The reasons for urea nitrogen lower than normal value may be reduced intake, increased excretion, liver function damage, etc.. 1. Decreased intake: the elderly, people who lose weight, pregnant women or patients with kidney disease, etc., protein intake is reduced, such as meat, dairy, etc., which will lead to a decrease in protein metabolites urea nitrogen. 2. Increased excretion: if normal people drink too much water or a large number of transfusions, etc., blood dilution, blood urea nitrogen levels will also be reduced; in addition, the early stage of diabetes mellitus or uremic patients with increased urinary output, urine dilution, urea nitrogen will also be reduced. 3. Liver function damage: patients with hepatitis, cirrhosis or liver cancer, ammonia can not be metabolized normally in the liver, resulting in reduced urea nitrogen synthesis and lower urea nitrogen levels. Therefore, urea nitrogen is lower than normal due to more reasons, patients should consult the doctor in time, early treatment, so as not to delay the condition and affect the prognosis.