Phenylalanine is an essential amino acid, which means that it is essential to human health but cannot be manufactured by the body. For this reason, phenylalanine must be obtained from food. Amino acids are the building blocks of protein. It is available in three chemical forms: L-phenylalanine, the natural form of phenylalanine found in proteins throughout the body; (2) D-phenylalanine, a mirror image of L-phenylalanine that is synthesized in a laboratory; and (3) DL-phenylalanine, a combination of the previous two forms.

The body converts phenylalanine into tyrosine, another amino acid essential for making proteins, certain brain chemicals, and thyroid hormones. Symptoms of phenylalanine deficiency include confusion, lack of energy, 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. Symptoms of PKU, which tend to appear between three and six months of age, include eczema, developmental delay, an abnormally small head, and hyperactivity. If it is not treated before three weeks of age, PKU can cause severe, irreversible mental retardation. In the United States, newborns are tested for PKU during the first 48 to 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 studies have not been entirely consistent, preliminary evidence suggests that D-phenylalanine may help reduce chronic pain associated with certain health conditions 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 pain-killers. Other 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
Evidence suggests that combining L-phenylalanine (oral and topical) with UVA radiation for people with vitiligo (a condition characterized by irregular depigmentation or white patches of skin) may lead to some darkening or repigmentation of the whitened areas, particularly on the face. Although preliminary information suggests that it is safe when used under appropriate medical guidance and supervision, more research is needed to assess potential side effects of this treatment approach.

Depression
Although far from proven, there is at least theoretic benefit in using phenylalanine as part of a comprehensive therapy for depression. In support of this theory, some 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.

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 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 or used as a topical cream.

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

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

Pediatric

• Birth to 4 months: 125 mg per kilogram of body weight per day
• Children 5 months to 2 years: 69 mg per kilogram of body weight per day
• Children 3 to 12 years: 22 mg per kilogram of body weight per day

The combination of oral and topical phenylalanine (together with ultraviolet light) have been used to treat children with vitiligo. The dose and appropriate length of time to continue the therapy would be determined by your physician.

Adult

• Teenagers and adults: 14 mg per kilogram of body weight per day

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 to 100 mg per kilogram have been used in studies of those with vitiligo. The most common amounts used range from 750 to 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 healthcare provider.

People with PKU and women who are lactating or are pregnant should not take phenylalanine supplements.

Little is known about the use of aspartame during pregnancy. Talk with your health care provider about using this artificial sweetener.

DL-phenylalanine may cause symptoms of anxiety, jitteriness, and hyperactivity in children. Children with 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 healthcare provider.

Antidepressant Medications, Monoamine Oxidase Inhibitors (MAOIs)
Phenylalanine may cause a severe increase in blood pressure in people taking MAOIs (such as phenelzine, tranylcypromine, pargyline, and selegiline). 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, 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, 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.

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