Most of the literature on fasting makes a distinction between fasting
and starvation. When you fast, you go through distinct stages. The first
day you're using up glycogen stores in your muscles and liver. You're
also burning fat for energy. The second day you're still burning fat,
but without glucose, your body breaks down some of your muscle to burn
the fat (not much muscle — not enough to worry about). Sometime during
the third day, you enter ketosis (where you are burning fat for fuel).
If you are like most of us, you have some fat on your body. So you have enough fuel for awhile. Eventually, if you keep fasting, you will run out of fat and your body will have to start breaking down muscle for energy.
The moment you burn your last gram of body fat, you are no longer fasting. As soon as you start breaking down muscle for energy, you're starving and it is unhealthy to continue. You should stop fasting and start eating.
I've never gotten that far, and you probably shouldn't either. But in the fasting literature, it says that you know it when you hit that point. You are suddenly no longer merely hungry. You feel a dangerous, urgent, starvation. They all say it is an undeniable, easily recognizable change and you will have no doubt about it.
Adam Khan is the author of Principles For Personal Growth, Slotralogy, Antivirus For Your Mind, and co-author with Klassy Evans of How to Change the Way You Look at Things (in Plain English). Follow his podcast, The Adam Bomb.