First, the anecdotal evidence. I'm a guy in my late thirties, with kids, without a bunch of free time. I'm a former national-class sprinter, so I suppose I've got genetics working for me, but age and lack of sleep were starting to take their toll. More urgently, I had relentless hunger all the time—not a problem back in the day when I was training hard 2-4 hours a day, but a serious issue now that I'm older (with a slower metabolism), have had knee surgeries (so can't run as hard anymore), and just don't have the time. So, long story short, I gained about 35 kilos over a few years. Some of it good weight, most of it not.
I lost the weight by going strictly low-carb for about 6 months. That worked, but I was hungry, irritable, and listless all the time; I thought about jumping off that particular wagon every single day. But I had bought a bunch of expensive clothes in my new, smaller sizes, so that wasn't an option. Lol (but srsly, I need bigger closets, folks).
So, like Milarepa of Tibetan lore, I went on a spiritual quest for (dietary) enlightenment. Tried a whole bunch of solutions, all of which left me either (a) hungry and listless, (b) gaining weight back, or (c) without energy for workouts.
Enter a trip where I was stuck in a delayed flight, and then an airport, for a total of over 24 hours. Without money in the local currency, and unwilling to venture outside (and have to return through int'l security) for sustenance. In other words, involuntary IF.
Well, hell, wouldn't you know, I wasn't hungry. At least not in the urgent, holy-crap, I-would-eat-that-plastic-table-right-about-now sense of "hungry" that had characterized all my previous attempts at dieting. It was like a revelation.
Sure I was somewhat hungry, but in a dull, gently urging way, and not in a proto-diabetic FEED ME NOW, MORTAL kind of way.
I had never felt that way before. Ever since retiring from track & field, I had spent eight years oscillating between "starving" and merely "really hungry", trying to stop eating when I reached the latter. The one time in those whole eight years when I truly felt "full", I had consumed about 6,000 calories in one hour.
And, to top it off, I had energy. This didn't make sense at first to my rational brain; I wasn't yet aware that the 12th to 20th hours of fasting provoke an increased adrenocorticoid response (= "adrenaline rush"). I wasn't passing out on the airport floor; I put together an entire thirty-page prospectus while I waited and waited and waited. A level of focus and energy I'd rarely felt outside of the "manic" stage (I'm bipolar).
So yeah, that's my experience.
I immediately adopted "IF" (though I didn't think of it that way) as a lifestyle thing.
On workout days, wake up, fast for 8-9 hours, eat a small amount of sugary cereal + BCAA, go work out like a beast, come home, f*** the all-too-sexually-voracious wife, then eat a satisfyingly huge meal. And pass out within 1 or 2 hours of that meal (I can't sleep more than 2 hours on an empty stomach).
On non-workout days, wake up, fast for 12-14 hours, eat all my food within 4 hours, and then pass out within 1-2 hours of that meal.
Since adopting that plan (with regular cheating) I have regressed to appx 7% body fat, and a hard-to-believe 13-inch drop from chest to waist (42" --> 29", good thing I can tailor my own clothes). My lipids and fasted blood sugar are better. I get to eat meals where I'm reasonably satisfied at the end, rather than perpetually being forced to eat what feels like 1/2 or 1/3 of a meal every time I eat. I'm not as much of a raving a**hole. The hunger doesn't gnaw at my heart and soul. And I've gained substantial strength from eating so many calories post-workout, despite averaging only about 4.5 to 5.5 hours of sleep a day.
There have literally been no downsides. Well, ok, occasionally it was awkward to go to dinner with friends or colleagues and have water or Diet Pepsi for dinner. But that's only weird if you decide it's weird; if you don't, it isn't.
Confounding variables: (I.e., other things that could be at least part of the actual cause here, besides the fasting thing)
* I plan meals more carefully now.
* I don't eat as much convenience food, because I cook huge amounts of "macro" food and then eat it over a few days as leftovers.
* Most importantly, I don't normally eat until after midnight, which means I eat essentially NO restaurant food anymore. That's probably a big factor, what with all the soy oil and sugar that restaurants put into EVERYTHING. (Also, I have more money with which to buy myself clothes.)
Oh, and my thyroid pills are more effective now, too, because I have a 18- to 20-hour fasting window for them to have full effect, rather than just a couple of hours like most people.
Amazing, folks.
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As for the evidence, OP, you are wrong about what you characterize as the paucity of that evidence.
The recent popularity of IF is, of course, a new thing. But IF is hardly some johnny-come-lately idea. Fasting diets of all kinds are as old as the hills.
Because of their connection to various religions, fasting has a much longer and more storied history than, well, any other type of diet plan imaginable. Probably more than ALL other diet plans, combined.
You can find studies on "IF" that go back decades and decades. I can't post links (I guess I need a longer posting history to do that), but one particularly fruitful place to look is studies of Muslims who fast for Ramadan—which, particularly if Ramadan falls during summer months, is almost an exact mirror of the 16/8 IF concept.
Because entire countries of people engage in that sort of "IF" together, there are thousands of such studies. Go look, you'll find 'em.
The only complicating factor is that the Ramadan fast also includes abstention from water, while most IF proponents advise an increase in water consumption during the fast (primarily to prevent constipation later, but also to mitigate the "empty stomach" feeling).
There are also loads and loads of "IF" studies that concern ordinary Americans. Poke around the nih.gov database and you'll find those, too.
