So, first let's assume you digest your protein into the usable building blocks: amino acids. Excess dietary amino acids cannot be stored for future use, not nor are they excreted unused. Instead, they are converted to common metabolic intermediates that can be either oxidized by the citric acid cycle or used to form glucose or fat. The excess nitrogen liberated by the metabolic degradation of amino acids is excreted in the form of urea via the urea cycle.

So, first let's assume you digest your protein into the usable building blocks: amino acids. Excess dietary amino acids cannot be stored for future use, not are they excreted unused. Instead, they are converted to common metabolic intermediates that can be either oxidized by the citric acid cycle or used to form glucose or fat. The excess nitrogen liberated by the metabolic degradation of amino acids is excreted in the form of urea via the urea cycle.