Phenylalanine is an essential amino acid that is the direct precursor to phenylethylamine, a neuromodulator that is commonly used as a dietary supplement. Aspartame, or NutraSweet, is a non-food source or phenylalanine. Those suffering from PKU (phenylketonuria) cannot metabolize phenylalanine. You can find more information here: http://en.wikipedia.org/wiki/Phenylalanine
