I know that fasting for too long will cause the body to break down the heart and intestines, etc. this is very bad for one’s health.
Will a three day fast for a person with 10% body fat cause this? Or is the body smart enough to go after biceps, leg muscles, etc. long before it goes after the heart?
Thank you! That makes me feel better. I believe you are correct, in particular because your answer goes along with common sense, however I am wondering if there is a good solid scientific answer for this?
Jason Fung might be a name to search. He’s done a lot of work in the area of fasting.
@Fry, it has come up a few times here, and in addition to actual case studies, that “common sense” you mention is very important, I’d say. 
In three days, not much changes for most people, not much at all. If one is very, very lean to start with, then the body is going to guard the fat stores strongly, and it will not “want” to burn much fat at all. (Being insulin resistant will make it hard to burn fat, regardless of one’s leanness, but I know of no better way to start addressing the insulin resistance problem than fasting itself.)
If a three day fast meaningfully harmed the heart or other vital organs, it would not bode well for our survival as a species, especially back in the days before the industrial revolution, when feast/famine was more prevalent. Practically, in the present time, hundreds of us on this forum alone have fasted for three days or longer, several or many times, and I’ve never heard a claim that there was lasting harm from it.
Taken to a much greater extreme, there’s the case of the man who fasted for 382 days, going from 456 lbs/207 kg to 180 lbs/82 kg, and maintained close to that weight for years afterwards. He was fine - it’s not like he “burned up his muscles” or “consumed and weakened his heart.” Our bodies are not going to do that, certainly not in fasts of the length we are talking about on this forum, almost always. I’ve read of Russian studies on people who starved to death - even with this very extreme case, they reported only a 3% loss in the mass of the heart.
I searched for more information on the Russian deal, there, but found nothing concrete today. In the Ukraine, in the early 1930s, there was a horrendous famine - that may have been where the study was done, but I can find nothing specific about it.
From what I’ve seen there are often rather alarmist, even outright fear-mongering claims from people - usually people who want to sell you their supplements or their diet program - that fasting by itself (which nets them no money) is bad for us.
Not saying that Dr. Phinney was specifically that way, but a quite from him is, “After just one day of fasting, you begin to lose body protein from lean tissue – from places like muscle, heart, liver, and kidneys. These organs and their functions are things that we want to preserve, and definitely should not be given up lightly. So while you may lose weight according to the scale, part of it will be at the cost of your important, metabolically active tissue.”
I think this is very misleading. “Places like muscle, heart, liver, and kidneys” need only mean “protein” to be technically correct, but this does not mean that the body will necessarily be consuming those organs. It really only means that protein is being supplied by the body, rather than being ingested from external sources.
Yes - protein is being consumed or cycled and recycled, because we can detect the byproducts of its metabolism, nitrogen for one. However, autophagy is the body breaking down damaged and/or old cellular components - a very beneficial thing for the body - so there’s nothing bad there, on that score alone.
Our skin is roughly one-sixth of our weight, and consuming skin cells is “using protein” and generating nitrogen - but of course this is not harming the heart or any other vital organs, including the skin itself. If we lose weight and don’t need so much skin, to be rid of the excess is a good thing.
Fasting is not for everybody, even those of us who are obese. Not saying it will be perfect or even “good” for you - that remains to be seen. Yet there is nothing I’ve ever seen that makes a rational case for a three-day fast harming your heart or other essential muscles.