that stated heavy whipping cream (in small quantities) had no impact on glycemic index. Anything with sugar or sweetener most definitely will. Additionally, this spike will cause you to be hungry and most definitely interfere will your ability to abstain. I agree with the others here, EXCEPT to say, if having coffee the way you love it will encourage you to remain fasted for the rest of the day, go for it. If you can do this and lose weight, why not? Sometimes total deprivation throws people into a bad mindset. Everyone is different.