Phenylketonuria, or PKU, is the most common amino acid disorder. It is characterized by a deficiency or complete absence of the phenylalanine hydroxylase enzyme, an enzyme involved in the conversion of the amino acid phenylalanine to the amino acid tyrosine. Because this enzyme is lacking or absent, there is a buildup of phenylalanine. Because there is no conversion to tyrosine, tyrosine becomes an essential amino acid, or an amino acid that must be ingested and cannot be synthesized in the body. This buildup, when untreated, can lead to mental retardation and severe behavioral problems, as well as neurological problems, eczema and seizures. Nutritional treatment requires a lifetime dietary restriction to achieve optimal health and quality of life.
Consult a physician or other medical professional before making any drastic changes to your diet.
Goals of Dietary Treatment
PKU treatment is based on limiting dietary protein consumption, which is the source of phenylalanine. Protein needs must be met with a formula that contains all amino acids except for phenylalanine. The goal of the dietary treatment is to have maximum dietary protein consumption for growth promotion while limiting the amount of phenylalanine to maintain the desired range of blood levels. The National Institute of Health recommends between two and six milligrams per deciliter (mg/dL) for children below 12 years, and a range of two to 15 mg/dL for those older than 12 years.
Determining Protein Recommendations
People with PKU are assessed by a professional to determine protein and calorie needs. The amount of protein allowed in the diet is based on the amount of phenylalanine hydroxylase enzyme, the enzyme involved in converting phenylalanine to tyrosine. Some people with PKU have almost no enzyme activity and need a high level of protein restriction; others can survive with fewer protein restrictions, because their systems have more phenylalanine hydroxylase activity. Monitoring PKU blood levels closely helps maintain control of the disease.
Protein Allowance
Protein is essential for adequate growth. To maintain proper blood levels of phenylalanine, it is more important to calculate the amount of phenylalanine in particular foods than to count the grams of protein. The amount of protein and subsequent phenylalanine ingestion allowed in the diet usually comes from a controlled amount of fruits, vegetables and some grain products. Most protein-rich foods like meats and dairy products, as well as many legumes, are highly restricted due to their high phenylalanine content.
Supplementation
After dietary protein needs are calculated, particular formulas may be added to the diet to supplement missing protein. Phenylfree is such a supplement. This is a phenylalanine-free powder that contains additional protein as well as vitamins and minerals. Because those with PKU may become calcium-deficient due to a lack of dairy intake, calcium supplementation may be recommended by a professional.
High- and Low-Phenylalanine Foods
High-phenylalanine foods include nuts, chicken, pork, eggs, cheese, beans, milk and beef. Low-phenylalanine foods include fruit juices, low-protein breads and pasta and some fruits and vegetables. It is important for people who have PKU to get a variety of low-phenylalanine foods while avoiding those foods that are the biggest offenders.
References
- "Nutrition Therapy and Pathophysiology"; Sara Long, Marcia Nelms, KathrynSucher; 2007
- Cristine M. Trahms Program for Phenylketonuria: What Is the Diet for PKU?
