This is something I don't quite get about Jason Fung. He is critical of calorie restriction being less effective (he says you will feel more tired). But I'd like to see strong arguments and evidence for that.
He sites several studies including the Minnesota Starvation Experiment[0]. It's pretty simple, if the body gets some calories but not enough, your metabolism is lowered and you have less energy along with a host of other symptoms. This also explains why this is a terrible dieting strategy, once your metabolism is lowered you regain the weight quite fast once you start eating more.
Oh I don't deny I felt like shit and every moment of it sucked. But like most things worth achieving you need to put something resembling effort into it. The results were definitely worth it.
It is the same thing if he his insulin homeostasis is out of whack. It's a constant struggle in that case. That's why most people who lose weight in the short term gain it right back when they "maintain".
A thin dude at work pretty much only eats junk food and washes it down with regular coca cola, of which he drinks 3-4 cans a day. Every single day. Doesn't exercise much either. And he doesn't gain weight. For me, god forbid I eat a single donut a week, I inflate like a balloon.
[0]https://en.wikipedia.org/wiki/Minnesota_Starvation_Experimen...