Phenylketonuria, or PKU, is an inherited disorder in which an enzyme deficiency causes buildup of the protein building blocking phenylalanine in the bloodstream. In high concentrations, phenylalanine damages the brain. Because early diagnosis and treatment are essential to prevent brain damage and mental retardation, all newborn babies in the United States are screened for PKU. Although phenylketonuria is incurable, brain damage can be prevented by following a strict, low-phenylalanine diet and taking prescription nutritional supplements.
No Meat, Fish, Eggs and Dairy Products
The amino acid phenylalanine is present in large amounts in foods that contain significant amounts of protein, including all animal meats, fish, seafood, eggs, milk and dairy products. Normally, the enzyme phenylalanine hydroxylase breaks down excess dietary phenylalanine. This enzyme, however, is lacking in people with PKU. With no mechanism to break down phenylalanine, blood levels climb with continued consumption of dietary protein. People living with phenylketonuria must follow the PKU diet carefully because even occasional lapses may adversely affect brain development and function. Medical protein supplements formulated without phenylalanine are used to provide this essential dietary nutrient. Vitamins and minerals are typically included in these supplements to prevent deficiencies of these micronutrients.
No Nuts, Seeds and Legumes
High-protein plant products --- including dried beans, peas, lentils, seeds, tofu and other soy products, nuts and nut butters --- contain large amounts of phenylalanine and must be avoided on a PKU diet. Throughout infancy, childhood and adolescence, doctors monitor blood phenylalanine levels to be sure they are neither too high nor too low. Although a high phenylalanine level can damage the brain, a low level may interfere with normal growth of other body tissues and organs.
No Aspartame
The artificial sweetener aspartame contains the amino acid phenylalanine and should be avoided by people with PKU. Diet sodas commonly contain aspartame as do many other sugar-free and reduced calorie products, such as candy, yogurt, desserts, jellies and sweet sauces. The U.S. Food and Drug Administration requires that all products marketed in the United States that contain aspartame include a warning statement noting the presence of phenylalanine for people living with PKU.
Restrict Wheat and Other Grains
Wheat and other grains contain moderate amounts of phenylalanine, necessitating restricted intake on a PKU diet. Practically speaking, this means that regular bread, pasta, cereals, crackers, cookies and other baked goods are off-limits or severely restricted on a PKU diet. Low-protein varieties of these products are available from specialty stores and online retailers, although intake amounts must be carefully coordinated with the health care team, based on the patient's blood phenylalanine levels.
Fruits and Vegetables
Fruits and vegetables are the primary staples of the PKU diet because most are low in phenylalanine. Due to variability in the amount of phenylalanine among different varieties of fruits and vegetables, some are preferred over others for regular inclusion in a PKU nutrition plan.
References
- Gene Reviews; Phenylalanine Hydroxylase Deficiency; John J. Mitchell, M.D., Charles R. Scriver, M.D.; May 2010
- Star-G: Phenylketonuria
- Baptist Health Systems; Phenylketonuria; Amy Marlow, M.P.H., R.D., C.D.N.; September 2010
- University of Washington: What is the Diet for PKU?
- University of Washington: PKU Management Guidelines
- National Institute of Child Health & Human Development: Phenylketonuria (PKU)
