Things I've faced so far: Constipation and Hyponatremia.

Catabolysis and heart damage is also possible. The body will consume fat first, so this is an avoidable outcome. I added the multivitamin and calcium/magnesium/zinc supplement to help avoid damage as well. Damaging my heart is my largest concern with this diet.

Losing fat inherently raises blood lipids (the fat has to be circulated), which is bad for the heart long-term, but not an acute danger. The body is evolved to live on body fat, it's fine and normal.

My dad survived stage 4 cancer... a bowel obstruction is what took him out... you don't want your bowels completely stopped, otherwise very, very bad things happen. That's why it's dangerous.

They don't completely stop though. You poop out liquid that (I've read) comes mostly from your body's dead blood cells.