L-Phenylalanine is an essential amino acid, which means l-phenylalanine must be received from your diet. L-Phenylalanine is connected to L-Tyrosine, another amino acid that is important to healthy body function. L-Phenylalanine has been associated with the neurotransmitters and brain function; elevating the mood. L-Phenylalanine is important to healthy body function and can be found in most foods that contain protein. There are two forms of l-phenylalanine, a L or D form; both are essential to your body.