After 3 days, I could easily do a week, and I managed to do a 30 day fast one year.
I don't recall feeling high, I just remember a lot of laying down, dreaming and thinking about food, baking, and not doing much of anything else.
I really rather prefer being able to eat everything I want, and spend 2-3 hours a week at the gym. But my weight has gone up by 10-15 lbs (and I have also aged from 17 years old to 29).
I can't say I'm the same size as when I was fasting, but I have a defined shape, whereas before I was bone and skinny fat.
I dropped from 140 lbs to 110 lbs in a month. I'm 5'4 female, and a 'skinny' weight for me is about 130 lbs as I am fairly muscular.
I used to want to look like a skeleton, I had no medical supervision and this was probably one of my worst eating disorder periods, the other being at college where I ate a microwaved apple or orange and 2 cups of broccoli / carrots only every day, for most of my freshman/sophmore year.
When I was ill, I knew I what I was doing with a disorder in mind, but I didn't admit it to anyone outside of me. I think most people who have an eating disorder know they have it inside, it's just something kept carefully secret.
I've been in recovery for about 2 years now, with healthy thoughts and no food restriction, excess exercising, starvation or purging. When I was sick I really didn't care if I died, as I have major depressive disorder comorbid, which I continue to have more difficulty beating than I did the eating disorder. I've fainted from fasting and I've been hospitalized from starvation. It's hell, pretty much.
I think it's important to take a step back from whatever the goal of these diets are, and look at the bigger picture. My weight only varied between 30 lbs over 15 or so years, and it controlled me entirely. It made me have to put a lot of my life on hold, when I had to focus what little energy I had towards getting better. I would really recommend anyone take a more careful, therapeutic look at their life if the above sounds tempting, and they find themselves going into cycles of fasting and 'losing' control over how they manage their diet.
For example my wife can eat is a notch over 5 foot tall, and eat as much as I do, and does not put on weight, however, if she misses a meal, she gets wobbly and dizzy. So fasting is definitely not for her!
I have a pet theory that some people are just very good at extracting energy from food, and some people are very bad at it. Perhaps it's related to a difference in gut bacteria mix or something.
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.
By the second day, you are no longer hungry anyway, so, for me, a 3 day fast is next to ideal. But I can do rather easily 5 days.
I mostly start on the monday morning and finish on friday, or saturday if I feel brave and there's nothing too exciting in the fridge!
As to why do do it? Well I like cooking, and eating, and drinking, and I rather hate counting calories and such, I'm much more of a 'on/off' sort of guy, and I rather like a bit of a challenge. That also helps me control my weight, without having to spend stupid amount of time in the gym.
Also, there /is/ a 'high' to a fast, I feel like on the third and fourth day you feel particularly sharper; it then decline a bit and you can feel a bit woozy. But the high is pretty cool...