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 these people cannot safely consume phenylalanine. For example, aspartame, a common sweetener, is a nonprotein source of phenylalanine. Aspartame is used in many diet soda pops and sugar-free foods.