The aspartic acid is primarily used for energy through conversion to CO2 in the Krebs cycle. The phenylalanine is primarily incorporated into body protein, either unchanged or as tyrosine.
People with the rare human genetic disease, phenylketonuria have a deficiency in their ability to metabolize phenylalanine and unable to dispose of any excess phenylalanine. Their intake of this essential amino acid must be very strictly controlled from birth to adulthood.
Therefore, they must include phenylalanine content of aspartame in their dietary calculations.
The accumulation of phenylalanine and its by–product is toxic to the developing nervous system, causing irreversible brain damage.
Aspartame is non-carcinogenic and has calorific value of approximately 3 cal/g. The level of daily consumption that is judged to be safe by the FDA is 50 milligrams per kilogram of body weight per day.
Aspartame metabolism in human body 
