Protein is made up of nitrogen, carbon, oxygen and hydrogen molecules. The body can metabolize all of those but nitrogen. Nitrogen is essential for forming many different amino acids necessary for body function, but excess nitrogen must be excreted. The liver converts excess nitrogen to urea, which then travels through the blood and to the kidneys to be filtered out. A high-protein diet may result in increased urea production; excess urea may show up in the urine.