Effects of a protein‐restricted diet on body weight and serum tyrosine concentrations in patients with alkaptonuria

In an open‐label, controlled study of nitisinone in alkaptonuria (SONIA 2), patients were advised to lower dietary protein intake to reduce serum tyrosine (s‐Tyr) levels and the risk of keratopathy. A body weight increase was observed in the nitisinone‐treated patients but not in the control group. To investigate the effectiveness and consequence of protein restriction in patients with alkaptonuria, a post‐hoc analysis of SONIA 2 was performed. One hundred and thirty‐eight patients were randomised (nitisinone: n = 69, controls: n = 69). Comparison of baseline and Month 12 data on 24‐h urinary excretion of HGA (u‐HGA24) and urea (u‐urea24, used as an approximate protein intake measure), tyrosine and body weight were performed using paired t tests. Comparisons of data between groups were made using 2‐sample t tests. We found that u‐urea24 decreased more in nitisinone‐treated than controls. The study centre with lowest average s‐Tyr and u‐urea24 (nitisinone arm) at Month 12 also had lowest keratopathy incidence (3.1%), while the centre with highest values showed the highest (14.6%). S‐Tyr was generally high in those with keratopathy, but those without keratopathy had similar elevated values. A similar pattern across centres was seen for body weight changes, with a statistically significant weight increase in nitisinone‐treated patients at centres with lower u‐urea24 values. Therefore, in nitisinone‐treated patients, protein restriction led to increased body weight but may also have lowered the risk of developing keratopathies. If introduced, a protein‐restricted diet should be supervised by a dietician and, when appropriate, include amino acid supplements deficient in tyrosine and phenylalanine, to avoid malnutrition and undesired weight increase.