Aspartame is a low-calorie sweetener used to sweeten a wide variety of low and reduced calorie foods and beverages, including low calories tabletop sweeteners. 

Also,spartame is composed of two amino acids : aspartic acid and phenylalanine, as the methyl ester. Amino acids are the building blocks of protein. 

Aspartic Acid and phenylalanine are also found naturally in protein containing foods, including meat grains and dairy products. Methyl esters are also found naturally in many foods such as fruits and vegetables and their juices. Upon digestion, aspartame breaks down into three components (aspartic acid, phenylalanine, and methanol), which are then absorbed into the blood and used in normal body processes.

Neither aspartame nor its components accumulates in the body.
These components are used in the body in the same ways as when they are derived from common foods. 

Source: www.aspartame.org/aspartame_facts.html