Phenylalanine is an essential amino acid, one of the building blocks of protein. The term “essential” means the body cannot produce it, so it must be obtained from food. This amino acid is fundamental for various physiological processes necessary for normal growth and development.
The Role of Phenylalanine in the Body
The primary purpose of phenylalanine is to build proteins, which are molecules the body uses to create and repair tissues. Phenylalanine’s importance extends beyond protein synthesis, as the body converts it into another amino acid called tyrosine.
This conversion is the first step in a pathway that produces several molecules. Tyrosine is used to synthesize neurotransmitters, chemicals that transmit signals between nerve cells. These neurotransmitters include dopamine, norepinephrine, and epinephrine. Dopamine is involved in regulating mood and motivation, while norepinephrine and epinephrine play roles in the body’s stress response and maintaining alertness.
Phenylalanine in the Diet
Phenylalanine is naturally present in a wide variety of protein-rich foods. Animal-based products such as meat, including beef and chicken, fish, eggs, and dairy items like milk and cheese are significant sources.
Plant-based sources also contain ample amounts of this amino acid. Soy products like tofu and tempeh, nuts, seeds, and legumes such as lentils and beans are all good dietary contributors. Grains like quinoa and oats also provide phenylalanine. A balanced diet containing a variety of these protein sources will supply all the phenylalanine the body needs.
A notable non-food source of phenylalanine is the artificial sweetener aspartame, marketed under brand names like Equal and NutraSweet. This sweetener is metabolized by the body, and one of its byproducts is phenylalanine. Aspartame is found in many “diet” or “sugar-free” products, including sodas, chewing gum, and some medications. Food labeling laws in many countries require products containing aspartame to indicate the presence of phenylalanine.
The Phenylketonuria (PKU) Connection
Warning labels on products containing phenylalanine are required because of a rare inherited disorder called Phenylketonuria (PKU). People with PKU are born without, or with a deficiency of, an enzyme called phenylalanine hydroxylase. This enzyme is responsible for breaking down excess phenylalanine in the body.
Without a functional enzyme, phenylalanine from food and other sources accumulates in the blood and brain to toxic levels. This buildup can lead to severe health consequences, including neurological damage and intellectual disability. Symptoms of untreated PKU can include seizures, developmental delays, and a characteristic musty body odor in sweat and urine.
To prevent these outcomes, newborn screening for PKU is a standard practice in many countries. A blood test performed shortly after birth can detect high levels of phenylalanine, allowing for immediate intervention. The warning label “PHENYLKETONURICS: CONTAINS PHENYLALANINE” is a safety measure for individuals with PKU and not a warning for the general population.
Management of PKU requires a lifelong, strict low-protein diet. This involves avoiding high-protein foods like meat, dairy, and nuts, and carefully controlling the intake of other foods like potatoes and cereals. Individuals with PKU must also consume a special medical formula to get the other amino acids and nutrients they need.
Phenylalanine Supplementation
Phenylalanine is also sold as a dietary supplement. L-phenylalanine is the natural form found in food, while D-phenylalanine is a synthetic, mirror-image version. A third form, DL-phenylalanine (DLPA), is a mixture of both. These supplements have been studied for several conditions related to phenylalanine’s role in neurotransmitter production.
Research has explored its use for several conditions, including:
- Depression, based on the theory that it supports dopamine production to improve mood.
- Chronic pain, as the D-form may block the breakdown of natural pain-relieving substances.
- ADHD.
- The skin disorder vitiligo.
However, the scientific evidence for these uses is mixed or limited. High doses may cause side effects like anxiety or jitteriness and can interact with certain medications, such as levodopa for Parkinson’s disease or some antipsychotics. Anyone considering taking phenylalanine supplements should first consult with a healthcare professional.