Phenylalanine can cause mental retardation, brain damage, seizures and other problems in people with PKU. Phenylalanine occurs naturally in many protein-rich foods, such as milk, eggs and meat. Phenylalanine also is sold as a dietary supplement.
The artificial sweetener aspartame (Equal, NutraSweet), which is added to many medications, diet foods and diet sodas, contains phenylalanine
Natural or artificial? http://en.wikipedia.org/wiki/Phenylalanine Excerpt: Phenylalanine is found naturally in the breast milk of mammals. It is used in the manufacture of food and drink products and sold as a nutritional supplement for its reputed analgesic and antidepressant effects. It is a direct precursor to the neuromodulator phenylethylamine, a commonly used dietary supplement.