Neuropsychological effects of subsequent exposure to phenylalanine in adolescents and young adults with early-treated phenylketonuria.

The team looked at teens and young adults who had been treated for PKU since babyhood.

All had stopped the special low-protein diet at age 10 and had eaten normal food for at least three years.

They gave each person a full battery of thinking, memory, and motor tests to see if the diet break had hurt their skills.

No clear losses showed up.

Scores for IQ, reaction time, and fine-movement speed looked the same as expected for their age.

The authors say stopping the diet at 10 seems safe for the brain.

Szempruch et al. (1993) used a similar teen-to-adult check-up design in autism.

They also ran long tests and found early scores could predict later life skills, just like P et al. tracked early diet choices into teen outcomes.

Baghdadli et al. (2018) followed autistic kids for 15 years and saw most stay on a low-growth path, while P et al. saw PKU kids hold steady after diet stop — the two groups end up different, but both studies warn us to watch the teen years closely.

Sievers et al. (2020) add that in genetic autism subtypes, the age a child first walks or talks shapes later IQ; P et al. show that in PKU, what happens at age 10 (diet stop) does not reshape later scores — timing matters, but in different ways for different conditions.

If you work with teens who have early-treated PKU, you can reassure families that stopping the diet at 10 likely will not harm thinking or motor skills.

Still, keep checking executive functions and self-care in session; rare problems could pop up and you will spot them faster than a yearly clinic visit.