As you might imagine, a double blind study on something like this would be challenging (or maybe impossible) to do. But that said, the study of caloric restriction/I.F. is a well established line of scientific inquiry, and research on this topic has been done with many species of animals including humans. As far as I'm aware, nearly all of the results across all animal species (including humans) all point in essentially the same direction: caloric restriction/I.F. seems to have considerable and quite potent health effects/benefits.

If you want to see for yourself, the best thing I have seen that has summarized the current state of this research has been this short BBC doc about I.F. (link below). In the documentary, there are a number of well-respected scientists that talk about their research (and their very intriguing results). Many of their findings are, in my view, simply astounding.

But take a look and decide for yourself. It is compelling stuff.
Link to BBC doc:
https://vimeo.com/170735109

As you might imagine, a double blind study on something like this would be challenging (or maybe impossible) to do. But that said, the study of caloric restriction/I.F. is a well established line of scientific inquiry, and research on this topic has been done with many species of animals including humans. As far as I'm aware, nearly all of the results across all animal species (including humans) all point in essentially the same direction: caloric restriction/I.F. seems to have considerable and quite potent health effects/benefits.

If you want to see for yourself, the best thing I have seen that has summarized the current state of this research has been this short BBC doc about I.F. (link below). In the documentary, there are a number of well-respected scientists that talk about their research (and their very intriguing results). Many of their findings are, in my view, simply astounding.

But take a look and decide for yourself. It is compelling stuff.
Link to BBC doc:
https://vimeo.com/170735109

PURPOSE:
We investigated the effect of a chronic dietary periodization strategy on endurance performance in trained athletes.
METHODS:
Twenty-one triathletes (V˙O2max: 58.7 ± 5.7 mL·min(-1)·kg(-1)) were divided into two groups: a "sleep-low" (SL) (n = 11) and a control (CON) group (n = 10) consumed the same daily carbohydrate (CHO) intake (6 g·kg(-1)·d(-1)) but with different timing over the day to manipulate CHO availability before and after training sessions. The SL strategy consisted of a 3-wk training-diet intervention comprising three blocks of diet-exercise manipulations: 1) "train-high" interval training sessions in the evening with high-CHO availability, 2) overnight CHO restriction ("sleeping-low"), and 3) "train-low" sessions with low endogenous and exogenous CHO availability. The CON group followed the same training program but with high CHO availability throughout training sessions (no CHO restriction overnight, training sessions with exogenous CHO provision).
RESULTS:
There was a significant improvement in delta efficiency during submaximal cycling for SL versus CON (CON, +1.4% ± 9.3%; SL, +11% ± 15%, P < 0.05). SL also improved supramaximal cycling to exhaustion at 150% of peak aerobic power (CON, +1.63% ± 12.4%; SL, +12.5% ± 19.0%; P = 0.06) and 10-km running performance (CON, -0.10% ± 2.03%; SL, -2.9% ± 2.15%; P < 0.05). Fat mass was decreased in SL (CON, -2.6 ± 7.4; SL, -8.5% ± 7.4% before; P < 0.01), but not lean mass (CON, -0.22 ± 1.0; SL, -0.16% ± 1.7% PRE).
CONCLUSION:
Short-term periodization of dietary CHO availability around selected training sessions promoted significant improvements in submaximal cycling economy, as well as supramaximal cycling capacity and 10-km running time in trained endurance athletes.

Sorry Jason, just noticed this thread. And for me it has been great, doing an average of 18 hour fasts, and "all" my training during that time. I do moderate to intense workouts, swimming and running usually, but throwing in some weights now too. I have never bonked in 2 hours of training, and generally feel great during and after. Actually never hungry right after, so usually take an hour or two before refueling.

Take a look at Gene who has been posting here, he is setting world marathon records fasted, and says he trains every day without breakfast, or hardly any calories during training, and he says he goes hard most of the time..

All my blood markers became a lot better, except the last test where my total cholesterol was up. T went up 18%, liver enzymes went down a lot, white cell counts went from high to low(thinking overall inflmation there), and a couple other good ones. I do drink straight black coffee every morning, and take one or two salt caps during the morning time. If I dont, then I start to feel cramps coming on, and this always fends them off until I can eat. Probably a combination of not eating for 18 hours, training and burning another 800 to 1000 calories, and the diuretic affect of the coffee.

Anyway it has been over a year, no problems. Lost a lot of weight in the first 6 months, but have since leveled off. Have to change what I actually eat to get those last 7 or 8 lbs. Which I'm just starting to do, heading towards a more plant based diet, with just occasional meat and dairy.

Hope that helps, let me know if you have any other questions...