If they want to talk about what is NOT sustainable long term, that would be calorie restriction. I would white-knuckle my way through constant hunger for weeks and lose... a pound.

I would run out of calories at 2 pm and if I could have managed it, I would have fasted until the next morning, but I could not, because I was so hungry.

I happen to be midweek with alternate day fasting (it's easy when I'm feeling well and keeping up with my niacin) and I'm not hungry. I test with some salt and tea with coconut oil if I do feel hungry... and then it goes away.