Nitrogen is a component of amino acids and therefore all proteins. It is absorbed into the body as amino acids and also as ammonia, a by product of bacterial fermentation. Ammonia is toxic and is converted to Urea by the liver after absorbtion. Various nitrogen / urea / ammonia cycles exist to convert nitrogen into amino acids and eliminate it as urea via the kidneys before toxic levels build up. Nitrogen is present in the blood as Urea and Blood Urea Nitrogen BUN is used as a measure of kidney function. BUN also rises with increasing levels of dietary protein. Feeding protein in excess of requirements leads to production of waste nitrogen which must then be converted to urea and voided in the urine. There is hence some metabolic cost to high protein diets which may give rise to an accumulation of waste Nitrogen in the body, and extra work for the liver and kidneys to remove it. The ammonia smell of soiled bedding is indicative of excreted nitrogen waste in the urine.
