The phenylalanine response curve in relation to growth and mental development in the first year of life.

Thirty-three children with classical phenylketonuria were treated from 9 to 21 days of age onward. The standard for phenylalanine concentration was 0.2-0.5 mmol/l. During the first years of life phenylalanine concentrations were measured once every two to six weeks. For each child these values were graphically plotted in a so-called phenylalanine response curve. Growth and mental development indices were assessed at about one and two years of age. Measurements for weight, height and head circumference were normal at one month of age, showed a significant decline at the age of one year but were restored for normal values at the age of two. No associations were found with phenylalanine parameters. Mental development indices at one and two years were not different from normal. Significant negative correlations were found between mental development indices at one year and phenylalanine values above 0.5 mmol/l. Changes in mental development indices between the first and second years of life are significantly related to the phenylalanine levels during the second year of life.