The phenylalanine restricted diet is used for people who lack the enzyme (phenylalanine hydroxylase) necessary to convert phenylalanine to tyrosine, causing a disorder called phenylketonuria (PKU). High amounts of phenylalanine are toxic to the brain and can cause mental retardation. Prior to newborn screening (most states started to screen in the 1960s), individuals with PKU were not identified until the brain damage had occurred. Even though a phenylalanine-restricted diet will not reverse the mental retardation that may have occurred, it may reduce some of the behavior problems these individuals may have. When newborn screening began, infants were placed on the diet at diagnosis and kept on the diet until 5 to 8 years of age when it was thought safe to go off the diet because the most rapid time of brain development was over. As more individuals have been off the diet for longer periods of time, it is apparent that high amounts of phenylalanine continue to be a brain toxin and can cause varying degrees of brain damage. Currently, individuals begin the diet at diagnosis and the recommendation is to continue the diet for life so brain damage does not occur.
Over the past few years, some new treatments for PKU have emerged. Tetrahydrobiopterin or BH4 is a cofactor required for the enzyme phenylalanine hydroxylase to work optimally. A synthetic form of this cofactor marketed under the name of Kuvan® was approved by the FDA in December 2007. Clinical trials indicate that Kuvan® taken orally enhances the efficiency of phenylalanine hydroxylase in some individuals with PKU. This can lead to decreased serum phenylalanine levels or increased natural protein intake.
Another relatively new treatment is the use of large neutral amino acids (LNAA). These are thought to lower the phenylalanine in the brain by competing with phenylalanine to cross the blood-brain barrier. These can be used by individuals who are off diet and find it difficult to return to a phenylalanine-restricted diet, but who wish to see if this improves their mood, memory, or behavior.
For more information Contact the Iowa Academy of Nutrition and Dietetics.