it depends on how much fat you have to burn. how healthy you are to start
ive been fasting 3+ days a week for over two months (plus some 5 and 7 day) and my last blood test had zero issues except a little low vitamin D (thanks to it being cold and i am not get much sun)
so heath wise it had zero effect except for me to lose a bunch of weight i really needed to start losing.
and i am personally not taking much for supplements at all, few electrolytes, one a day vitamin it pretty much it.
IDK, Im doing rolling 72’s and so far, so good. That means I eat one meal, fast for 72 hours, eat one meal, fast for 72 hours. Have been going for a little over 2 weeks now having considerable fat to lose
Generally speaking, for someone at healthy weight and without an eating disorder (so will actually eat enough to maintain weight), a 1d/week is probably the max I’d recommend.
2d/week which I have done for years takes an insane amount of effort to make work. You need to eat a huge amount, and basically eat continuously on feeding days to ensure maximum nutrient uptake and protein synthesis. Otherwise just fasting 2d/week can drive healthy people to anemia in as little as 7 weeks (as in this study):
<https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4143608/>
These are very high levels of restriction. 1-2d per week is like 15-30% of your eating, so you have to start eating really massive amounts to make up for it.