Excess amino acids are converted into urea by

A supply of amino acids in the blood is kept at a normal level. Any spare which has not been absorbed cannot be stored,  but is converted into the waste products, called urea,  when at the liver, and is then sent to the kidneys to be removed from the body as urine. The remainder of the amino acid molecule is not wasted;   it is changed into a carbohydrate,  that can be used.