I have a hard time believing that drinking water or black coffee will set you back in any meaningful way. Maybe it disrupts autophagy for a short time, but what are we talking about, 15 minutes?
I mean, you have to drink something, so water intake is the most natural thing in the world, right?
I would keep your water intake up at a couple of litres per day or whatever is recommended for your weight index and not worry about it.
My 2 cents.
Due to a health issue and recent non-response to a treatment that should have worked, I have made it a mission to ascertain as much as possible to give my body an edge in achieving optimal health.
I am currently a huge fan of various fasting protocols and I am now doing 19/5 or 20/4 IF.
Based on my research, water with the big 3 electrolytes is a must.
Black coffee and tea are fine - as long as there are ZERO sugar, almond milk, etc. If it has a calorie - avoid it.
And forget about artificial sweeteners. Even though they have no calories they are sweet to the taste. This can elicit cephalic insulin response. Your body, as a response to the sweet taste, releases insulin which probably does inhibit autophagy.
As a notoriously wet part of the human body I’d be surprised if it was good for your eyes to be dehydrated in any way.
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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7551870/ This paper describes several benefits of intermittent fasting on eye health markers in mouse models, and if you follow through each of those references in the eye section you can see that the mice are always given 24/7 unrestricted access to water i.e. this is about water fasting not dry fasting.
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Stay hydrated.