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Signs & Symptoms of Low L-Phenylalanine

By Owen Bond

Phenylalanine is an essential amino acid, which are building blocks for protein that you must get from dietary sources. L-phenylalanine is found in most foods that contain protein. Your body needs adequate amounts of phenylalanine in order to manufacture certain brain chemicals that are important for mood. An inability to metabolize phenylalanine is a rare genetic disorder that leads to mental retardation in newborns if not treated.

Functions of Phenylalanine

Phenylalanine is a precursor for tyrosine, another type of amino acid that is needed to make neurotransmitters, such as L-dopa, thyroid hormones that affect metabolism, and adrenal hormones such as noradrenaline and adrenaline. L-dopa and noradrenaline affect mood, which is why different forms of phenylalanine have been proposed to treat depression, according to “Biochemistry of Human Nutrition.” Phenylalanine is also needed to make melanin, the skin pigment.

Symptoms of Phenylalanine Deficiency

According to the University of Maryland Medical Center, infants require 125 mg of phenylalanine per kilogram of body weight, children need between 22 and 69 mg/kg and adults need at least 14 mg/kg and maybe as much as 39 mg/kg. Symptoms of phenylalanine deficiency include confusion, decreased alertness, faulty memory, depression, sluggish metabolism, lack of energy, reduced appetite and vitiligo. Vitiligo develops when your skin doesn’t produce enough melanin pigment, so patches of white skin develop. A combination of oral and topical phenylalanine is used with UV light to treat children with vitiligo.

Food Sources

L-phenylalanine is found naturally in the breast milk of all mammals and in many protein-rich foods such as beef, chicken, pork, fish, milk, yogurt, eggs, cheese, soybeans, tofu and some nuts, seeds and legumes, according to “Contemporary Nutrition.” The artificial sweetener aspartame metabolizes into phenylalanine and other compounds.


Phenylketonuria, or PKU, is a genetic disease that occurs in newborns who are missing an enzyme needed to properly use phenylalanine. Consequently, high levels of phenylalanine build-up and become toxic, leading to permanent mental retardation if not detected and treated within three weeks following birth, as cited in “Human Biochemistry and Disease”. In the United States, newborns are tested for PKU within 72 hours of birth. People with PKU must adopt a low-phenylalanine diet and take tyrosine supplements.

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