Posting to follow.
I currently practice intermittent fasting (~16 hours per day), and low carb / ketogenic dieting. However, I am looking to further optimize my fasting duration and intervals. My main reservation with longer fasting periods is muscle loss. That being said, I have been inspired to start a 40hr fast tomorrow.
How did you decide on the 5 and 15 fasting duration periods? Do you have any good references to scientific studies on different fasting durations / frequency?
-You are on a similar path to the one we took to fasting, started by skipping breakfast, then days of eating, so on and so on. It's been a real eye opener to see this "side" of diet/nutrition/exercise.
-You should not lose muscle on a fast. The body will break down old cells/junk cells (autophagy) and use it to maintain your muscle/joints etc. etc. etc. If you lose muscle (especially on anything shorter than 40 days or so...) you might have a hormonal imbalance or other issue that might make fasting a bad idea anyway.
-This is one of those things that seems to make sense, don't eat, the body will catabolize ANYTHING including muscle BUT, when you really mull it over, it doesn't hold up from an evolutionary standpoint. If we started losing muscle within a few days of a fast, we would not lose the ability to chase/kill/eat the food we needed to break the fast. Now, if we drop to 3-4% body fat, maybe you start to catabolize muscle but for most of us, it's not an issue. Dr. Jason Fung covers this is the complete guide to fasting. One of the co-authors actually had a scan done and found that he GAINED muscle during a fast.
From my own experience, I have dropped weight and gotten more "cut" during a fast, my lifts have not been compromised (I don't lift heavy, moderate weights with higher repetitions because I focus on strength endurance rather than hypertrophy/bulking). I have zero indication that I've lost any muscle through fasting, in theory, we might put more on due to the increase in human growth hormone (HGH) while fasting. It makes sense, if you are living in a cave and you are out "working out" by running after a dinosaur for dinner, the body might support that effort, releasing endorphins, testosterone, adrenaline and HGH (among other things). By exercising, we simulate that effort and reap the benefits.
Eat stop eat and leangains are great sites for this topic.
-Deciding on a duration and frequency protocol, it has been through trial and error.
Like I said, we "laddered up" into this process. We started by cutting sugars and simple carbs (we don't do white carbs etc. anyway) and then moved into very low carb, and then in and out of that protocol for a time. Once I got interested in IF (it seemed sooooo crazy initially) we started skipping breakfast etc.
We continued to develop our protocol, extending our fasts a little bit at a time. Skip breakfast, then breakfast and lunch, then do 24 hours (oh my god, we'll die!) then 48 hours (insane!) then, the big one, 72 hours (call the psychologist, they've LOST it!). We did those at random intervals, interspersed around vacation/trips/holidays/work luncheons etc. eventually we did two or three 72 hours periods and we found that welt like CRAP day 1-2 and started feeling pretty good day 3, and then we ATE! We'd do a 24 hour one week, wait a week, do a 48 hour, wait a week, do a 72 hour etc.
The problem is that we were eating carbs (not crazy amounts but some) and sugar (bowl of ice cream Sunday night, a cliff bar here and there) in between sessions (oh, and drinking beer). Which leads me to believe that we were entering a nutritional ketosis state on day three, and then jumping right out. So, why do all the "work" of getting into keto to only "enjoy" it for one day? That's why we decided to try a five day fast period. We wanted to go longer but, due to work luncheons, the wife's holiday baking weekend, and the holidays coming up, we decided on the five day period, for now. We'll do a 15 day fast in February after I complete a foot race I'm training for.
Short answer, we just settled on 5-15 days due to other considerations/lifestyle factors. I can't support those exact durations for any particular reason.
I will say, once I start fasting and get into ketosis, I want to extend that period as long as possible. Case in point, this past five day fast, I was working in the yard on day six and I seriously considered extending the fast because I felt so good. I had been DREAMING about the steak I was planning to eat later that day but here I was on day six, giving serious thought to skipping that steak (likely a result of the endorphins/HGH release from running/lifting earlier in the day). I discussed it with the spouse and I chose to eat due to my long training run on Sunday morning.
Essentially, once you put in the "work" of going 24/48/72 hours or more, it just kind of make sense to extend it based on your goals/reason behind fasting in the first place (for me it's autophagy and mental clarity/level of energy, the wife adds weight loss to that list) all other factors considered.
I think the ladder up approach is sensible BUT, once you have your fasting "legs" under you, I'd advice going as long as you want; as long as you have a healthy BMI, are not bulimic/suffering from other eating disorders, and are not at 5% or lower body fat. Just my two cents...
-How is the 40 hour fast going?