I've dabbled with fasting in the past - but I'm getting into it more seriously now.
I recently read The Obesity Code by Dr. Jason Fung. He makes a compelling case that the old advice "Eat less, Exercise more" is ineffective. And this is backed up by studies showing almost nobody keeps off weight with that approach.
If you don't feel like reading an entire book (although it's great!), then check out this video... hits the key points.
The book is mostly not about fasting - he actually doesn't bring it up until the very end. Instead it explores the advice we've generally been given, and the science of why we are fat.
His claim is that the root of the problem is hormonal, and primarily insulin.
Boiling it down: we are fat because of what we eat and when.
The What: Sugar and refined carbs. These create elevated insulin responses that ultimately lead to reduced insulin sensitivity, which result in more insulin, which makes us fat. He also takes aim at artificial sweeteners, which provoke an insulin response despite having no sugar in them (!).
The When: Snacking, snacking, snacking. We eat too much, and the advice to "graze" makes it even worse. Our bodies are flush with insulin, always, which just makes the body numb to it... which again leads to even more insulin, which makes us fat.
This is where fasting comes in. There are various approaches, but in all cases the idea is to reduce insulin for long periods of time by not introducing new glucose into the bloodstream, thus no insulin response. This in turn has been shown in studies to increase insulin sensitivity, which he argues is the cure for obesity.
My Weight, and How I Will Apply It
I was an outlier in 2014... and had a bit of swagger about it. I had lost weight and held it off for 10+ years. I did it through calorie reduction and exercise. And it was bloody hard.
I'm not anymore. I weigh 212lbs, vs the 235lbs I weighed at my peak - but I'm almost 30lbs above the 185lbs that I need to be "Normal" rather than "Overweight", and a good 40lbs above where I'd like to be.
Fasting - in the past I had done 16:8 - basically not eat except between noon and 8pm.
But I think the 24h/36h fasts he suggests sound like they may be more effective. Fast for 24h 2-3 times a week, rinse, repeat.
Dr. Fung is very clear that exercise is good for you - it has a lot of hormonal effects, including increased insulin sensitivity and reduced Cortisol (stress hormone).
However he is very down on the role it plays in weight loss. Burning calories via exercise is not the way you will lose weight.
He mentions exercising while fasted is a "good time" - but it sounds like he's mostly talking about gym rats (and he even references the Lean Gains guy).
... so where does endurance exercise fit in?
I found this very excellent chart.
At low intensity, the body burns mostly fat (which I have a lot of!), so it's possible to go for a long time without using up the body's glycogen or consuming additional sugar.
At high intensity, the body burns mostly carbs, so it either needs to be short or you can't be fasted.
I've done runs as long as 16km while fasted for 16h before - they were slow and pretty horrible. I don't know if they get better, I think it's just something I'll have to experiment with.
One more beautiful chart, from the same source...
... so as you can see, the probability of hitting "the wall" (running out of carbs) is nearly zero when the "running intensity" is under 40% of VO2 max...
This is probably down to one's personal fitness/genetics/weight/etc - but I this all points at it being possible to run long slow runs, fasted, with nothing but water and electrolytes.
My goal right now is to lose weight, not to run a marathon - so I'm not so worried about this. If I can run long I'll run long. If I can't I'll put that on the back-burner until I can.
It will be an interesting journey, if nothing else!