http://www.livepositively.com/en_us/healthy_alternative?WT.srch=1#/healthy_alternative/aspartame
Aspartame, the sweetener in Diet Coke, is one of the world's most thoroughly studied food ingredients, with more than 200 scientific studies confirming its safety. It has been approved for use in carbonated beverages by the US Food and Drug Administration (FDA) for more than 25 years. More than 100 other countries also permit its use in a variety of foods and beverages.

Aspartame is made of two amino acids, the building blocks of protein. Proteins are found in a variety of common foods including dairy, meat, nuts and soy. As with other proteins in the diet, the amino acids in aspartame are digested and used by the body in the same way. Products sweetened with aspartame state on their labels that they contain phenylalanine—one of the two amino acids that make up aspartame. This is important information because individuals with a rare genetic condition called PKU (phenylketonuria) need to avoid all dietary sources of this amino acid.