I think it's highly likely that Autophagy is the most important effect of fasting. Here is a good article that explains the metabolic process if you're interested. In short, when you don't eat, your body cleans house and recycles junk in your cells like protein aggregates and damaged mitochondria. In the ancestral environment, people didn't eat ever waking hour so this process would presumably run pretty frequently. But now that we have food at our disposal all the time, it doesn't happen as much unless we make deliberate dietary interventions like fasting. Autophagy also occurs during chronic calorie restriction, which may account for its observed effects on longevity in mice. However, if we can get the benefit from a type of intermittent fasting without chronic CR, then that would be a net win because CR has many known harmful side effects such as lethargy and impaired thinking.

Personally, I do the LeanGains 16/8 fast/eat program every day. My feeding window is 12:30pm to 8:30pm and I don't eat at all outside that window. Within the window, I eat as much generally healthy food as I want. It feels great once you're used to it after a week or two (hormonal entrainment of meal patterns and all that), allows you to not worry about breakfast, and (hopefully) allows your body to clean up the intracellular junk better than if you don't do it. It can be sustained indefinitely without much effort and doesn't get in the way of anything else I want to do with my life. Just take the 80/20 or 90/10 rule to it and don't worry about it if you need to "break" it every once in a while. If you have company come visit and they want to have an early breakfast or a late dinner, then just do it and don't worry about it.

In my opinion, it's by far the most sustainable and least disruptive to lifestyle of the fasting protocols and I think that frequent mini-fasts like this are probably healthier and more effective than infrequent large/intense fasts. I encourage you to look into it.

Edit: I agree with others here. Fruit juice is liquid candy. Just eat whole fruit instead, primarily berries if possible as they have the lowest sugar(fuctose) to mass ratio and are packed with anthocyanins.

I have been doing Bulletproof Intermittent Fasting for over a year. For six days a week I try to limit my consumption of carbohydrates and protein to a six hour window. I do this in part by having in the morning coffee that contains MCT oil and butter from grass fed cows. My hope is that this is increasing my lifespan by significantly increasing autophagy. I am far from an expert on biology or nutrition, but from what I have read autophagy seems fantastically beneficial.

One immediate benefit I think I'm getting is that my colds are much more mild. I used to get colds that lasted 6-10 days and made me miserable. I could often tell I was getting a cold because my wife or son would usually get one first and then I would get a very dry throat. Since doing intermittent fasting my colds only last a few days and just make me feel a bit worse. I take this as a sign that I'm undergoing increased autophagy.

I twice did 24 hour fasts before I did intermittent fasting and both times I got a cold at the end of the fast. From what I have read this might have been because when you undergo a really long fast your body shuts down autophagy when the fast concludes.

I am keto-adapted, but the transition was difficult in part because I didn't know at the time that there might be a transition. Look up Atkins Flu to learn more about the transition.

Fruit Juice = Liquid Candy. Avoid!

My non-professional opinion is that much of the benefits of fasting come from what I call "running cool," i.e. eating so that you are at the bottom of your metabolic range. My thinking is that when you eat more than the minimum you need (but not so much more that you gain weight), the body adjusts by increasing its metabolic rate which puts more wear and tear on things and also leaves more energy free for mischief.

So that instead of fasting you might consider moderate calorie restriction -- you could call it micro-fasting between meals.

I'm also unclear how to measure and track the effect of this diet.

Yes, that's a real conundrum. Probably it can take years or even decades for the effects of one's diet to manifest themselves.

One idea I had is to regularly measure the uppermost frequency you find audible. Since this is know to decrease with age, perhaps it can be used to measure the rate at which you are aging. On the other hand, if you test yourself regularly in this way, possibly you will get better at perceiving high frequencies which would throw off your results.

Also, it might lead you into a lifestyle which is great for your hearing but otherwise counter-productive or even destructive. I believe Seth Roberts used to regularly test his ability to quickly do mental math and discovered great improvements from eating half a stick of butter a day. Until one day he had a heart attack and die.

The general problem of fasting and extremely reduced intakes is that the metabolism slows down. Also you first lose a lot of muscles, then a bit of fat - when going on eating normal you burn less than before (because of less muscles) and therefore gain more fat. So for your body's composition I would say: Don't do it. (There is this great German book called "Die Physik des Abnehmens" = "the physics of weight loss" in which a physicist explains all this things very nicely)

However: I personally tried fasting for the mental effect, I was able to concentrate even better than normal, I liked the feeling of being "independent" from food and of being able to get through all of this things. (But I will not do it again because I try to get less body fat :) )