Phenylketonurics" is the term used to refer to people that have the metabolic disorder Phenylketonuria, or PKU for short. and the warning on diet soda cans is included merely to inform people that the product contains aspartame. People that have the disorder PKU cannot consume any product that contains aspartame. Phenylalanine is an essential amino acid that is in nearly all forms of protein - meats, dairy products, eggs, nuts, legumes... etc. Some researchers claim that the amino acid phenylalanine is a natural muscle relaxer and sleep aid. We've heard a rumor that this could be why turkey, which is a high phenylalanine food, always seems to make people sleepy (in addition to the amino acid tryptophan, which is also a natural sleep aid). For people without PKU or other metabolic disorders, it is very important to eat a balanced diet, and this certainly includes high protein (high phenylalanine) foods.