Jan, in comments, wrote:
I've been doing intermittent fasting of 15-18hrs most days a week (for about month or so), but I can't seem to get past that 18hr mark. So 24-30 seems unimaginable!
Well, I was going to respond in comments but figured this could be of help to others who may not see it, so here goes.
My guess with regard to getting past the 18hr mark is that, as it turns out, fat burning during a fast — though there's some prior — really kicks in earnestly at around 18 hours and increases more and more up to about 30 hours. This is why I do 30 hour fasts — in order to take maximum advantage of fat burn. At about 30 hours, your body begins to increasingly adapt, and fat burning tapers off to a steady state rather than increasing rate. What you seem to be indicating is that your body is really complaining about getting into a full-on fat burn. It's not used to it, and it's no surprise. Depending on your age, you've likely been feeding regularly and continuously for decades, day in and day out. Your body isn't even aware that it can go months without anything but water. In a sense, it's not much different than any other kind of addiction — and we all have them. But don't be your body's fool — or anyone's, for that matter. You know better, and it's just like getting off caffein, nicotine, alcohol, assorted drugs — prescription or otherwise — that your body has become comfortably accustomed to. Your body is going to put up a big fuss when denied.
If you want to get really technical, a 30 hour fast is really about a complete 24-hr fast, which is to say that the fast really begins about 6 hours after your last meal — at about the point you would have your next meal — 'cause you're hungry and ready to eat again. You can only be fed or fasted:
Think of it this way: everything begins with hunger. An animal — any animal, including the human kind — can only exist in two states with respect to food: fed or fasted. From the point we’re hungry to the point when we eat, we’re in a fasted state. From the point we eat until we’re hungry again, we’re in a fed state.
In any case, I don't advocate the daily fasts lots of people do because I consider it chronic fasting, not intermittent or flexible. Now, the fact that I do two per week in a rather programmed and organized way is also less than total intermittency — but I'm doing it to lose fat. Another 15 pounds and I'll be around my goal of 10% BF and my fasting will become once weekly and will be totally intermittent, totally flexible, i.e., sometimes 24 hours, sometimes 36. …Maybe even a 48 every now and then — and perhaps a 3-4 day once or twice per year. I'm trying to model an evolutionary, primitive existence, as that is what programmed our genes.
All that said, here's how I've found to make a 30-36 hour fast as easy as I think it can be: have your last meal mid-afternoon, like around 2pm — or maybe 3-4pm initially — and go until dinner the next day. The reason is that if you have a good breakfast, then a good lunch at 2pm, it's not a big deal to go to 10 or 11 pm without food. You may be a bit hungry, but it ought not be gnawing and super unpleasant. Then, you go to bed, sleep like a baby, get up 6 or 7ish, your body has prepped you for fasting during sleep (hGH release), and you're already at about 18 hours. Then, just keep yourself busy until dinner around 7pm or later, depending on how long you wish to extend it. When you get really hungry, get really busy, or, depending on your personal circumstances, take a nap. If you can, get in a workout early afternoon, around the 22-24 hour mark. If you get your intensity up (lotsa weight, little to no rest between sets), it will kill your hunger for a few hours, though the first 5-10 minutes can be a bit daunting. What I've found, however, is that the hunger that returns in the later afternoon is of a very pleasant and warm kind. That signals that you've broken through and your body is now adapted to the fast pretty well and is easily releasing enough fat to keep your energy levels up and hunger at bay. Take a 3-5 minute dip in 50 degree water in your bathtub for an even more pleasant afternoon leading up to dinner. I guarantee that you will be amazed at how your body responds by kicking up the fat burning, thereby keeping you warm, cozy, and ferociously content. You will begin to gain insights into your long ancestral past and how wondrous it must have been to exist completely by one's wits. It's deeply contemplative at a point. That's the source of my "spirituality," if I've got any. And it's the real thing.
I don't claim this will be easy the first, second, or even third time; but just like anything else, you get good at it and it becomes a familiar path. Then, enjoy. And every once in a while, throw your body a curve and even extend the fast to breakfast the next day. It ain't gonna kill ya. We've got millions of years of evolutionary adaptation that says your body is built for survival in very sparse and inhospitable conditions. Fasting is the path to expressing those long dormant genes, and you will not believe how it will Free the Animal in you — and by this I mean a languid, content, focussed Animal — one completely free of rage, frustration, and other unhealthy toxins.