Indeed, Paul, but brutal malnutrition like after being a concentration camp for a long time is far different from modern willful fasting. There are conditions that make refeeding syndrome much more likely - much more likely than an otherwise normal person fasting for even 30 or 40 days - malnutrition (usually accompanied by being underweight), alcoholism, anorexia nervosa, some types of cancer.
Thanks, Nutbar, that is a good description. Yes - phosphorus (and magnesium, potassium and calcium) are mostly inside our cells (as opposed to sodium - where most of it’s outside the cells); blood levels of them are quite low, and those levels need to be controlled closely. Long fasts feature plasma as a declining percentage of blood.There can be problems with blood volume being gained fast upon starting to eat again (an example would be a lot of salty food that makes the body absorb a lot of water), this would reduce blood concentration of those elements - especially if eating carbohydrates with the subsequent higher insulin and other hormone response that has the effect of driving more of the ions (P, Mg, K and Ca) out of the blood and into the cells.
Some people do better/feel better by supplementing with salt or salt plus other electrolytes during a fast. But I don’t see it as necessarily needed, especially not if we’re talking about fasts of less than 10 days (or maybe 2 weeks). There are people who go 30 or 40 days with just water.
Here’s a guy’s account of a 40 day water-only fast. He’d previously ran a marathon on the 30th day of a fast, so it’s surprising what is possible. https://medium.com/@scottragsdale/40-days-and-40-nights-water-fast-fa493222e6a6
Bone broth (or anything containing protein) - Dr. Fung doesn’t mention autophagy here, but it’s one of the main benefits of fasting, in my opinion, and protein is the strongest suppressor of autophagy. If I felt like I needed it on a very long fast, like 2 weeks+ (never done it), then perhaps, but otherwise, no.
Now, if bone broth means a person can fast versus not fasting, and they desire to, then I’d say certainly - do it.
Taking vitamins - if there is a known deficiency, okay. Otherwise, if a person is feeling all right, then I would not worry about it all unless we’re talking about fasts of multiple weeks.
Usual activites, exercising, work, etc. - definitely. Exercise amplifies and intensifies the benefits of a fast.
I once ended a 10 day fast (stupidly, no question about it) by eating 4 Burger King Whoppers, buns and all. That’s a 200+ gram whack of carbhydrates. I did feel sluggish for about an hour afterwards. No other problems than that, but I should know better.