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Meta-Analysis
. 2021 May 19:12:667179.
doi: 10.3389/fimmu.2021.667179. eCollection 2021.

Tryptophan Catabolites in Bipolar Disorder: A Meta-Analysis

Kaat Hebbrecht  1   2 , Katrien Skorobogatov  1   2 , Erik J Giltay  1   3 , Violette Coppens  1   2 , Livia De Picker  1   2 , Manuel Morrens  1   2
Affiliations
Meta-Analysis

Tryptophan Catabolites in Bipolar Disorder: A Meta-Analysis

Kaat Hebbrecht et al. Front Immunol. .

Abstract

Objective: Tryptophan catabolites (TRYCATs) are implicated in the pathophysiology of mood disorders by mediating immune-inflammation and neurodegenerative processes. We performed a meta-analysis of TRYCAT levels in bipolar disorder (BD) patients compared to healthy controls.

Methods: A systematic literature search in seven electronic databases (PubMed, Embase, Web of Science, Cochrane, Emcare, PsycINFO, Academic Search Premier) was conducted on TRYCAT levels in cerebrospinal fluid or peripheral blood according to the PRISMA statement. A minimum of three studies per TRYCAT was required for inclusion. Standardized mean differences (SMD) were computed using random effect models. Subgroup analyses were performed for BD patients in a different mood state (depressed, manic). The methodological quality of the studies was rated using the modified Newcastle-Ottawa Quality assessment Scale.

Results: Twenty-one eligible studies were identified. Peripheral levels of tryptophan (SMD = -0.44; p < 0.001), kynurenine (SMD = - 0.3; p = 0.001) and kynurenic acid (SMD = -.45; p = < 0.001) were lower in BD patients versus healthy controls. In the only three eligible studies investigating TRP in cerebrospinal fluid, tryptophan was not significantly different between BD and healthy controls. The methodological quality of the studies was moderate. Subgroup analyses revealed no significant difference in TRP and KYN values between manic and depressed BD patients, but these results were based on a limited number of studies.

Conclusion: The TRYCAT pathway appears to be downregulated in BD patients. There is a need for more and high-quality studies of peripheral and central TRYCAT levels, preferably using longitudinal designs.

Keywords: bipolar disorder; depression; immune; inflammation; kynurenine; tryptophan.

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Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

Figure 1
Figure 1
Kynurenine Pathway [previously published in Morrens et al. (4)].
Figure 2
Figure 2
Flowchart.
See this image and copyright information in PMC

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