I started outlining and writing a piece about the biochemistry behind it, but (1) it took me down a way deep rabbit hole, and (2) I'm not very clear that I don't understand the topic well enough to write about it.
That said, when doing a basic intermittent fasting protocol a la Leangains.com, it's been relatively easy to consistently march down towards lower bodyfat and greater health and wellness.
I ran a caloric deficit of between 500 and 1000 calories each day, eat all those calories spaced into a 4-6 hour window.
That was pretty good, and remains pretty good.
I've recently (six weeks ago) started experimenting with fasting 24+ hours, sometimes to the 60 to 70 hour range once a week.
The benefits are absolutely tremendous in terms of mental clarity. From my research (note: please do your own research, carefully, and don't trust people on the internet with your health -- it's precious) -- from my research, it seems like under 70 hours of fasting combined with good hydration and generally eating a healthy mix of food when eating is fine. As far as I can tell, the "starvation mode" thing about the metabolism is largely a myth, so long as you're not running too deep of a deficit at a single time.
I was going to try to analyze what's going on chemically, with noradrenaline and dopamine and what not, but it's way over my head. But suffice to say, I'm a huge fan of fasting. The easiest way to start would be doing intermittent fasting where you still eat a normal day's worth of calories for yourself, but in a condensed window. After you're comfortable with that, you can stretch the fasting window out longer.
Leangains has some good guidelines and science to start learning. I'm a fan.