I have personally experimented with most of the diets mentioned on this site; from paleo, keto to carb cycling. I haven't practiced the fast-5 or read it's literature but I did actively follow the warrior diet for a period of 6 months beggining last spring. For those of you unfamiliar with WD, it's similar to fast-5 in that you essentially fast for 18-20 hours everyday, and restrict yourself to only eating in that 4-6 hour time frame. The founder of the warrior diet advocated "feasting" right before bed time, and specified in what order to eat protein, carbs and fats during this "feast".
At the beggining of the diet I felt amazing, I could similarly describe it as the fast-5 diet has been described on this board. I would usually workout out (running, swimming, and heavy lifting) prior to my meal, and then feast from 7-11pm. My overall strength greatly increased, while I was breaking new PR's in time and distance for both running and swimming.
While on this eating plan I often noticed that in the middle of the day I would feel a surge of energy, like drinking a giant cup of coffee. I would become more focused, energetic and genuinely positive.
I noticed no increase in bodyfat % and it made everyday more exciting because I knew at the end of the day, bounty-full feast awaited me.
But after a few months, I started to obsess about my feast meals. All I would think about all day was what I was going to eat later. I would literally rush home and start stuffing my face as fast as I could and nearly once lost a finger when I too excitedly cut up a chicken for dinner.
Before I knew it I began to cheat, I started eating two giant meals a day instead of one. Food began to occupy every thought I had. This loss of control lasted for a couple months until I moved back to LA from Boston. My Friends and family unanamously agreed that I had gotten fatter. I wasn't anywhere near obese but my body fat % had skyrocketed from where it was when I started. (I didn't take exact measurements but I can say it went from somewhere around 9% to 15%)
I then realized that this diet was no longer for me, and spent the entire summer losing what I had gained and switching up my diet regiment.
The point of this story is to emphasize the pyschological aspect of diets like this and how easily it can go awry. I've always had exceptional self-control, until it fundamentally crumbled under this eating plan.
I put a lot of the blame on my disregard for how many calories I was eating per meal. I mistakenly assumed my body would simply tell me when to stop eating, which it did at first, but this communication slowly diminished over time.
Diets like fast-5 can be beneficial, but they require the same will power and dedication that other diets do. I completely agree that no one diet works for everyone. I believe they need to be taylored to each individuals needs and physical/mental make-up.
I'm currently on my own combination of carb cycle/stream-line metabolsim plan. I focus on eating 4-5 small meals a day and try and restrict carbs to meals either immediately prior to, or after my workouts. So far this has worked best for maintaing low body fat and getting in better shape, regulalry eating small meals throughout the day de-fantasizes food for me; and if I have any foods I want to indulge in I sync them with my workouts in controlled quantitites.
I'm curious as to whether anyone else has had such an experience with fast-5; or if anyone has engineered their own personalized eating plan that they've found beneficial.