Rare conditions with nutrition implications: Phenylketonuria

All infants born in the United States are screened at birth for phenylketonuria (PKU). This stems from a single inherited genetic error and changes nutritional requirements for life.

What is PKU?

PKU is caused by a mutation in the PAH gene, which codes for the production of phenylalanine hydroxylase. This enzyme converts phenylalanine into different byproducts, including tyrosine and melanin. When this enzyme is absent or present in insufficient amounts, phenyalanine can build up in the bloodstream and cause damage.

The symptoms of PKU can range from mild to severe, depending on the level of activity, if any, of phenylalanine hydroxylase enzyme. Infants are screened and often diagnosed at birth. If the condition goes undiagnosed or untreated, symptoms include a musty odor of the breath, skin, or urine; seizures; skin conditions, including eczema; delayed intellectual development; and even brain damage.

Nutrition implications of PKU

Because PKU is caused by an inability to break down phenylalanine, medical nutrition therapy involves restricting phenylalanine. Phenylalanine is an amino acid and thus, found in high-protein foods, including animal meats, fish, nuts, beans, soy, and dairy. Interestingly, some artificial sweeteners also contain phenyalanine. Because of the underlying risk to individuals with PKU, the FDA requires food manufacturers to disclose if their product contains phenylalanine on the food label.

Following a PKU-friendly diet is easier said than done. Protein cannot simply be eliminated, as it is essential to normal growth and development, and to fuel daily body processes. Fortunately, there are specialized formulas and supplements made for infants, children, and adults with PKU that restrict phenylalanine while supplementing protein and other nutrients.

In addition to a low phenylalanine diet, adults with PKU may be appropriate for neutral amino acid therapy, which helps block some of the body’s absorption of phenylalanine. Likewise, sapropterin (Kuvan®) is an FDA-approved medication to treat PKU in adults. The drug is a synthetic version of BH4, a compound integral to the conversion of phenylalanine to tyrosine. Both neutral amino acid therapy and sapropterin should be used under medical supervision and in conjunction with a low phenylalanine diet.

How dietitians can help

Navigating PKU can be difficult and overwhelming. Guidance from a registered dietitian is essential to successfully mitigating the risks of untreated PKU. Dietitians are trained to educate clients and caregivers about dietary sources of phenylalanine and appropriate foods that are phenylalanine-free. In pediatric patients, dietitians can monitor growth trends and detect deviations and deficiencies early.

Acute care dietitians have a different role to play. Here, dietitians should focus education efforts on the foodservice team and make sure they know what foods are safe to serve. Early identification of patients with PKU is essential, as is coordination with the nursing staff to prevent the delivery of unsafe meals or snacks to the patient. Encourage the patient’s family members to bring PKU-friendly formulas and supplements from home to ensure the patient’s nutritional needs are met while in-house.

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References

KUVAN. How KUVAN works. https://www.kuvan.com/about-kuvan/how-kuvan-works/. Accessed June 7, 2019.

Mayo Clinic. Phenylketonuria (PKU). https://www.mayoclinic.org/diseases-conditions/phenylketonuria/symptoms-causes/syc-20376302. Updated January 27, 2018. Accessed June 7, 2019.

NIH U.S. National Library of Medicine. Genetics Home Reference. Phenylketonuria. https://ghr.nlm.nih.gov/condition/phenylketonuria. Reviewed October 2017. Accessed June 7, 2019.