thirtiestxt

In 1934 Folling discovered abnormal metabolites of phenylalanine in the
urine of two mentally retarded children Guttler (1984). Phenylketonuria
was the first of the inborn errors of metabolism to be identified. The
abnormal metabolites in urine were derived from a faulty enzyme in the
liver. These metabolites cross the blood-brain barrier in young children
and are toxic to the brain.

Neuropathology of phenylketonuria indicates failure of myelin nerve
sheaths to form. Brain damage can be prevented by restricting
phenylalanine in the diet for about the first five years of life, after which
time the blood-brain barrier has matured and abnormal metabolites are
excluded entry into the brain.

It has been found that women with phenylketonuria must go back on a
low phenylalanine diet during pregnancy because the abnormal
metabolites do cross the placenta and damage the developing brain in
many ways similar to the effects of prenatal exposure to alcohol.