Blood phenylalanine levels and intelligence of 10-year-old children with PKU in the National Collaborative Study.

Dietary intakes and blood phenylalanine levels of 125 10-year-old children with PKU are reported. Of those patients, 59 discontinued the special diet at 6 years, 16 discontinued at 8 years, 5 discontinued at 6 years but returned to the diet at 8 1/2 years, and 45 continued the diet to 10 years. Phenylalanine intake was determined by 2-day diet records. Blood phenylalanine concentrations revealed a lack of clear distinction between the diet groups; therefore, regression analysis was performed on the group as a whole. The analyses revealed significant relationships between blood phenylalanine levels from ages 6 to 10 years and each of the cognitive outcome variables, after the influences of parental IQ and the age at which treatment was initiated were controlled for. Intelligence test scores were also related to phenylalanine levels between ages 3 1/2 and 5 1/2 years. The findings suggest that dietary restriction of phenylalanine should continue in patients with PKU through at least 10 years of age.