My two cents for this sort of stuff when it comes up, folks can get a little too wrapped up in the exact physiological aspects when it’s the intent and goals of their fasting that ultimately matters. There’s no big switches in the body that go ‘well you’ve eaten exactly 51 calories of Skittles, so now no more fat burning’. It’s more dials and preferences.
If your goal is weight loss and that oat milk is keeping you from otherwise eating until your next meal, then there’s an argument for it. Hell, if it’s just an enjoyable aspect of your life and part of just overall sustainability that’s also true. Otherwise, it is sort of empty calories and you may want to think about cutting it.
More intent and goals, less finding the physiological edge cases.
If you would continue with fasting if you are drinking oat milk, then it is something to hang on to. But, technically, any food ingested breaks a fast. Your digestive enzymes and insulin are involved in the process. How much of involvement, will it affect me etc are all subjective to your health (insulin resistance), starting weight, exercise level etc. my personal suggestion is to do a clean fast (even cutting out black coffee) if possible. I also suggest that you keep it a shorter fast if 16 hours is too long to go without oat milk, initially. I fasted such that I was asleep for the last 6 hours of my fast to control my coffee cravings. I hope this helps.
What I’ve tried and works for me is alternating between a clean fast and “dirty” fast throughout the week. Sometimes I add a little keto sugar free creamer to my coffee. The other days I have plain Yerba Mate or black tea for that morning caffeine kick. Just find what works for you 😄
I stick with black coffee but after the first month of 16:8 I relaxed a bit and sometimes end my fast 30 minutes or so early. Some days I end up with a 18:6 fast. It took me a little bit to figure out that aiming for perfection isn’t really attainable. I’m down 6 Kgs in 4ish months with literally no change in diet other than the time I eat and little to no exercise. (I quit going to a gym because of Covid, and it’s too cold in the winter where I live to go jogging most days)