Beginners can start with just intermittent fasting.
If this not affordable, then have to afford a more costly one - chronic diseases.
There are a few studies knocking around about regarding it, but I've only got a minute so this is the best I could do for the moment-
"Findings indicated that pre-exercise feeding enhanced prolonged (P = .012), but not shorter duration aerobic exercise performance (P = .687). Fasted exercise increased post-exercise circulating FFAs (P = .023) compared to fed exercise."[1]
I experimented at the start of the year and didn't experience any drop in performance - office or gym - with water fasts less than 24 hours.
Fasting between 24-72 hours, though, really affected my sleep and made me feel weaker during workouts (though it didn't really affect me mentally). Curious to see if others experienced the same.
Can't comment on longer fasts as I didn't exceed 72 hours (though I did read many reports of it actually getting easier after that point in time).
Currently doing IF (very rough 20/4) which is working pretty well.
I fast once every 10 days minimum (arbitrary number, but it keeps me disciplined). If I've had a few weeks of excess (lots of drinking and greasy takeaways) I might do 3-4 days. At least 2-3 times a year I like to do a 6-8 day fast. I've done this for the past 4 years and I've felt remarkably better for it, with the most noticeable thing being reduction in scar tissue (from surgery) and inflammation symptoms.
In terms of how it makes me feel while fasting, it depends on the day. A one day fasts tends to make me just feel hungry around meal times, assumedly because I'm still largely running on the glycogen in my muscles. I used to find day 2 pretty rough, as I'm guessing that's how long it would take for the process of ketosis to start in me. As your body adapts to frequent fasting though it becomes plain sailing though. I'll often arrange my fast to coincide with a large stint of work, as I feel my mental clarity is much higher - akin to taking Modafinil if you've ever tried it. Hunger isn't an issue for me at all once I'm in a state of ketosis.
I highly recommend doing an extended fast at some point. If anything it allows you to become much more satiated by smaller portions, and it eliminated any sugar cravings I used to have.
This was my first reservation about fasting and while everybody is different and you'll need to see how your own body responds, I am happy to report that the following activities:
- mid to long distance runs (4-8 miles) - 60-90 minute power movement weightlifting session - routine crossfit workout and/or plyometrics - BJJ class with 5 minute sparring rounds
... are not only not impacted, but are in many cases enhanced by being in a fasted state. The most positive impact is in my weightlifting workout[1] which, ironically, is the activity I thought would be most negatively impacted.
I can't speak to 50 mile ultra runs and I know I can't do all of the above in a fasted state - but I never attempted to do all of the above on the same day anyway.
Just try it.
[1] Power movements, relatively heavy weights and relatively low reps. Just the kind of workout you'd think would be difficult while in a fasted state.