L-Phenylalanine is an essential amino acid and it is classified as an aromatic amino acid. L-Phenylalanine is metabolized to L-tyrosine in the liver and is known as a precursor to catecholamines (such as noradrenaline and dopamine). Essential amino acids are nutritional components that must be ingested from the diet and are present as protein constituents or free amino acids in various foods such as meat, seafood, eggs, and dairy products. The mean intakes of L-phenylalanine in Japanese were reported to be 3.51 g/day for males and 2.97 g/day for females (Kato et al., Jap. Soc. Nutri. Diet. 71 (2013)). In recent years, L-phenylalanine has been used as a supplement for physiological activity that it has been reported to modulate cerebral function (Birkmayer et al., J Neural Transm. 59 (1984)), analgesia (Walsh et al., Arch Phys Med Rehabil. 67 (2013) and skin conditions ((Antoniou et al., Int J Dermatol. 28 (2014)).
The following reports on the safety of L-phenylalanine are available. In a non-clinical safety study, rats were exposed to L-phenylalanine at 0.5, 1.5, and 5.0% diet for 4 weeks. From the results of this study, the NOAEL for L-phenylalanine was reported to be 1.5% diet (1,548 mg/kg/ days) in males and 0.5% diet (1,555 mg/kg/ days) in females (Shibui et al., Fund Toxicol Sci 1 (2014)). In a human safety study, 6 healthy adult males received a single dose of 3 g/day of L-phenylalanine. The results of this study indicate that no adverse events were observed after a single dose of 3 g/day of L-phenylalanine (Ueda K et al., J. the Int. Soc. Sports Nutri.14 (2017)).