Fasting with just water is a bit tough, as you can get a metallic taste in your mouth that is rather disagreeable. So that's my 'fix' for that problem.
In the absence of metabolic disease, the body breaks down fat to use the resulting energy. Think about it evolutionarily; "there's no food right now, time to jettison my energy stores"?
It is mostly unrelated to diabetes as that relates to insulin response.
Also, how do you keep sufficient energy to perform daily tasks and things like intense programming sessions?
But as I mentioned, the fruit juice input in the morning does help get the day started. Oh, and the coffee contains some nutrients too, so it's not like I'm totally starving.
I personally don't mind taking fruit juice, as it does give a small energy boost, especially in the morning.
If you really goose your ketogenic metabolism, your body gets better at using ketones and the taste will go away. You should also be sure to eat/drink extra salt during a fast or ketogenic diet due to water loss.