Nitrogen is an indicator of protein status [35] and has been used to determine protein requirements and lean tissue growth rates in pigs. Blood UN, as the ultimate and major nitrogenous product of protein and amino acid catabolism, is synthesized in the body via the ornithine cycle [36], [37]. Plasma UN concentration reflects the balance status of amino acids, and is often used as an indicator of kidney and liver function, as well as an indicator of relative hydration status in animals. Low blood UN indicates a good balance of amino acids, and suggests relatively low urea synthesis and hydration in the liver and relatively high dietary protein efficiency [38]. In the present study, we found that Landrace pigs fed the NRC diet had lower plasma concentration of UN, whereas Bama mini-pigs fed the same diet had higher plasma concentration of UN, than did those fed the GB diet. These findings suggest an interaction effect between breed and diet. In addition, increased protein consumption by mammals results in increased fecal ammonia, which is a polluting substance [39], [40]. Therefore, limiting protein ingestion will also limit the ammonia excretion in fecal substances; lower plasma concentrations of UN could decrease the emission of ammonia from pig production, thereby reducing environmental pollution.