The plasma phenylalanine level was measured on at least two occasions, usually more,
in 81 patients with phenylketonuria on unrestricted diets. The distribution of the
individual levels was gaussian and the mean (31 mg/100 ml) did not change significantly
over an average span of 12.7 years. Persistent individual differences were found but
so were unexpected and inexplicable high or low plasma phenylalanine levels in particular
patients. Variations in dietary phenylalanine intake did not explain the differences
in plasma phenylalanine levels between individuals. There was no clear relationship
between the degree of mental disability and the average plasma phenylalanine except
that those with the least brain damage had slightly lower average levels (24.2 mg/100
ml). The difficulties of demonstrating genetic heterogeneity in phenylketonuria are
illustrated even though this is likely present. It is argued that the assumption of
a direct quantitative relationship between the degree of brain damage and the height
of the plasma phenylalanine level is an oversimplification.
Mental retardation - phenylketonuria - phenylalanine
