The role of tryptophan degradation in the association between inflammatory markers and depressive symptoms in chronic dialysis patients

Background: Among chronic dialysis patients, associations have been found between inflammatory markers and depressive symptoms. In this population, no studies have examined the mechanism linking the association between inflammatory markers and depressive symptoms. We examined whether the association between inflammatory markers and depressive symptoms is mediated by tryptophan (TRP) degradation along the kynurenine (KYN) pathway. Methods: The data are part of an observational, prospective cohort study in five urban dialysis centres in The Netherlands. Depressive symptoms were determined with the Beck Depression Inventory. Peripheral blood was collected before dialysis to measure inflammatory markers [high sensitivity Creactive protein (HsCRP), interleukin (IL)-1b, IL-6, IL-10 and tumour necrosis factor-A (TNF-A)], TRP, KYN and 3-hydroxykynurenine. The KYN/TRP ratio was used as a measure of TRP degradation. The association between inflammatory markers and depressive symptoms was determined using linear regression analysis and adjusted for the KYN/TRP ratio. Results: In total, 490 chronic dialysis patients were included. HsCRP [b=3.8; confidence interval (CI): 1.0-6.6], IL-6 (b=9.1; CI: 4.0-14.1) and TNF-A (b=1.3; CI: 0.9-1.7) were associatedwith the KYN/TRP ratio. We found significant associations between HsCRP (b=0.8; CI: 0.3-1.3) and IL-6 (b=1.2; CI: 0.3-2.2) levels and depressive symptoms. However, this association was not attenuated after adjustment for the KYN/TRP ratio. Also, no significant associations were found between the KYN/TRP ratio and depressive symptoms. Conclusion: The association between inflammatory markers and depressive symptoms in chronic dialysis patients was not mediated by TRP degradation along the KYN pathway.