Supplements: Phenylalanine

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Phenylalanine is an essential amino acid that you need to get from food because your body cannot make enough of it. In healthy people, the body changes phenylalanine into tyrosine, which in turn makes important hormones, such as norepinephrine and epinephrine. Adults use about 90 percent of the phenylalanine consumed to make tyrosine, children about 40 percent. Phenylalanine may be used to treat pain, depression, multiple sclerosis, Parkinson's disease, rheumatoid arthritis, osteoarthritis, and even cancer.

Phenylketonuria (PKU) is a disorder in which the body fails to turn phenylalanine into tyrosine properly. This disease appears in infants about 3 to 6 months old, often causing severe mental retardation. It may also cause seizures and hyperactivity. Some people with PKU have a skin rash, such as eczema. PKU occurs in approximately 1 in 10,000 Caucasian infants and 1 in 132,000 African-American infants. In the United States, newborns are tested for PKU during the first 48 hours of life. PKU must be treated before the infant is 3 months old if it is to be treated successfully.

People with PKU must eat a phenylalanine-restricted, tyrosine-supplemented diet to have optimum brain development and growth. Experts disagree about whether people with PKU can discontinue this diet without problems and, if so, at what age. Mental performance and intelligence is better in those who have stayed on the diet, according to some studies. Consult your health care provider about the pros and cons of treatment.

Pregnant women with untreated PKU give birth to small infants with birth defects. These birth defects are often severe, and these infants may not live long. If you have PKU and you are, or may become, pregnant, you should be on a phenylalanine-restricted diet.

Too little phenylalanine may cause confusion, emotional agitation, depression, decreased alertness, decreased memory, behavioral changes, decreased sexual interest, bloodshot eyes, cataracts, decreased insulin, decreased skin melanin (pigment), and increased appetite. If you are getting too little phenylalanine, you should take supplemental phenylalanine and tyrosine. Otherwise, you may fail to gain weight or grow taller, lose your hair, have problems with your bones, get anemia, or even die.


  • Cancer: You may reduce tumor growth and metastasis, particularly in malignant melanoma (skin cancer), by getting less phenylalanine and tyrosine.
  • Depression
  • Inflammation
  • Multiple sclerosis: You may improve bladder control, increase mobility, and reduce depression.
  • Pain: You may be able to reduce chronic pain, particularly in osteoarthritis.
  • Parkinson's disease: You may improve rigidity, walking disabilities, and speech difficulties.
  • Vitiligo: You may improve the condition with a combination of oral L-phenylalanine, topical cream containing phenylalanine, and ultraviolet-A radiation.

Dietary Sources

Cheeses Nuts and seeds Milk chocolate Meat (excluding fat), particularly organ meats Poultry (excluding skin) Fish, including shellfish Milk Eggs Aspartame (Nutrasweet)

Some of the most concentrated sources of phenylalanine are torula yeast, soybean protein isolate and concentrate, peanut flour, dried spirulina seaweed, defatted and low-fat soybean flour, dried and salted cod, dried and frozen tofu, Parmesan cheese, almond meal, dry roasted soybean nuts, dried watermelon seeds, and fenugreek seeds.

Other Forms

D-phenylalanine L-phenylalanine D,L-phenylalanine (50/50 blend of D-phenylalanine and L-phenylalanine) Topical creams

How to Take It

The recommended dietary allowances for phenylalanine plus tyrosine include the following.

  • Birth to 4 months: 125 mg per kilogram of body weight a day
  • 5 months to 2 years: 69 mg per kilogram of body weight a day
  • 3 to 12 years: 22 mg per kilogram of body weight a day
  • Adults and teenagers: 14 mg per kilogram of body weight a day

Some experts say that adults need 39 mg per kilogram of body weight a day.

Talk with your health care provider about dosages for specific uses. Generally, nutritional doses are 0.75 to 2 g a day and therapeutic doses are 2 to 3 g a day. Supplements are usually taken 15 to 30 minutes before meals.


Anxiety, headaches, and hypertension are possible side effects. People with PKU and women who are lactating or are pregnant should not take phenylalanine supplements. L-dopa competes with phenylalanine for absorption and should not be taken at the same time of day. Little is known about the use of aspartame (Nutrasweet) during pregnancy. Talk with your health care provider about using this artificial sweetener. Doses in excess of 5 g a day may be toxic.

Possible Interactions

Vitamins B6 and C help the body absorb phenylalanine. Increased amounts of other amino acids will inhibit phenylalanine absorption.

This document contains information relating to general principles of medical care that should not in any event be construed as specific instructions for individual patients. The reader is advised to check product information (including package inserts) for changes and new information regarding dosage, precautions, and contraindications before administering any drug. No claim or endorsements are made for any drug or compound currently in investigative use. No responsibility is assumed by the publisher for any injury and/or damage to any person or property as a matter of product liability, negligence, or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in any material herein.

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Previously Published in OSA Today Reproduced by permission
Updated on: 02/01/10