Three days is the longest people should go for if they haven't tried fasting before and don't have experience with gradual refeeding. Any longer than five days and you risk running into the refeeding syndrome.
If you’re anorexic, fast for 40 days, and don’t supplement with salts (particularly potassium) you’re going to have refeeding syndrome.
If one’s worried about it, just get ones blood work done before one starts eating again. It takes about an hour and will show you exactly what you are low on what to supplement to get to baseline. To the extent people are advising one not to do an extended fast without medical supervision - eh - basically the extent of the medical supervision is "let's run a panel every few days and see if anything is dangerously out of whack" - and maybe confine you to a hospital for the duration - but if you're not outside and walking a normal amount and say confined to a room you absolutely will lose a large amount of muscle.