L-Phenylalanine is an essential amino acid and it is the precursor of the amino acid tyrosine. The body cannot make phenylalanie but it needs phenylalanie to produce proteins. Thus, human needs to obtain phenylalanie from food. 3 forms of phenylalanie are found in the nature: D-phenylalanine, L-phenylalanine, and DL-phenylalanine. Among these three forms, L-phenylalanine is the natural form found in most foods that containing proteins, including beef, poultry, pork, fish, milk, yogurt, eggs, cheeses, soy products, and certain nuts and seeds.