There are three forms of phenylalanine: L-phenylalanine, D-phenylalanine, and DL-phenylalanine. L-phenylalanine is the natural form found in some foods and is a building block for proteins in the body. D-phenylalanine is a lab-created form that closely resembles L-phenylalanine in structure. DL-phenylalanine is a combination of both forms.