Rice Bran Arabinoxylan Compound and Tryptophan Metabolism on Quality of Life of Cancer Patients: A Secondary Analysis of the RBAC-QoL Study

Tryptophan metabolism, with its breakdown along the kynurenine pathway, is linked to the diminishing quality of life (QoL) of cancer patients. Rice bran arabinoxylan compound (RBAC) is a plant-based immunomodulator shown to improve the global QoL and functioning beyond the placebo during systemic cancer treatment in a randomised-controlled trial (RBAC-QoL study). However, whether RBAC improved QoL through the tryptophan pathway and the role of tryptophan and its kynurenine metabolite in these QoL outcomes was not investigated. In this study, serum tryptophan and kynurenine were analysed using samples collected from the RBAC-QoL study with a validated liquid chromatography method and monitored via fluorescence detection. This secondary analysis was conducted using repeated-measures ANOVA, Spearman’s correlation, and linear mixed models.The results show that RBAC supplementation had no significant effect on tryptophan metabolism compared to the placebo. Tryptophan significantly (p ≤ 0.05) correlated positively with global QoL, physical and social functioning and negatively with fatigue, dyspnoea, appetite loss and diarrhoea. Kynurenine also demonstrated significant (p ≤ 0.05), but weaker correlations with physical and social functioning (positively), as well as fatigue and dyspnoea (negatively), albeit weaker than tryptophan. The kynurenine-to-tryptophan ratio exhibited no significant correlations with QoL. Stepwise reduction of a linear mixed model of haematological, renal, liver, and immune markers revealed that tryptophan and gamma-glutamyl transferase are prominent predictors (p < 0.001) of QoL. RBAC and serum tryptophan appeared to have an additive effect on QoL. Future research should investigate the combined impact of RBAC and tryptophan to assess any potential synergistic effects. Trial registration: The RBAC-QoL study was prospectively registered on the Australian New Zealand Clinical Trials Registry (ANZCTR Reg No: ACTRN12619000562178p, 10/04/2019).