Phenylalanine is an essential amino acid (building block for proteins in the body). It is essential to human health but cannot be manufactured by the body. For this reason, phenylalanine must be obtained from food. It is available in three chemical forms, including L-phenylalanine (the natural form of phenylalanine found in proteins throughout the body), D-phenylalanine (a mirror image of L-phenylalanine that is synthesized in a laboratory), and DL-phenylalanine, a combination of the previous two forms.

The body converts phenylalanine into tyrosine, another amino acid essential for making proteins, brain chemicals including dopamine and norepinephrine, and thyroid hormones. Symptoms of phenylalanine deficiency include confusion, lack of energy, depression, decreased alertness, decreased memory, and diminished appetite.

On the other hand, a rare metabolic disorder called phenylketonuria (PKU) occurs in people who are missing an enzyme that is needed to properly metabolize phenylalanine, causing high levels of phenylalanine in the body. Symptoms of PKU, which tend to appear between 3 and 6 months of age, include eczema, developmental delay, an abnormally small head, and hyperactivity. If it is not treated before 3 weeks of age, PKU can cause severe, irreversible mental retardation. In the United States, newborns are tested for PKU during the first 48 - 72 hours of life.

People with PKU must eat a phenylalanine-restricted, tyrosine-supplemented diet to have optimum brain development and growth. Rarely, over-restriction of phenylalanine in the diet can lead to deficiency of this amino acid, with the same symptoms described above.

Chronic pain

Although results of clinical studies have not been entirely consistent, preliminary evidence suggests that D-phenylalanine may help reduce chronic pain associated with certain health conditions, such as multiple sclerosis (MS) and fibromyalgia, by stimulating nerve pathways in the brain that control pain. Some scientists, for example, report that they have observed enhanced pain relief when D-phenylalanine is used together with prescription painkillers, including opiates. Other clinical studies have found D-phenylalanine to be no more effective than placebo in reducing pain. Further research is needed to determine the safety and effectiveness of this amino acid for pain.

Parkinson's disease

One animal study suggests that D-phenylalanine may improve rigidity, walking disabilities, speech difficulties, and depression associated with Parkinson's disease. It is not clear whether these results translate into a possible treatment for people with this disease, however. Further studies in people are necessary before supplementation with this amino acid can be recommended for individuals with Parkinson's disease.

Vitiligo

Clinical evidence suggests that combining L-phenylalanine (oral and topical) with UVA radiation for people with vitiligo, a condition characterized by irregular depigmentation (loss of color) or white patches of skin. L-phenylalanine may lead to some darkening or repigmentation of the whitened areas, particularly on the face. Although preliminary clinical information suggests that it is safe when used under appropriate medical guidance and supervision of a health care professional, more research is needed to assess potential side effects of this treatment approach.

Depression

Some clinical evidence suggests that phenylalanine may be effective as part of a comprehensive therapy for depression. Individuals have reported improvement in mood when taking phenylalanine. This is thought to be due to enhanced production of brain chemicals, such as dopamine and norepinephrine. More research is needed in this area.

L-phenylalanine is found in most foods that contain protein such as beef, poultry, pork, fish, milk, yogurt, eggs, cheese, soy products (including soy protein isolate, soybean flour, and tofu), and certain nuts and seeds. The artificial sweetener aspartame is also high in phenylalanine.

D-phenylalanine is synthesized in the laboratory but is not found in food.

• D-phenylalanine
• L-phenylalanine
• DL-phenylalanine (50/50 blend of D-phenylalanine and L-phenylalanine)

Phenylalanine can be taken as a capsule, powder, or tablet. It can also be used as a topical cream.

Recommended dosages of phenylalanine vary depending on the health condition being treated. Supplements are recommended 15 - 30 minutes before meals.

According to the National Research Council, the daily recommended dietary allowances (RDAs) for phenylalanine are as follows:

Pediatric

For infants 0 - 4 months: Phenylalanine, 125 mg per kilogram of body weight per day, may be given.

For children 5 months - 2 years: Phenylalanine, 69 mg per kilogram of body weight per day, may be given.

For children 3 - 12 years: Phenylalanine, 22 mg per kilogram of body weight per day, may be given.

The combination of oral and topical phenylalanine (together with ultraviolet light) are used to treat children with vitiligo. The dose and appropriate length of time to continue the therapy is determined by your doctor.

Adult

Teenagers and adults: Phenylalanine, 14 mg per kilogram of body weight per day, may be taken.

Some experts suggest that adults may need as much as 39 mg per kilogram of body weight per day for general health and doses as high as 50 - 100 mg per kilogram have been used in studies of those with vitiligo. The most common amounts used range from 750 - 3,000 mg per day for adults.

Because of the potential for side effects and interactions with medications, dietary supplements should be taken only under the supervision of a knowledgeable health care provider.

People with phenylketonuria (PKU) and women who are lactating or are pregnant should not take phenylalanine supplements. Aspartame, found in artificial sweeteners such as Nutrasweet, is a source of phenylalanine for the body. Therefore, the use of aspartame is not recommended in individuals with PKU. Little is known about the use of aspartame during pregnancy. Talk with your health care provider about using this artificial sweetener.

DL-phenylalanine should not be used in individuals diagnosed with schizophrenia, as it may cause symptoms of tardive dyskinesia. Tardive dyskinesias (TDs) are involuntary movements of the tongue, lips, face, trunk, and extremities that generally occur in patients treated with long-term antipsychotic drug use.

DL-phenylalanine may cause symptoms of anxiety, jitteriness, and hyperactivity in children. Children with attention deficit hyperactivity disorder (ADHD) may need to avoid foods containing aspartame, which is made from phenylalanine.

Doses in excess of 5,000 mg a day may be toxic and can cause nerve damage. High quantities of DL-phenylalanine may cause mild side effects such as nausea, heartburn, and headaches.

If you are currently being treated with any of the following medications, you should not use phenylalanine without first talking to your health care provider.

Monoamine Oxidase Inhibitors (MAOIs) -- Monoamine oxidase inhibitors (MAOIs) are a class of rarely used antidepressants drugs including phenelzine (Nardil), isocarboxazid (Marplan), and tranylcypromine sulfate (Parnate). Phenylalanine supplementation may cause a severe increase in blood pressure in people taking these drugs. This severe increase in blood pressure (also called "hypertensive crisis") can lead to a heart attack or stroke. For this reason, individuals taking MAOIs should avoid foods and supplements containing phenylalanine.

Baclofen -- The absorption of baclofen (Lioresal), a medication used to relieve muscle spasms, may be reduced by phenylalanine. Therefore, it is best to avoid taking this medication with a meal, especially one that is high in protein content, or with phenylalanine supplements.

Levodopa -- A few case reports suggest that phenylalanine may reduce the effectiveness of levodopa (Sinemet), a medication used to treat Parkinson's disease. Some researchers speculate that phenylalanine may interfere with the absorption of this medication. Therefore, phenylalanine should not be taken at the same time as levodopa.

Selegiline -- L-phenylalanine and the selective MAO inhibitor selegiline (Eldepryl, Deprenyl) may increase the antidepressant effects of phenylalanine. Therefore, phenylalanine should not be taken at the same time as selegine.

Neuroleptic drugs -- L-phenylalanine may enhance the tardive dyskinesia side effects of neuroleptic drugs, such as phenytoin (Dilantin), valproid acid (Depakene, Depakote), or carbamazepine (Tegretol), if used together. Therefore, phenylalanine should not be taken at the same time as neuroleptic drugs.

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