Phenylalanine is one of the eight essential amino acids necessary for the basic functioning of human cells that leads to normal growth and development. It is found in most foods, especially those high in protein, such as meat, cheese, and milk.

Food labeling laws require products that contain phenylalanine to be labeled with a warning for people who have the inherited disorder phenylketonuria (PKU), because they cannot safely consume phenylalanine. For example, aspartame (Nutrasweet), a common sweetener, is nonprotein source of phenylalanine. It is used in many diet soft drinks and sugar-free foods.