Altered levels of phenylalanine and its metabolites in blood and cerebrospinal fluid have previously been reported in schizophrenia. This study attempted to examine whether phenylalanine kinetics is altered in schizophrenia using the ¹³C-phenylalanine breath test (¹³C-PBT).

Subjects were 20 patients with schizophrenia and the same number of controls. ¹³C-phenylalanine was administered and then ¹³CO₂ concentration in breath was monitored for 120 minutes. The Δ ¹³CO₂ at each collecting time, the maximal Δ ¹³CO₂ (C_max), the time to reach C_max (T_max), the area under the curve of time course of Δ¹³CO₂ (AUC), the cumulative recovery rate (CRR) at each collecting time of the ¹³C-PBT were calculated for each subject.

Body weight (BW) and diagnostic status were significant predictors for C_max. BW, age and diagnostic status were significant predictors for AUC and CRR at 120 minutes (CRR_0-120). A repeated measures ANCOVA controlling for age and BW revealed a different pattern of change in CRR over time between the patients and controls and that Δ¹³CO₂ in schizophrenia were lower than that in healthy control at all sampling point during 120 min, with an overall significant differences between healthy control and schizophrenia. The ANCOVA controlling for age and BW, showed that C_max, AUC and CRR_0-120 were significantly lower in schizophrenics than in controls.

Our data indicate the different change of Δ¹³CO₂ and CRR over time and the decreased C_max, AUC and CRR_0-120 of ¹³C-PBT in schizophrenia patients compared to healthy controls, suggesting the altered phenylalanine kinetics in schizophrenia.