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Phenylalanine

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.

Uses

Dietary Sources

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

How to Take It

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

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.

Precautions

Possible Interactions

The absorption of baclofen, a medication used to relieve muscle spasms, may be inhibited by phenylalanine. Therefore, it is best to avoid taking this medication with a meal, especially one that is high in protein content.

Phenylalanine may reduce the effectiveness of levodopa, a medication used in combination with carbidopa for the treatment of Parkinson’s disease.

Supporting Research

Bugard P, Bremer HJ, Buhrdel P, et al. Rationale for the German recommendations for phenylalanine level control in phenylketonuria 1997. Eur J Pediatr. 1999;158:46–54.

Cejudo-Ferragud E, Nacher A, Polache A, Ceros-Fortea T, Merino M, Casabo VG. Evidence of competitive inhibition for the intestinal absorption of baclofen by phenylalanine. Int J Pharmaceutics. 1996;132:63-69.

Doudet DJ, McLellan CA, Aigner TG, Wyatt RJ, Cohen RM. Delayed L-phenylalanine infusion allows for simultaneous kinetic analysis and improved evaluation of specific-to-nonspecific fluorine-18-DOPA uptake in brain. J Nucl Med. 1992;33(7):1383-1389.

Ensminger AH, Ensminger ME, Konlande JE, Robson JRK. Foods & Nutrition Encyclopedia. 2nd ed. Baton Rouge, Fla: CRC Press, Inc; 1994:1,2:60–64, 1,748.

Garrison Jr RH, Somer E. The Nutrition Desk Reference. 3rd ed. New Canaan, Conn: Keats Publishing, Inc; 1995:39–52.

Haas EM. Staying Healthy With Nutrition. Berkeley, Calif: Celestial Arts Publishing; 1992.

Herbert V, Subak-Sharpe GJ, eds. Total Nutrition (Mount Sinai School of Medicine). New York, NY: St. Martin's Press; 1995:318–320.

Newstrom H. Nutrients Catalog. Jefferson, NC: McFarland & Co; 1993:303–312.

Pietz J. Neurological aspects of adult phenylketonuria. Curr Opin Neurol. 1998;11:679–688.

Pietz J, Dunckelmann R, Rupp A, et al. Neurological outcome in adult patients with early-treated phenylketonuria. Eur J Pediatr. 1998;157:824–830.

Shils ME, Olson JA, Shike M, Ross AC. Modern Nutrition in Health and Disease. 9th ed. Baltimore, Md: Williams & Wilkins; 1999:41, 1,010.

Start K. Treating phenylketonuria by a phenylalanine-free diet. Prof Care Mother Child. 1998;8:109–110.

Werbach MR. Nutritional Influences on Illness. 2nd ed. Tarzana, Calif: Third Line Press; 1993:159–160, 384, 434, 494–495, 506, 580, 613–614, 636.

Woodward WR, Olanow CW, Beckner RM, et al. The effect of L-dopa infusions with and without phenylalanine challenges in parkinsonian patients: Plasma and ventricular CSF L-dopa levels and clinical responses. Neurology. 1993;43:1704-1708.

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