Kynurenine pathway in depression: A systematic review and meta-analysis

•We conducted the first meta-analysis on metabolite levels of kynurenine pathway in patients with depression.•Out of 899 initial records, 22 articles were identified.•Kynurenic acid and kynurenine levels are decreased in patients with depression.•Quinolinic acid levels are increased in unmedicated patients with depression.•Future research should examine relationships between treatment and kynurenine pathway.