Final answer:
When amino acids are used as fuel, the nitrogen ends up in the form of urea and is excreted from the body.
Step-by-step explanation:
The nitrogen attached to an amino acid used as fuel ends up in the form of urea, which is excreted from the body through urine. Amino acids are primarily used for synthesizing proteins and other nitrogen-containing compounds. However, when there is a shortage of carbohydrates and fats, amino acids can be used as an alternative source of energy.