"I tried Atkins for half a day, but it made me feel bad so i stopped."
The medical shyster who said the above (approximately) has got to be the poster-child for stoopid when it comes to diet. One can't realistically try ANYTHING for just a few hours and expect to succeed.
ESPECIALLY when it comes to diet.
No matter what the change, if it's radical enough, the body HAS to make adjustments like upregulating enzyme and hormone production/use, specific to the macronutrient content of the new regimen. Such a process takes some time. To expect to feel good immediately on the new food ... well, just shows how nutritionally-ignorant that self-promoting, egomaniacal Dr. Ass is.
And this takes us back to those people who think they "CAN'T fast".... Have they ever actually tried? Have they read up on how to do it? More importantly, have they learned how to break their fasts? Or have they listened to people who have FAILED at fasting and been discouraged? We all know how people who have failed at anything enjoy it when we fail too -- it bolsters their damaged egos.
Learning how to follow any regimen properly is central to being able to do it successfully. I'm sure all of us have heard of people who tentatively began low-carbing, but who were still afraid to add natural fats; the double restriction results in ASSURED failure! Or those who called their eating style "low carb" but who were inhaling fruit and reduced-carb baked goods in astonishing quantities.... When we don't do our due diligence, our chances of getting the results we desire are infinitesimal.
When i first went on the Strong Medicine regimen a couple of years ago, even though I followed instructions to the letter, i had to tweak and practice patience before I got to a point where it operated smoothly. Even though the doctor who presented the diet had treated countless patients with the plan and perfected details as much as he could, individualization is essential. So it is also with fasting.
You have to juggle liquid intake and mineral supplementation. In opposition to Conventional Wisdom, i firmly believe that sodium/salt is GOOD for you! Sometimes potassium supplementation is the right answer, as is magnesium, zinc, etc etc etc.... If you're doing an "all meat" diet, you must not dilute your stomach acid -- so minimal beverages during meals is the way to go, and extra betaine-HCl might be necessary. Making sure your meats are very fresh to minimize histamine issues helps, too. If you're completely fasting, liquid is your friend, but it's still possible to drink so much that your stomach feels funky. A LITTLE coffee/tea is very valuable as an agent of lipolysis but a lot can cause trouble.
It's well known that a low-carber needs to plan to increase carbohydrates for a couple of days before taking a glucose-tolerance test. Same principle. If you get a huge blood-sugar spike out of the blue when your body isn't used to it, you'll feel like death-warmed-over while your organs are scrambling to cope with such an unaccustomed load.
The upshot is, you should allow your body plenty of time to get used to any new regimen. If you've been a glucose-burner for decades, you can't swap over to burning fat/ketones immediately. "Atkins flu" IS the period of adjustment, and "keto-adaptation" does NOT happen overnight. Two or three days are necessary to go the other direction, too.
The time between starting a new dietary style and beginning to feel really good on it is like driving along on a bright sunny day and suddenly entering a tunnel. You're going to feel uncomfortably out of your element for a little while -- your body is scrambling to adjust to the abrupt change, and it won't be efficient. You'll feel systemically drained until your fat-digesting and -mobilizing enzymes reach optimal function, or alternatively you'll feel wired then starved till your sugar-using mechanism is hitting on all eight. In either case, it requires a period of feeling gawd-awful before you settle into smooth operation. If we're prepared for it, the experience is less alarming.
I figure that people who don't properly learn how to follow a special dietary regimen, who do it half-assedly and then proclaim that "it doesn't WERRRRK" (like Dr. Ass) really didn't want to do it in the first place -- why they wasted their time (and ours) is all about their prejudices and intellectual laziness, nothing more -- we can't help but suspect some of them of having ulterior motives.