We tested the hypothesis that resuming dietary control in early-treated phenylketonuria (PKU) is associated with improvements in white matter integrity, using data from the ReDAPT study, which previously demonstrated cognitive and psychiatric improvements with reduced phenylalanine (Phe) levels.

We re-initiated dietary control for early-treated patients with PKU and assessed the T1w/T2w ratio from standard T1- and T2-weighted magnetic resonance images, a marker of myelination and microstructural integrity. General linear mixed model (GLMM) analyses were performed to assess change in the T1w/T2w ratio from baseline over twelve months after resumption of dietary control.

Seven participants (mean age 31 years; five female) with neuroimaging were included, with a mean of 16 years off diet and baseline Phe levels of 1157 µmol/L. GLMM analyses showed significant increases in T1w/T2w ratio over time for the whole brain (β = 0.47 [95%CI = 0.28, 0.66]), left hemisphere (β = 0.36 [95%CI = 0.19, 0.54]), and right hemisphere regions of interest (β = 0.52 [95%CI = 0.30, 0.72]). Longer time off diet was also positively associated with greater T1w/T2w changes. There was no evidence for the effects of gender or age at baseline.

This study demonstrated significant increases in the T1w/T2w ratio in PKU patients as they resumed dietary control over a 12-month period. Raw Phe levels were not strongly associated with neuroimaging measures. These findings support the importance of lifelong treatment for PKU, and also demonstrate the potential reversibility of white matter changes in the disease.