What prompted me to write this question was an old news that said an old man died trying to fast for 40 days. I kept thinking of the possibilies why that happened.
Not taking electrolytes.
One mistake that’s especially easy for people to make is avoiding sodium. This would be particularly likely for an older person.
Even if he took electrolytes, he might have avoided salt. However, if he took potassium, but not salt, the potassium could easily have become imbalanced.
Potassium poisoning is fatal and not easily detected (and likely wouldn’t be checked for).
Salt is what gives your blood volume so that it can be pushed along through the blood vessels. Potassium, on the other hand, softens the walls of the blood vessels and allows water to penetrate cell membranes and refresh them.
When you get an imbalance of these two, the blood vessels and cell walls become too open and receptive to water.
The cells absorb so much water that they eventually burst, and you die. It looks like a heart attack upon first examination.
People with diabetes or on medication for heart disease or blood pressure shouldn’t be fasting without a doctors help. Fasting could be fatal for people who are underweight, possess preexisting medical conditions, or have blood sugar issues in particular.
As others have said, electrolytes, water or lack of available macros (aka being underweight). All three of these will show up as other problematic symptoms before death though - so as a proximal cause, I’d guess “ignoring significant pain, nausea or other bodily symptoms” is the factor that leads to death.
I would say that besides ignoring pre existing conditions, just ignoring what your body is telling you. If something doesn’t feel right just stop. You are not supposed to be puking half way through, you are not supposed to be in pain. Weak? Sure. Lethargic? That happens to me from time to time while eating. Confused? Nope.