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, were not previously investigated. In this
study, serum tryptophan and kynurenine were analysed in samples collected from the RBAC-QoL study using a validated
liquid chromatography method with ultraviolet and 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 statistically significant effect on tryptophan metabolism compared to the placebo. Tryptophan
significantly (P ⩽ .05) correlated positively with global QoL, physical and social functioning and negatively with fatigue,
dyspnoea, appetite loss and diarrhoea. Kynurenine also demonstrated significant (P ⩽ .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 of QoL (P < .001). 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.