In the United States a heel stick test (using a needle prick on the baby’s heel for a small blood sample) is done at the ripe old age of 3 days old to test for Phenylketonuria (PKU) and other disorders. PKU is a metabolic disorder that shows up when the gene is inherited from both parents of the newborn, which is an enzyme deficiency that is needed for proper metabolism of the amino acid phenylalanine.

People with PKU must avoid foods that contain phenylalanine altogether from birth, however, in order to survive since side effects can include pigmentation loss in the skin/eyes/hair, a “mousy” odor, muscles pains and aches, seizures, and mental retardation.

Newborn screening and phenylalanine-restricted diets for PKU patients

Phenylalanine is an essential amino acid that is in foods, including proteins like meats, dairy products, beans, eggs, tofu, nuts, and many others, including aspartame (the sugar substitute in diet soda). An “essential” amino acid means that the body cannot produce this amino acid on its own so it must be gotten from food. Aminos phenylalanine and tyrosine are both associated with PKU and are tested for in all newborn babies at clinics and hospitals.

According to the American Academy of Family Physicians, it is Dr. Richard Koch from the University of Southern California School of Medicine in Los Angeles, California, who promotes that repeat testing should occur in any child that tests positive for PKU. The amino acid phenylalanine in foods should be avoided and a special diet should begin from birth so as to prevent mental retardation.

Koch says, “Occasionally, cases of PKU are missed by newborn screening. Thus, a repeat PKU test should be performed in an infant who exhibits slow development.”

Phenlyketonuria (PKU) is a recessive defect in the enzyme phenylalanine hydroxylase

Koch discusses the enzyme deficiency: “Phenylketonuria (PKU) is caused by an autosomal recessive defect in the enzyme phenylalanine hydroxylase, which is required for converting phenylalanine to tyrosine. (Five percent of natural protein is composed of phenylalanine.)”

PKU is caused by a mutation that is located on chromosome 12, although the specific type of mutation may vary, which results in severity that is variable among those with the PKU disorder. Phenylalanine containing foods should be avoided in all of these cases. Of course, normal people who do not have PKU need phenylalanine and should have a diet consisting of enough proteins to provide this essential amino acid.