How do you get a zero 24 hour urine urea nitrogen result?

well, you get it if you basically consume too little protein.

I mean, the body will break down 0.1*ideal body weight - 2 grams of nitrogen.

if you get 3-6 you get essential the exact amount of protein you need.

about this level, it means you had extra protein which was converted to energy.

below, you had so little that your body basically used up the regular amount it supposed to excrete.