L-Phenylalanine is an essential, nine-carbon amino acid and is a precursor to the amino acid tyrosine. Because tyrosine is necessary for the synthesis of proteins and the production of the neurotransmitters dopamine and norepinephrine, phenylalanine is an important nutrient that must be obtained through the diet or supplementation. As the building blocks of protein, amino acids are vital to health. Next to water, amino acids in the form of proteins make up the greatest portion of our body weight. They comprise tendons, muscles and ligaments; organs and glands; hair and nails; important bodily fluids, and are a necessary part of every cell in the body.