Aspartame crystals, coloured scanning electron micrograph (SEM). Aspartame (Nutrasweet) is an artificial sweetener used to flavour foods. Its chemical name is L-alpha-aspartyl-L-phenylalanine methyl ester. Aspartame is around 200 times sweeter than sugar. It is used in foods as it has no energy value, and thus can help weight loss. All products that contain aspartame must carry a warning, as it contains the amino acid phenylalanine. People with the disease phenylketonuria (PKU) cannot metabolise phenylalanine, and over-consumption can lead to mental retardation. Magnification unknown.