Low urea nitrogen refers to low serum urea nitrogen, the clinical significance of low urea nitrogen has the following aspects, first, the reduction of urea nitrogen production caused by the reduction of urea nitrogen, such as liver insufficiency, cirrhosis, poisoning, severe cirrhosis, hepatitis of patients, etc., will cause the reduction of urea nitrogen; second, low protein diet, such as vegetarian people and less soy products, peanuts, melon seeds, walnuts and other plant protein, the intake of protein Insufficient intake of protein causes lower blood urea nitrogen; third, the protein demand is higher in late pregnancy and infancy, and insufficient supplementation causes lower urea nitrogen; fourth, excess urea nitrogen excretion causes lower urea nitrogen, such as polyuria, uremic patients, etc.